Dorcas Mrs

Mr Pwavodi

Dr Paul Mamman

Cos

Dr Kayode Oshinubi

Dr Rose Bote

Subduction zones are the loci of active seismicity and large tectonic deformation. The foot of the accretionary prism has long been considered as aseismic zones where earthquake energy dissipates, but recently, mega-earthquakes have activated large displacements near subduction trenches, generating devastating tsunamis. It is important to better understand the seismotectonics of these zones. The Nankai subduction zone is one of the most suitable study sites: in addition to the seismic hazard of mega-earthquakes (M w > 8) for densely populated Japanese coasts, it is also the site of slow slip. This was the motivation for the NanTroSEIZE project, a flagship project of the International Ocean Discovery Program (IODP), which consisted of 13 expeditions from 2004 to 2019. A detailed understanding of the hydraulic regime is needed to understand the diversity of behavior at Nankai. Indirect evidence suggests that this subduction zone is particularly fluid rich. To refine these conclusions, we innovatively integrate drilling and logging data acquired in the NanTroSEIZE project, in the frontal region (sites C0024 and C0006) and in the "megasplay" fault region (site C0004). We obtain a high-resolution hydraulic properties profile along the NanTroSEIZE drillings, to finely characterize the fault architecture in the Nankai subduction zone. This will help to estimate: (a) pore pressure profiles across the accretionary prism (b) high-resolution hydraulic profiles to identify the damaged zone around the detachment fault, and verify whether it is a significant drainage path. (c) permeability evolution. This allows inferring the origin of the fluids (diffusion along the décollement or local fluid emission following compaction of the sedimentary pile), and extrapolating the data near the hole to deeper seismically active zones. Mud pressure reanalysis allows us to estimate the inflow of fluids from the formation to the borehole. To verify its predictions, 2 methods have been used to predict pore pressure: (a) Eaton-type methods based on drilling or seismic data (b) empirical relationships between pore pressure, porosity, and P-wave velocity, obtained from consolidation experiments.

Permeabilities can be calculated by combining flow and pressure estimates. At Site C0024, a fluid influx (> 0.05m 3 /s) is localized in the damaged zones of the décollement at 813 mbsf. It is associated with a significant overpressure (P * ≈2.38 -4.79M P a) below the plate boundary.

The décollement acts as a vertical hydraulic barrier, but as a longitudinal drain that diffuses overpressures from the site where earthquakes and slow slides occur. In contrast, at Site C0006, fluid influxes remain weak, and localized just below the frontal thrust at 711 mbsf. There is no apparent increase in pore pressure within the accretionary prism, perhaps because a fault between C0024 and C0006 drains the overpressurized fluids. At Site C0004B, a large fluid flow (0.07m 3 /s) and high pore pressure are also observed in the fault zone at about 310 m depth. The large fluid flow is evidence of high pore pressure, facilitating propagation of the coseismic slip to the seafloor. i

The megasplay region is then more favorable than the foot of the prism region for generating tsunamigenic earthquakes.

R ÉSUM É

Les zones de subduction sont le site d'une intense activité sismique et de grande déformation tectonique. Le pied du prisme d'accrétion ont été longtemps considérées comme des zones asismiques où l'énergie des séismes de dissipait. Mais récemment, des mégaséismes ont activés des grands déplacement près de fosses de subduction, générant des tsunamis dévastateurs. Il est important de mieux comprendre la sismotectonique de ces zones. La zone de subduction de limit is below 150°C and the downdip limit is above 350°C [START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF]. . .

2.6

Asperity model of the interplate interaction in subduction zones [START_REF] Kanamori | Rupture process of subduction-zone earthquakes[END_REF], [START_REF] Lay | The asperity model and the nature of large subduction zone earthquakes[END_REF] (2012) with the results of the pore pressure ratio results estimated at the frontal thrust region and the megasplay fault region (areas in green rectangles). The results are expressed as pore pressure ratio. Modified after [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] These zones are known to host varying destructive earthquake magnitudes [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF][START_REF] Kanamori | Rupture process of subduction-zone earthquakes[END_REF], 1971, 1972) (Figure 2.2). Other non-classical earthquakes, like slow slip events (SSE), very low frequency earthquakes (VLFE) and tremors [START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF] are also hosted in these zones.

In subduction zones, earthquakes occur along the subduction plate interface within the fully or partially locked seismogenic zone and the two bounding aseismic zones: the updip direction towards the trench and the downdip portions of the seismogenic zone. Generally, subduction zones demonstrate large variability of seismic behaviours at local and regional scales [START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF]. Some zones are largely seismic or aseismic with large, intermediate or small magnitude earthquakes. [START_REF] Scholz | Evidence for a strong San Andreas fault[END_REF] identified that large magnitude earthquakes in subduction zones occur on preexisting faults that undergo seismic failure in a simultaneous multiple seismic cycles. Fault slip behaviours are influenced by elevated pore pressures and high pressure fluids trapped at depths [START_REF] Hyndman | Thermal constraints on the zone of major thrust earthquake failure: The cascadia subduction zone[END_REF][START_REF] Magee | Evidence for a weak interplate thrust fault along the northern Japan subduction zone and implications for the mechanics of thrust faulting and fluid expulsion[END_REF][START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF]. Understanding the interplay of fluid flow, high pore pressure, and the resulting decrease in effective stress is key to understanding tectonic deformation and earthquake occurrence in subduction zones [START_REF] Jaeger | Friction of Rocks and Stability of Rock Slopes[END_REF]. Fluids are introduced in subduction zones as fluid-rich sediments from the incoming plate underthrust or are scrapped off to build up the accretionary prism [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]. Fluids are also released by mineral dehydration or sediment compaction, diagenesis, rapid burial and tectonic loading associated with disequilibrium compaction [START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF]. In addition to these sources, the fluid distribution is controlled by fluid migration. Fluids can escape through the porous network of rocks or be channeled along fractures or fault zones, whose permeability can be influenced by the maturity of the fault zone architecture [START_REF] Caine | Fault zone architecture and permeability structure[END_REF].

Elevated pore pressure and fluid flow substantially influence earthquake cycles and tectonic deformations. Hence, it has become important to understand the spatial distribution of in situ hyraulic properties and how it influence fault mechanics and seimogenesis in subduction zones.

To understand these properties, several drilling campaigns were launched over the years at different subduction zones with the basic objective of understanding the fault mechanics and seismogenesis of subduction thrust (Tobin andKinoshita 2007a, Tobin et al. 2009). In Nankai subduction 1.2 -RESEARCH OBJECTIVES zone, the multi expedition drilling campaign was undertaken under the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project of the international ocean discovery program (IODP) (Tobin andKinoshita 2007a, Tobin et al. 2009). The NanTroSEIZE project has hosted thirteen IODP expeditions between 2004 to 2019. Nankai subduction zone has been identified as the locus of slow slips, low frequency earthquakes and very large classical magnitude earthquakes exemplified by M w > 8 1946 Nankaido earthquake and 1944 Tonankai earthquake [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF][START_REF] Kanamori | Tectonic implications of the 1943 Tonankai and the 1946 Nankaido earthquakes[END_REF][START_REF] Obara | Very low frequency earthquakes excited by the 2004 off Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism[END_REF][START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF].

Although the Nankai subduction zone is one of the most investigated subduction zones, many questions remain unanswered about the hydrogeologic behavior of the subduction megathrust and the aseismic-to-seismic transition of the megathrust system (Tobin andKinoshita 2007a, Saffer andTobin 2011). In order to gain a better knowledge of the hydraulic properties that are present at the fault zones, accretionary prism, and underthrusting sediments, a number of boreholes have been drilled in the Nankai subduction zone (Tobin and Kinoshita 2007a, Tobin et al. 2009, Kinoshita et al. 2009a[START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]). The goal is to learn more about how plate boundaries and active megasplay faults contribute to the genesis of tsunamis, SSEs, and VLFEs (Tobin and Kinoshita 2007a).

Research Objectives

The dynamics of earthquakes are thought to be significantly influenced by high pore pressure and considerable fluid flow. Since the hydraulic regime of the Nankai accretionary prism is crucial to comprehending this variety of behaviors, a complete understanding of the hydraulic regime is required. Our studies contributes to understanding the hydraulic regime in Nankai Trough by innovatively integrating drilling and logging data acquired during the NanTroSEIZE project, from the frontal thrust region (Sites C0024 and C0006) and the megasplay region (Site C0004

). Therefore, we combine drilling engineering methods with geophysical approaches to provide high-resolution continuous quantitative profile of hydraulic properties at metric scale with focus on of the décollement fault zone at the toe of Nankai accretionary prism and the megasplay faults region. Our study helps to resolve:

• High resolution hydraulic profiles by characterising the structure of the damage zone of the décollement and identifying whether the décollement or the megasplay faults acts as a significant flow channel. Given the large spectrum of faults in the subduction zone (the décollement, the megasplay faults, the out-of-sequence thrust (OOST) faults, metric fractures etc ), hydraulic properties must be resolved continuously, with a resolution on the metric scale to discriminate and quantify both contributions to fluid flow and high pore fluid pressure generation.

• Pore pressure profiles along the accretionary prism, hence assessing the origin of the fluids, especially whether they diffuse from the décollement, or whether they are inherited from sedimentary compaction or from tectonic loading.

• To investigate how fluid flow and high pore fluid pressure are affected by the faults and 1.3 -THESIS OUTLINE sediments permeability variations along the borehole.

• To connect the fault zone as a hydraulic channel, moderate pressure at shallow depth suggests high pressure at moderate depth, could be associated with the generation of VLFE and SSE within the Nankai accretionary prism.

Thesis Outline Thesis Outline

• The first chapter is based on the general introduction with context of the study. The research objectives and the thesis outline.

The part A of the thesis groups the state of art about the subduction zone hydrogeology. This is categorized into three chapters.

• The chapter two is based on the seismicity of subduction zones, earthquake generation and the role of fluids and pore pressure.

• The chapter three captures the existing methods to study hydrology of subduction zone.

• The chapter four details the Nankai subduction zone geology, its seismic behaviours and the NanTroSEIZE project.

The part B groups the main methodology used for the research.

• The chapter five details the introduction to drilling mud and circulations, the methodology for modeling fluid flow from mud pressure and proxy estimates for predicting pore pressure along the borehole.

The part C groups the main hydraulic profile results and application to the Nankai subduction zone at the frontal thrust region (Sites C0024 and C0006) and the megasplay fault region (Site C0004).

• The chapter six details the results and discussion of the hydraulic profile from Site C0024 located at the toe of the accretionary prism. The Discussion part captures the relevance of the hydraulic profile, the fault architecture and sesimogenic potential.

• The chapter seven details the results and discussion of the hydraulic profile from Site C0006 located at the toe of the accretionary prism. The Discussion part captures the relevance of the hydraulic profile and the fault architecture.

• The chapter eight details the results and discussion of the hydraulic profile from Site C0004 located at the megasplay region. The Discussion part captures the relevance of the hydraulic profile, the fault architecture and tsumagenic potential.

The part D captures the regional discussion of the hydraulic profile of the décollement and the megasplay fault.

• The chapter ten details the discussion about the heteregenous décollement at the toe of the acrretionary prism, the fluid rich and complex megasplay fault.

Finally, the part E captures the conclusion, scientific contributions to the hydrogeology of the Nankai Subduction Zone, limitatations of study and the future work.

Part A

CONTEXT OF THE STUDY CHAPTER 2

Introduction to seismicity of subduction zones and the importance of hydrogeology

Seismic behavior in subduction zones has been shown to vary on both local and regional scales, as evidenced by numerous studies [START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF]. Therefore, some of the subduction zones such as Nankai, Columbia, southern Chile, Alaska, and the Aleutians (Fig-

ure 2.
2) can experience large magnitude earthquakes above M W > 8.2, while other subduction zones like Mexico, the Mariana Islands, and the New Hebrides experience earthquakes with magnitude below M W < 8.2 [START_REF] Kanamori | Rupture process of subduction-zone earthquakes[END_REF][START_REF] Kanamori | Great earthquakes at island arcs and the lithosphere[END_REF]. Most subduction zones with large magnitude earthquakes are due to slip and variable mechanical coupling strength between the downgoing dense oceanic plate and the overriding continental plate [START_REF] Kanamori | Great earthquakes at island arcs and the lithosphere[END_REF], 1977a,b, Uyeda 1982[START_REF] Uyeda | Back-arc opening and the mode of subduction[END_REF][START_REF] Kelleher | Why and where great thrust earthquakes occur along island arcs[END_REF]. Strong coupling occurs in subduction zones that experience frequent large earthquakes, while weak coupling occurs in regions that do not [START_REF] Uyeda | Back-arc opening and the mode of subduction[END_REF]. Tectonic features at plate borders are reliant on interplate contact [START_REF] Uyeda | Subduction zones: An introduction to comparative subductology[END_REF][START_REF] Uyeda | Subduction zones: Their diversity, mechanism and human impacts[END_REF], hence earthquakes occur along the interface of the subducting plate during subduction. The subducting plate boundary can be classically delimited in three zones [START_REF] Hyndman | Thermal constraints on the zone of major thrust earthquake failure: The cascadia subduction zone[END_REF][START_REF] Tichelaar | Depth of seismic coupling along subduction zones[END_REF][START_REF] Shimamoto | A Simple Rheological Framework for Comparative Subductology[END_REF][START_REF] Ruff | What Controls the Seismogenic Plate Interface in Subduction Zones?[END_REF]): (a)

The updip aseismic zone (b) the seismogenic zone (c) the downdip aseismic zone (Figure 2.4).

The seismogenic zones are bounded updip towards the trench and downdip by aseismic zones (Figure 2.4).

2.1 The Seismicity Structure of Subduction Zones

Seismogenic Zone

A high rate of seismic activity is characteristic of the seismogenic zone, which denotes the area as being particularly prone to the occurrence of earthquakes (Figure 2.3a). This zone is characterized by significant frictional and seismic heterogeneities [START_REF] Dixon | The Seismogenic Zone of Subduction Thrust Faults: Introduction[END_REF]. At a depth range of 5-10 km updip portion of the seismogenic zone, the temperature range is approximately between ∼100º -150°C (Figure 2.3b) while at 40-50 km downdip portion of the seismogenic zone, the temperature is approximately between ∼350 • C -450 • C [START_REF] Hyndman | Thermal constraints on the zone of major thrust earthquake failure: The cascadia subduction zone[END_REF], Hyndman et al. 1995, 1997[START_REF] Currie | Thermal models of the mexico subduction zone: Implications for the megathrust seismogenic zone[END_REF], Wang et al. 1995) (Figure 2.3b). The seismogenic zone can be fully locked, partially coupled, or freely sliding [START_REF] Scholz | Earthquakes and friction laws[END_REF][START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF]. When the seismogenic zone is fully locked, elastic stress and strain accumulate during the inter-seismic phase (stick), which is normally for long periods [START_REF] Scholz | Earthquakes and friction laws[END_REF][START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF]. Fault slip occurs when the accumulated strain and stress exceeds the frictional forces locking the fault, hence generating earthquakes during a co-seismic slip phase.

After the earthquake rupture, the post-seismic phase involves adjusting the fault, the crust and the mantle wedge back to initial stress state [START_REF] Scholz | Earthquakes and friction laws[END_REF][START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF].

The seismogenic zone undergoing stick-slip is characterized as fictionally unstable [START_REF] Scholz | Earthquakes and friction laws[END_REF] with velocity weakening implying that as the slip accelerates due to decreasing frictional The seismogenic zone can be identified by the following: mechanically, by large regions or asperities that, when they fail, result in a frictionally unstable or velocity-weakening situation manner. The aseismic area updip and downdip of the seismogenic zone or within the frictionally stable or velocity-strenghtening manner [START_REF] Lay | Great Earthquakes on Plate Boundaries[END_REF].

strength when the slip velocity increases (Scholz 1998, Marone andSaffer 2007) (Figure 2.4a, b).

The locking of the seismogenic zone can be spatiotemporally heterogeneous (Figure 2.4b).

Updip and Downdip Aseismic Zones

This is the aseismic zones, within which no stress build-up occurs and thrust events (great or small) do not occur to the trench [START_REF] Byrne | Loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones[END_REF]). These zones are characterized as frictionally stable with velocity strenghtening [START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF] implying the frictional strength increases with slip velocity. Recent work in the Nankai subdution zone have associated the occurrence of SSEs and VLFE with the aseismic zones [START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF][START_REF] Takemura | Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes[END_REF]. The seismogenic zone is bounded by two aseismic zones: (a) Updip Aseismic Zone and (b) Downdip Aseismic Zone.

Updip Aseismic Zone

The Updip Aseismic Zone is located (Figure 2.5a) beneath the accretionary prism sediments [START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF]. It is normally about 40 km away from the trench. For example, it is about 30 km in Nankai subduction (Ando 1975, Obara and[START_REF] Obara | Very low frequency earthquakes excited by the 2004 off Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism[END_REF] (Figure 2.5 c). The temperature range varies between 100-150°C (Figure 2.5c) and this might influence mineral dehydration and transformation of clay content in this zone. [START_REF] Oleskevich | The updip and downdip limits to great subduction earthquakes; thermal and structural models of cascadia, south alaska, sw japan, and chile[END_REF] defined the extent of the aseismic zone as the rupture limit of great earthquakes or defined by smaller earthquakes. This zone is believed to behave aseismically due to the presence of high porosity, fluid-rich sediments, clay mineral rheological behavior or diagenesis [START_REF] Byrne | Loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones[END_REF]).

Downdip Aseismic Zone

It is located down-dip portion beyond the seismogenic zone (Figure 2.5b). It ranges between 10-50 km and is characterized by a temperature above 350°C (Blanpied et al. 1991, 1995, Tse and Rice 1986) (Figure 2.5 c). In young subduction zones, the downdip limit is thermally influenced and limited [START_REF] Tichelaar | Depth of seismic coupling along subduction zones[END_REF][START_REF] Hyndman | Thermal constraints on the zone of major thrust earthquake failure: The cascadia subduction zone[END_REF][START_REF] Ruff | What Controls the Seismogenic Plate Interface in Subduction Zones?[END_REF].

The above classical categorization above has been shattered by the Tohoku earthquake 2011

where a large portion of slip occurred in the shallowest portion of the suduction interface. It was not expected. This lead to the seminal paper by [START_REF] Lay | Great Earthquakes on Plate Boundaries[END_REF] with the heterogeneous patches at the limit between the two zones. Note that this observation raises the need to better understand shallow subduction zone and this motivates this thesis.

Earthquake Generation: Seismic and Aseismic Processes

Earthquake generation and magnitude is closely related to the diverse nature and characteristics of the subduction zone and subduction processes [START_REF] Uyeda | Back-arc opening and the mode of subduction[END_REF]. The rate at which seismic moment is released from the seismogenic zone determines the size of earthquakes. [START_REF] Wells | New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement[END_REF], Kanamori (1977a), [START_REF] Kanamori | The physics of earthquakes[END_REF] propose that the earthquake magnitude increases with the area of the fault rupture. [START_REF] Pacheco | Nature of seismic coupling along simple plate boundaries of the subduction type[END_REF] identified that the subduction thrust with earthquake magnitudes (M < 8) have small seismic patches (Figure 2.6) supported by a small moment release rate. Large magnitude (M > 8) earthquakes have large seismic moment release with seismic efficiency close to unity. Scholz (1998) identified that seismic areas with large asperities can easily undergo seismic rupture nucleation and propagation, while areas with small asperities are limited in terms of seismic rupture propagation. In [START_REF] Lay | An asperity model of large earthquake sequences, in earthquake prediction (eds d. w. simpson and p. g. richards)[END_REF], [START_REF] Lay | The asperity model and the nature of large subduction zone earthquakes[END_REF].'s asperity models, large magnitude earthquakes result from the simultaneous failure of several asperities (Figure 2.6a,b).

The condition of simultaneous failure requires equal stress on surrounding asperities and may be the result of synchronization over multiple earthquake cycles [START_REF] Ruff | What Controls the Seismogenic Plate Interface in Subduction Zones?[END_REF]. Seismic coupling occupies the large portion of the convergence plate and increase of any remainder fraction of the convergence plate, it is hypothesized to be accommodated by aseismic slip [START_REF] Ruff | Seismic coupling and uncoupling at subduction zones[END_REF][START_REF] Jarrard | Terrane motion by strike-slip faulting of fore-arc slivers[END_REF][START_REF] Pacheco | Nature of seismic coupling along simple plate boundaries of the subduction type[END_REF]. Therefore, aseismic slip may occur in postseismic transient slip, slow slip events and steady creep motion [START_REF] Hyndman | Seismogenic zone of subduction thrust faults[END_REF][START_REF] Hyndman | The Seismogenic Zone of Subduction Thrust Faults: What We Know and Don't Know[END_REF]).

2.1.4 Mechanics of Faults Slip and Earthquake Generation [START_REF] Scholz | Evidence for a strong San Andreas fault[END_REF] identified that large magnitude earthquakes in subduction zones occur on preexisitng faults that undergo seismic failure in a simultaneous multiple seismic cycles. For a stable fault slip, the fault undergoes a steady stress loading, while, for unstable fault slip, it undergoes spontaneous transient slips (elastic energy release) due to fault weakening, which in turn generates either earthquakes (seismic) or slow slips [START_REF] Marone | Fault Friction and the Upper Transition from Seismic to Aseismic Faulting[END_REF]. Several factors affect the stability of faults such as the fault-zone rheology, frictional properties, stress state and continuum interactions with its surroundings [START_REF] Marone | Fault Friction and the Upper Transition from Seismic to Aseismic Faulting[END_REF]Saffer 2007, Hyndman et al. 1997). (Hyndman Fig. 2.6 Asperity model of the interplate interaction in subduction zones [START_REF] Kanamori | Rupture process of subduction-zone earthquakes[END_REF], [START_REF] Lay | The asperity model and the nature of large subduction zone earthquakes[END_REF]. coefficients due to the material at fault zone such as clay, silica gel [START_REF] Vrolijk | On the mechanical role of smectite in subduction zones[END_REF]; and (c) dynamic weakening [START_REF] Brune | Elastohydrodynamic lubrication of faults[END_REF][START_REF] Shi | Generation of high pore pressures in accretionary prisms: inferences from the Barbados Subduction Complex[END_REF][START_REF] Brodsky | Elastohydrodynamic lubrication of faults[END_REF][START_REF] Ma | Terrane motion by strike-slip faulting of fore-arc slivers[END_REF]. The effective normal stress, and by extension the true fault strength, are directly related to the pore pressure, hence an increase in pore pressure has a major impact on fault mechanical behavior [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Scholz | Earthquakes and friction laws[END_REF][START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF], as well as the characteristics of fault slip and sediment compaction [START_REF] Liu | Spontaneous and triggered aseismic deformation transients in a subduction fault model[END_REF][START_REF] Kodaira | High pore fluid pressure may cause silent slip in the Nankai Trough[END_REF][START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF][START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF][START_REF] Scholz | Earthquakes and friction laws[END_REF]. The effect of the elevated pore pressure on faults strength is of interest in this work.

Role of Fluids and Pore Pressure in Subduction Zones

Fluid trapped at depths and elevated pore pressure largely influence fault mechanics, tectonic deformation and earthquake cylces in subduction zones [START_REF] Jaeger | Friction of Rocks and Stability of Rock Slopes[END_REF][START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF][START_REF] Osborne | Mechanisms for generating overpressure in sedimentary basins[END_REF][START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF], Rubey and King Hubbert 1959, Flemings (a) They may be introduced in subduction zones as fluid-rich sediments from the incoming plate underthrust or are scrapped off to build up the accretionary prism [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]). (b) Fluids are also released by mineral dehydration, sediment compaction, diagenesis, loading associated with disequilibrium compaction [START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF]. Each of the sources is responsible for certain amount of fluid contribution within the subduction system. In conditions where the temperatures (> 60 • C) and pressure are very high, sediments that are subjected to dehydration might contribute high amount of fluids than sediments undergoing normal compaction and diagenetic process [START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF].

In addition to these sources, the fluid distribution is controlled by fluid migration. Fluids can escape through the porous network of rocks or be channeled along fractures or fault zones (décollement, splay faults and OOST) (Figure 2.7), whose permeability can be influenced by the maturity of the fault zone architecture [START_REF] Caine | Fault zone architecture and permeability structure[END_REF][START_REF] Ranero | Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis[END_REF]. Along the décollement, fluids can flow laterally assuming it serves as a channel or it can leak to the overlying accretionary sediments through vertical dewatering [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]Tobin 2011, Gamage and[START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF]. The existence of fluids and fluid migration has been related to the slow rupture of shallower and deeper extensions of the megathrust seismogenic zones, as well as stress conditions in the vicinity of the megathrust zones [START_REF] Shelly | Non-volcanic tremor and low-frequency earthquake swarms[END_REF][START_REF] Katayama | Episodic tremor and slow slip potentially linked to permeability contrasts at the moho[END_REF][START_REF] Audet | Possible control of subduction zone slow-earthquake periodicity by silica enrichment[END_REF][START_REF] Kodaira | High pore fluid pressure may cause silent slip in the Nankai Trough[END_REF][START_REF] Chaves | Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise[END_REF][START_REF] Tonegawa | Fluid migration before and during slow earthquakes in the shallow nankai subduction zone[END_REF]. Warren-Smith et al.

(2019) hypothesized that slow earthquakes are triggered at the plate boundaries whenever there is upward fluid migration along the faults zones from the downdip end to the updip end in the direction of the trench.

CHAPTER 3

Existing methods used to study hydrology of subduction zone

Estimating the spatial variation of the hydraulic properties (pore pressure, permeability) within the accretionary prism and the décollement zone is difficult [START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF]. Hence, several methods that have been used to quantify in situ pore pressure and flow within the Nankai accretionary prism, with a particular emphasis on understanding the plate boundary are discussed below. They can be categorized in three kinds of techniques:

1. A combination of laboratory consolidation experiments coupled with numerical simulation of building of the accretionary prism. Hydro-mechanical properties from cores collected during the consolidation tests are used to calibrate and constrain numerical models.

• Numerical models were combined with the critical taper Coulomb wedge theory [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF][START_REF] Dahlen | Critical taper model of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF][START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF][START_REF] Matmon | Hydromechanics of a high taper angle, low-permeability prism: A case study from Peru[END_REF][START_REF] Wang | Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge[END_REF][START_REF] Flemings | Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach[END_REF].

• Simulations of regional scale hydrogeological models employing laboratory data as constraints for computing transient fluid flow models and estimating sediment and fault permeabilities. Such studies have been done for 1D [START_REF] Shi | High pore pressure generation in sediments in front of the Barbados Ridge Complex[END_REF][START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF][START_REF] Daigle | Data report: permeability, consolidation properties, and grain size of sediments from Sites U1420 and U1421, offshore southern Alaska[END_REF][START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF], 2D [START_REF] Rowe | Coupled fluid flow and deformation modeling of the frontal thrust region of the Kumano Basin transect, Japan: Implications for fluid pressures and decollement downstepping[END_REF], and 3D geometries [START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF]Ge 1997, Spinelli et al. 2006).

The use of borehole observatories (circulation obviation retrofit kits [CORKs]

) to provide direct in situ measurements of elevated pore-fluid pressures, fluid flow, fluid chemistry, and temperature variation [START_REF] Davis | Formation Temperatures and Pressures in a Sedimented Rift Hydrothermal System: 10 Months of CORK Observations, Holes 857D and 858G[END_REF][START_REF] Henry | Fluid flow at the toe of the Barbados accretionary wedge constrained by thermal, chemical, and hydrogeologic observations and models[END_REF][START_REF] Jannasch | CORK-II: Long-Term Monitoring of Fluid Chemistry, Fluxes, and Hydrology in Instrumented Boreholes at the Costa Rica Subduction Zone[END_REF][START_REF] Becker | A review of CORK designs and operations during the Ocean Drilling Program[END_REF][START_REF] Kastner | New insights into the hydrogeology of the oceanic crust: Through long-term monitoring[END_REF], Sawyer et al. 2008[START_REF] Wheat | Subseafloor seawater-basalt-microbe reactions: Continuous sampling of borehole fluids in a ridge flank environment[END_REF][START_REF] Bekins | Reanalysis of in situ permeability measurements in the Barbados décollement[END_REF][START_REF] Fisher | Design, deployment, and status of borehole observatory systems used for single-hole and cross-hole experiments, IODP Expedition 327, eastern flank of Juan de Fuca Ridge[END_REF][START_REF] Hammerschmidt | Fluid pressure and temperature transients detected at the Nankai Trough Megasplay Fault: Results from the SmartPlug borehole observatory[END_REF][START_REF] Kinoshita | Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring[END_REF].

The CORK monitoring system has also been investigated as a precursor for determining whether detectable inter-seismic changes or precursory phenomena exist prior to large subduction earthquakes (Tobin andKinoshita 2007b, Kinoshita et al. 2018).

Similarly, industry tools were used at times to determine hydrological properties (eg, nuclear magnetic resonance (NMR) logging tools and modular formation dynamics Tester (MDT)) [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF][START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF], Saffer et al. 2013), but their use was limited in academic studies due to their large cost and they cannot provide a complete profile along a borehole.

3. A third series of studies rely on seismic velocity (Tobin andSaffer 2009, Tsuji et al. 2008).

Strong calibration based on samples is needed to convert seismic anomalies into hydrogeologic quantities. These calibrations are performed on core samples, with the same issue of representativeness as for the first type of study. All these methods are summarily discussed below with regards to their contribution in understanding the hydraulic properties within the Nankai accretionary prism:

Laboratory Compaction Experiments coupled with Numerical Modeling

Core samples are acquired along the borehole during drilling campaigns. These samples are preserved and subjected under consolidation experiments in the laboratory. Laboratory consolidation test refers to the compressive and volume-reduction process that occurs when pore fluid is drained from the sediment matrix in response to external stress (Terzaghi andPeck 1948, Knappett and[START_REF] Knappett | Craig's soil mechanics (8th edn)[END_REF]. During sediment loading the pore pressure is increased and its dissipation is determined by the permeability and flow path within the sediments [START_REF] Knappett | Craig's soil mechanics (8th edn)[END_REF].

Common consolidation experiments are: a) one-dimensional consolidation test like oedometer test b) constant rate of strain loading test.

Oedometer test method:

This test simulates one-dimensional consolidation state of a rock sample and the drainage conditions under variable applied load stress [START_REF] Knappett | Craig's soil mechanics (8th edn)[END_REF]Craig 2012, ASTM 2012). Pressure is applied on the specimen which is already placed inside a metal ring (figure 3.1). The pore fluid within the sample dissipates based on the applied stress and the permeability within the sample (figure 3.1). Total fluid dissipation within the sample is achieved when the applied total stress is equal to the vertical effective stress [START_REF] Knappett | Craig's soil mechanics (8th edn)[END_REF]. The dial gauge on the loading cap records the compression pressure the sample is subjected to [START_REF] Knappett | Craig's soil mechanics (8th edn)[END_REF]Craig 2012, ASTM 2012).

Constant rate of strain (CRS) method:

This method is designed to determine the magnitude and consolidation rate of saturated sample using controlled continuous strain axial compression (ASTM 2012, [START_REF] Olson | State of the art: consolidation testing[END_REF][START_REF] Tsuji | Identification of the static backstop and its influence on the evolution of the accretionary prism in the nankai trough[END_REF][START_REF] Dugan | Near-seafloor overpressure in the deepwater mississippi canyon, northern gulf of mexico[END_REF]. In a constant rate of strain (CRS) test, the sample is confined in a load cell (steel ring) and is subjected to a uniaxial strain using a controlled loading frame (figure 3.2). During the consolidation test, the height (m) of the sample, total and effective axial stresses, basal excess pore pressure (Pa), axial deformation, pressure in the chamber are calculated. The details of the device and how the experiment can be carried out are discussed in [START_REF] Ediments | of N Ankai a Ccretionary P Rism[END_REF], ASTM 2012). Pressure builds up is observed when the sample is progressively loaded and compressed, hence the permeability (m 2 ) can be estimmated as a function of the sample consolidation state [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF]:

k = v • ε • l • l 0 2 • U b (3.1)
v is the fluid dynamic viscosity (Pa.s), ε is the strain rate (1/s), l is the height of the sample (m), U b is the basal excess pore pressure (Pa) and l 0 is the height of the sample at the start of the experiment (m).

Consolidation properties of rock samples such as porosity, permeability, flow, effective stress and pressure are determined from the experiements. These are used as input for building the different numerical models of the subduction zone. To build large numerical models that properly represent the subduction zone, the critical taper theory [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF]) serve as a basis for understanding the architecture of the accretionary prism, underthrusted sediments and the plate boundary. Before discussing the types of numerical models used in the subduction zone, the critical taper theory by [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF] and [START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF] is discussed below.

Critical taper coulomb wedge

The critical taper wedge is a mechanical model of the structural evolution of fold-thrust belts and accretionary wedges along the plate boundary [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]) (Figure 3.3). This model describes either extensional or compressive stress state of a taper on the verge of failure [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]. The theory assumes that the internal stress state of a wedge material deforming is on the verge of failure everywhere based on Mohr-Coulomb criterion [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]. The stress state and the elevated pore pressure influence the mechanical strength of the basal décollement, thus supporting the movement of large thin sheets [START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF]King Hubbert 1959, Dahlen 1990). The theory of a critically tapered Coulomb wedge has been widely employed., with great success in understanding deformation processes, geometry, and pore pressures in the accretionary prism systems [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF][START_REF] Dahlen | Critical taper model of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF][START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF][START_REF] Matmon | Hydromechanics of a high taper angle, low-permeability prism: A case study from Peru[END_REF][START_REF] Wang | Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge[END_REF][START_REF] Flemings | Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach[END_REF]. Critical taper theory predicts that to main small taper angles, high pore pressures above hydrostatic pressure is needed [START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF]. The model describes the total taper angle (sum of the surface slope (α)

and basal décollement dip (β) as a function of the pore pressure ratio as defined by [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF] and [START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF]:

λ = (P f -ρ w gD) (σ z -ρ w gD) (3.2) λ b = (P b -ρ w gD) (σ z -ρ w gD) (3.3)
Where P f is the fluid pressure within the wedge (P f = λρgz cos α), P b is the fluid pressure within the basal décollement, σ z is the lithostatic pressure, P w is the hydrostatic pressure (which is considered as the reference level), ρ w density of water, g is the acceleration due to gravity, D is the depth of the water column.

The theoretical critical taper angle α and β of a non-cohesive Coulomb wedge (figure 3.3) is calculated by:

α + β = (1 -λ b )µ b + (1 -ρw ρ b )β (1 -ρw ρ b ) + (1 -λ)K (3.4)
Where α is angle of the surface slope, β is the dip of the basal décollement, ρ b is the density of the bulk sediment, µ and µ b are the coefficients of internal friction within the wedge and the basal fault (décollement) respectively. Hence, µ(1 -λ) and µ b (1 -λ b ) determine the strength of the wedge material and the fault, K is the hydraulic conductivity. [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF] presented a simplified model of the taper wedge that focuses on physical concepts and geological observations. However, under appropriate boundary conditions, the stress state within the wedge was estimated by solving static equilibrium equations [START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF].

The solution presented by [START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF] considered the stress orientation angle within the wedge (ψ) and the décollement (ψ b ):

α + β = ψ b -ψ 0 (3.5)
The angle of stress orientation is defined based on Mohr's geometry in Figure 3.4. The underlying assumption is that faults are created as new fractures that coincide with slip lines in a

Coulomb material [START_REF] Dahlen | Mechanics of Fold-and-Thrust Belts and Accretionary Wedges: Cohesive Coulomb Theory[END_REF].

Recent advances combined with critical taper wedge

Recent advances in critical taper theory considered other factors that influence pore pressure and taper angle such as fault and sediment permeability, plate convergence rate, incoming sediment thickness, sediment partition between subducted and accreted sediments [START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF][START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF][START_REF] Matmon | Hydromechanics of a high taper angle, low-permeability prism: A case study from Peru[END_REF][START_REF] Wang | Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge[END_REF]. [START_REF] Flemings | Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach[END_REF] used the porosity data from drilling to estimate the magnitudes of pore pressure and stress within the Nankai accretionary prism. These studies used numerical models of fluid flow in accretionary prism. The conceptual framework for the pore pressure models is based on a dynamic balance between the hydrologic forcing, fluid escape and fluid generation [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]Bekins 2002, 2006).

The numerical model is divided into three hydrologic regimes: underthrusting sediments, décollement, and accretionary. Some boundary conditions were also considered. The prism has thirteen rows, the underthrust has eleven, the décollement has six closely spaced elements with a higher permeability. Also tightly spaced are the three parts directly above and below the décollement. The boundary conditions are also adopted (Modified fromSaffer and Bekins ( 2006)). [START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF], [START_REF] Matmon | Hydromechanics of a high taper angle, low-permeability prism: A case study from Peru[END_REF], [START_REF] Flemings | Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach[END_REF] detail this method using core samples from Ocean Drilling Program Sites 1174 and 808. The estimated pore pressure in Nankai accretionary prism supports 70% to 90% of the overburden (λ u = 0.7 to 0.9), for about 5-30 • friction angles [START_REF] Flemings | Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach[END_REF]. Because of the elevated pore pressures, the wedge becomes mechanically weak and maintain the narrowly tapered shape [START_REF] Flemings | Pressure and Stress Prediction in the Nankai Accretionary Prism: A Critical State Soil Mechanics Porosity-Based Approach[END_REF].

1D Numerical Model

The core samples are used in the laboratory to measure the permeability and porosity, hence a permeability-porosity relationship is developed which is used as an input to build a 1D model (Gamage andScreaton 2006, Skarbek andSaffer 2009). The basic objective of the model is to simulate the evolution of excess pore pressure and porosity that will result from rapid sedimentation and tectonic loading within Nankai accretionary prism (Gamage andScreaton 2006, Skarbek andSaffer 2009). The samples used by [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF], [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] are from the drilling site 808, 1173 and 1174. [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] simply used the permeability-porosity relationship to understand pore pressure variation at the frontal thrust. They also tested their results with bulk permeabilities, lateral stresses and hypothetical low permeability at the décollement. [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] made attempts to predict pore pressure variation from loading and pore pressure diffusion by making projections within the underthrust sediments and along the décollement. As an input to their model permeability was determined by (a) constant-flow tests (steady state flow) experiments [START_REF] Ediments | of N Ankai a Ccretionary P Rism[END_REF]) (b) constant rate of strain (CRS) experiments. Here we discuss only constant-flow test, while the notes on CRS can be found in the introduction text of consolidation test above.

Constant-flow tests method:

This method is used to measure the hydraulic gradient by pumping fluids in and out of the core sample within the instrument (figure 3.7) [START_REF] Ediments | of N Ankai a Ccretionary P Rism[END_REF]. The sample used for this test is normally fully saturated. The sample is placed inside the cell and the fluid pressure within the cell is increased while the sample backpressure is kept constant. This will inturn, increase the sample's effective stress. The cell pressure and backpressure are maintained when the targeted effective stress is attained. An appropriate flow rate can be selected by conducting a constant pressure gradient test, which serves a guide for subsequent constant-flow tests. Pressure gradient heads can be monitored during the flow rate tests and the permeability can be determined. The estimation of the flow rates and permeability is based on the Darcy's law:

Q = -KA dh dl (3.6)
where K is the hydraulic conductivity (m/s), A is the sample area ( m 2 ), dh is the difference in sample head (m), and dl is the sample length (m). Permeability is derived from the conductivity using the equation below:

k = Kµ ρg (3.7)
Where, µ is the water viscosity (0.001 Pa•s), ρ is the density (1027 kg/m 3 ), g is the acceleration due to gravity (g = 9.81 m/s 2 ).

The permeability of sediments is a key factor that controls the magnitude of pore pressure; therefore, a relationship between permeability and porosity was derived from laboratory experiments [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]Bekins 1998, Gamage and[START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF]. This relationship was defined by [START_REF] Neuzil | How permeable are clays and shales?[END_REF] with permeability decreasing with decreasing porosity. This was derived based on several laboratory experiments on a variety of argillaceous clays (Figure 3.6). Mathematically represented:

log(k) = log(k 0 ) + φn (3.8)
Where; log(k 0 ) is the logarithm permeability and φ is a parameter that describes the rate of change of permeability with porosity.

The model combines a program that simulates excess pore pressure due to increasing sedimentation rate and initial subduction due with preexisting fluid flow [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF], [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF]. [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] did not consider the lateral fluid flow, while, [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] solved the problem of fluid flow by considering it in terms of hydraulic head, hydrostatic pore pressure increment with burial depth and incremental changes in the overburden:

The model assumes that additional nodes represent additional sediment layer in the accretionary prism. Different boundary conditions were considered by [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] and [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] to build the model. Further discussion of the method can be found in [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF], [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF]. Additionally, both studies performed different sensitivity analysis to determine the reliability of their models and the different parameters that have been estimated. of [START_REF] Neuzil | How permeable are clays and shales?[END_REF] in Figure 3.6. The pore pressure estimates by [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] showed that the pore pressure ratios at site 1173 may have been underestimated (Figure 3.9).

However, the results of the pore pressure ratios (λ b = 0.68-0.77 and λ * b = 0.30-0.60) reported by [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] at the décollement are consistent with the predicted values (λ * b = 0.56 at Site 808) by [START_REF] Saffer | Episodic fluid flow in the Nankai accretionary complex : z a[END_REF] but lower than the values (λ * b = 0.82) which is 20km away from the trench [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF].

The results from [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] further show that the magnitude of the pore pressure (Figure 3.9) in the underthrust is slightly sensitive to lateral stresses in the prism. Identi- fied that the maximum excess pore pressure ratios are within the décollement zone, hence might contribute to stable sliding of the décollement zone [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF]. The results of [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF], [START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] (Figure 3.8) demonstrates further that pore pressure within the underthrusted section at subduction zones is strongly dependent on the low-permeability conditions of decollement acting as a "pressure cap" that prevents vertical dewatering of fluids from below.

2D Numerical Model

Effective stress is very important because it influences the shear strength and the spatial evolution of mechanical strength of sediments [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]. In two-dimensional method pore pressures are coupled with the applied stresses using mechanical properties of sediment [START_REF] Screaton | Fluid expulsion and overpressure development during initial subduction at the Costa Rica convergent margin[END_REF]. The two-dimensional models are simulated with inputs data from laboratory experiments.

Non-deformed portions of the core samples is determined using the computed tomography (CT) scan, after which the selected portions are used for uniaxial consolidation and permeability test [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]. During the consolidation experiments, sediment response is based on the applied Fig. 3.9 Simulated porosity and pore pressure ratio (λ * ) results profile for Sites 1173, 1174 and 808 [START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] external stress, compressibility and history of loading the sample underwent insitu [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF].

Maximum and minimum vertical effective stresses values are obtained beyond which the sediments void ratio is no longer plotted on the rebound curve or virgin consolidation curve. Porosity loss is estimated based on assumed relationship between effective stress and porosity, while the pore pressure values are increased to account for the increasing stress values [START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF]. In simulating the 2D models, three hydrological regimes are assumed for the accretioary prism, as discussed in the 1D model. Each nodes of the model is assigned a predefined value of porosity and porosity. The permeability values used can either be inputs from the laboratory experiments or varying from suite of permeability values considering the linear relationship between porosity and permeability [START_REF] Neuzil | How permeable are clays and shales?[END_REF]. The porosity and permeability values of the decollement are set at a higher values normally reasonable values to the [START_REF] Spinelli | Hydrogeologic responses to threedimensional temperature variability, Costa Rica subduction margin[END_REF][START_REF] Bekins | Episodic and constant flow models for the origin of low-chloride waters in a modern accretionary complex[END_REF]. The pore pressure ratio values are from derived values from critical taper wedge. These values are compared to the model domain geometry and if inconsistent then the parameters and assumptions used for the models are unrealistic combination of permeability and hydrologic forcing [START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF].

P f = σ overburden -σ ′ (3.9)
Where P f is the pore fluid pressure, σ overburden is the total overburden stress and σ ′ is the effective stress.

e = e 0 -C c log(σ ′ ) (3.10)
Where e is the void ratio, e 0 is the void ratio when effective stress is zero, C c is the compression index, and σ ′ is the effective stress.

C c,calculated = e sample-a -e sample-b log(

P ′ c,sample-a σ ′ sample-b ) (3.11)
Where C c calculated is the void ratio difference between two samples and P ′ c is the in situ effective stress.

Pore pressures can also be estimated from inferred effective stress (equation 3.9). The effective stress can be calculated from both measured (equation 3.10) and calculated (equation 3.11) values of the consolidation index during the laboratory experiments reflecting undrained conditions [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]. [START_REF] Saffer | Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts[END_REF] showed that the evolution pore pressure within the accretionary prism is controlled by the hydrologic forcing and permeability, at the same time proposed that pore pressure evolution can also be influenced by the thickness and stratigraphy of the incoming sediments from the underthrusting plate.

Borehole Observatories using Circulation Obviation Retrofit Kit (CORK)

CORK is a device designed to obtain direct measurements of pore pressure, temperature, fluid chemistry and their variations over time (Shipley et al. 1995, Davis and[START_REF] Davis | Formation Temperatures and Pressures in a Sedimented Rift Hydrothermal System: 10 Months of CORK Observations, Holes 857D and 858G[END_REF] To detect the hydrological transients pressure the recorded time series data are further caliberated and processed for temperature compensation and pressure signal due to periodic and aperiodic signals [START_REF] Kinoshita | Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring[END_REF]. Detailed data filtering, detiding, and removal of interfering signals aids proper accountability of seafloor and ocean tidal pressure signals [START_REF] Kinoshita | Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring[END_REF][START_REF] Ariyoshi | Precise monitoring of pore pressure at boreholes around nankai trough toward early detecting crustal deformation[END_REF][START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF]. Processed data (Figure 3.16a) are easily compared with earthquake seismic waves to identify and understand the transient responses of the insitu fluid pressure and temperature conditions [START_REF] Hammerschmidt | Fluid pressure and temperature transients detected at the Nankai Trough Megasplay Fault: Results from the SmartPlug borehole observatory[END_REF][START_REF] Hammerschmidt | Limitation of fluid flow at the Nankai Trough megasplay fault zone[END_REF][START_REF] Dutilleul | Tidal analysis of the nantroseize long term borehole monitoring system (ltbms) pore pressure records at the nankai margin, sw japan[END_REF]. This comparison makes a fundamental distinction between the types of seismic waves that were responsible for the pressure transients that lasted for several hours to days. [START_REF] Hammerschmidt | Fluid pressure and temperature transients detected at the Nankai Trough Megasplay Fault: Results from the SmartPlug borehole observatory[END_REF]. The pressure shifts were connected to meteorological occurrences and waves generated by tsunamis [START_REF] Hammerschmidt | Fluid pressure and temperature transients detected at the Nankai Trough Megasplay Fault: Results from the SmartPlug borehole observatory[END_REF]. The effects of earthquakes almost often take the form of an increase in pore pressure and a decrease in the loading efficiency [START_REF] Kinoshita | Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring[END_REF].

Seismic Velocities Anomalies

The estimate of anomalous pore pressure can be achieved from reflectivity [START_REF] Moore | Structure of the nankai trough accretionary zone from multichannel seismic reflection data[END_REF], Park et al. 2002, Bangs et al. 2004, 2009) or can be derived from the interval velocities model by velocity stacking, where the reflections are flat (Cochrane et al. 1994a,b, Bangs et al. 2009, Park et al. 2010). This is basically due to variations in reflection amplitudes due to energy dispersion by geologic interfaces (Bangs andWestbrook 1991, Lindsay and[START_REF] Lindsay | Pore pressure influence on rock property and reflectivity modeling[END_REF]. Evidence of pore pressure using seismic velocity can either (a) qualitative (b) quantitative approach.

Qualitative evidence of pore pressure and fluid flow from reflection and seismic velocity

The evidence of anomalous pore pressure in the accretionary prism is attributed to reverse polarity and a low seismic velocities zone (Cochrane et al. 1994b,a). The change in polarity of the seismic response results from the change in acoustic impedance contrast from an increase to a decrease in a normal SEG convention (Cochrane et al. 1994b,a). [START_REF] Pennebaker | Seismic data indicate depth, magnitude of abnormal pressure[END_REF] establishes the relationship between the velocity change with depth. To model pore pressures from velocity, the velocities of different layers are considered to obtain a high vertical depth resolution with velocities (Figure 3.18b). In order to maintain the resolution of the interval velocity data, standard deviations of the interval velocity data is used which represent the systematic errors due to approximation of hyperbolic travel time, random errors due to noise corrections, and variation of the lithologies [START_REF] Cochrane | Velocity and inferred porosity model of the oregon accretionary prism from multichannel seismic reflection data: implications on sediment dewatering and overpressure[END_REF]).

Velocity values are measured from groups of common mid points (CMPs) (Figure 3.18c) [START_REF] Cochrane | Velocity and inferred porosity model of the oregon accretionary prism from multichannel seismic reflection data: implications on sediment dewatering and overpressure[END_REF], Park et al. 2002, Tsuru et al. 2002). Variation in interval velocity is observed from the CMPs all through the accretionary prism. [START_REF] Bangs | Seismic modeling of the décollement zone at the base of the barbados ridge accretionary complex[END_REF], Moore 3.3.2 Quantified evidence of pore pressure and effective stress seismic velocity and porosity

Here we look at the advances employed and results by several studies [START_REF] Tobin | Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone[END_REF][START_REF] Tsuji | Effective stress and pore pressure in the Nankai accretionary prism off the Muroto Peninsula, southwestern Japan[END_REF][START_REF] Bak | Model based design of electro-hydraulic motion control systems for offshore pipe handling equipment[END_REF]) that combined porosity and velocity to estimate pore pressure. Tobin and Saffer ( 2009) combined a pre-stack depth migration seismic velocity with borehole-based transformations inputs as constraints for the model. To model the pore pressure evolution at the décollement in Nankai accretionary prism, [START_REF] Tobin | Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone[END_REF] combined the relationship between: (a) P-wave velocity to porosity [START_REF] Hoffman | An empirical relationship between velocity and porosity for underthrust sediments in the nankai trough accretionary prisms[END_REF], and (b) porosity to effective stress relationship [START_REF] Saffer | Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai[END_REF]. Pore fluid pressures are simply estimated using [START_REF] Terzaghi | Soil Mechanics in Engineering Practice[END_REF] equation. It is assumed that consolidation and velocity increases with depth, hence the transform of velocity-porosity by [START_REF] Hoffman | An empirical relationship between velocity and porosity for underthrust sediments in the nankai trough accretionary prisms[END_REF] was used to calculate porosity distribution. They compared their predicted results with measured data from core samples [START_REF] Hoffman | An empirical relationship between velocity and porosity for underthrust sediments in the nankai trough accretionary prisms[END_REF] from Sites 1173, 1174, and 808 to minimise misfit for the porosity models. The porosity models are further inverted to get effective stress, while the total lithostatic load were calculated from using the porosity derived from drilling and integrated bulk density data. The results of [START_REF] Tobin | Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone[END_REF] showed that excess pore pressure is attributed to the wave tomography approach collects velocity information from refracted and broad reflected waves. The detail approach used for building a velocity model was described by [START_REF] Kamei | On acoustic waveform tomography of wideangle obs data-strategies for pre-conditioning and inversion[END_REF][START_REF] Kamei | Waveform tomography imaging of a megasplay fault system in the seismogenic nankai subduction zone[END_REF], [START_REF] Tsuji | Pore pressure distribution of a mega-splay fault system in the nankai trough subduction zone: Insight into up-dip extent of the seismogenic zone[END_REF]. To increase the accuracy of the pore pressure estimation, the poisson's ratio (Vp/Vs ratio) was determined. Since, S-wave property decreases with increasing pore fluid pressure values [START_REF] Dvorkin | Overpressure detection from compressional and shear-wave data[END_REF]. For pore pressure estimation several other properties were determined from borehole logging and laboratory experiments. The relationship between effective stress and pore pressure was determined from the logging data by [START_REF] Tsuji | Pore pressure distribution of a mega-splay fault system in the nankai trough subduction zone: Insight into up-dip extent of the seismogenic zone[END_REF]. [START_REF] Tsuji | Pore pressure distribution of a mega-splay fault system in the nankai trough subduction zone: Insight into up-dip extent of the seismogenic zone[END_REF][START_REF] Tsuji | Effective stress and pore pressure in the Nankai accretionary prism off the Muroto Peninsula, southwestern Japan[END_REF]) also derived the effective stress from seismic velocity using rock physics based approach. They estimated the insitu effective stress and pore fluid pressure by fitting the P-wave velocity derived from differential effective medium (DEM) [START_REF] Berryman | Single-scattering approximations for coefficients in biot's equations of poroelasticity[END_REF] and the WT-derived velocity distribution [START_REF] Kamei | On acoustic waveform tomography of wideangle obs data-strategies for pre-conditioning and inversion[END_REF][START_REF] Goodwyne | Pressure prediction and underbalance drilling in the deepwater niger delta[END_REF], Tsuji et al. 2014). The results of [START_REF] Tsuji | Pore pressure distribution of a mega-splay fault system in the nankai trough subduction zone: Insight into up-dip extent of the seismogenic zone[END_REF] revealed high pore pressure that stretches towards the Trough from the megasplay fault footwall. Their results (Figure (3.23) further showed that predicted pore pressure at the footwall of the megasplay using the S-wave velocity (l = 0.7 -0.95 ) is higher than pore pressure derived from P-wave velocity (Figure 3.23d). 3.4 Other Methods 3.4.1 Vp, porosity and effective stress empirical relationship [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] and [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF] proposed an empirical relationship to estimate the in situ stress and pore pressure condition in Nankai Trough. The methodology uses a combination of calibrated relationship between P-wave velocity, porosity and effective stress derived from representative core samples of the Nankai accretionary prism [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF]. The core samples used were subjected under suite of laboratory triaxial deformation tests. The core samples used were from Site C0011 [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] and Site C0002 [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]). As a first step the porosity was derived from the P-wave velocity data.

The compressional velocity is gotten from published seismic reflection, ocean bottom seismometer (OBS) studies or using the empirical relationship of [START_REF] Erickson | Velocity-porosity relationships for water-saturated siliciclastic sediments[END_REF] for highly consolidated sediments:

V p [km/s] = 1.11 + 0.178 ϕ t + 0.305 (ϕ t + 0.135) 2 + 0 : 0775 + 0.61 (v sh -1) X m (3.12)
where ϕ t is the total porosity, v sh is the shale volume, and X m accounts for the brutal change in behavior at a critical porosity ϕ c = 0.39, above which stress is supported by the fluid rather than by the grain skeleton of the rock:

X m = tanh [20 (ϕ t -ϕ c )] -∥ tanh [20 (ϕ t -ϕ c )] ∥ (3.13)
The porosity was derived from laboratory measurements of shale samples (Kitajima and Saffer 2012, [START_REF] Kinoshita | Expedition 316 scientists[END_REF][START_REF] Strasser | Proceedings of the integrated ocean drilling program[END_REF][START_REF] Tobin | Proceedings of the integrated ocean drilling program[END_REF] or derived from density logs assuming a grain density = 2.65 g/cm3 and a water density = 1.024 g/cm3 [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]). The Vp-porosity relationship is independent of the stress path, while the void ratio depends on both differential stresses and the mean effective stress:

e = ϕ 1 -ϕ (3.14)
Void ratio e is the volume of voids relative to the volume of solids, whereas porosity ϕ is the volume of voids relative to the total volume.

The second step consists in using experimental relationships between effective stress and the porosity, derived from consolidation experiments [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] under "triaxial" conditions (the principal stresses are so that σ 1 > σ 2 = σ 3 ). In this case, two quantities characterise stress: (1) the effective mean stress

p ′ = 1 3 (σ 1 + σ 2 + σ 3 ) -P f (3.15)
and the differential stress

q = 1 2 (σ 1 -σ 2 ) 2 + (σ 2 -σ 3 ) 2 + (σ 3 -σ 1 ) 2 (3.16)
. Two end members are considered for both triaxial compression (σ 1 > σ 2 = σ 3 ) and extension (σ 1 = σ 2 > σ 3 ):

1. 1-D vertical consolidation without radial (horizontal) strain (K 0 ). This loading is similar to the loading occurring during sediment burying in a sedimentary basin. In these conditions, [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] found the following relationships for samples from the Shimano basin, in the Philippine Sea Plate :

p ′ = 10 0.89-e 0.44
(3.17)

q = 0.375 × p ′ (3.18) 2. 1-D radial consolidation (1/K 0 ) without axial strain in triaxial extension (S H = S h > S v )
3. critical state loading (CSL) in which the sample is subjected to maximum differential stress (q) and continues shear deformation without changes in stress and volume in triaxial compression and extension.

p ′ = 10 0.79-e 0.40

(3.19) q = p ′ (3.20)
To evaluate pore pressure, first [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] derived the principal stress directions ( σ H and σ v ) from the mean effective stress under critical state condition σ H = 1.58 p ′ and σ v = 0.42 p ′ . After which the pore pressure for the low velocity zones (LVZ) were determined from σ v by relating it to σ v0 (derived from the bulk density of the sediments) and hydrostatic pressure (P p-hydro ). The Excess pore pressure (P * = σ v-hydro -σ v , where σ v-hydro =σ v0 -P ( p -hydro)).

The results of the excess pore pressure (Figure 3.24a) varies P * between 17-87 MPa. Their results

shows an upper bound for non critical state sediments. In such state the mean effective stress at specific Vp or porosity value. At the other hand the pore pressure ratio (λ * ) was derived from the excess pore pressure (P * ) and σ v-hydro . Pore pressure ratio (λ * = P * /σ v-hydro ). The results of λ * ranged between 0.51-0.77 with the high values at 25km away from the trench (Figure 3.24c). [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] identified the LVZ area, with the highest pore pressure region at 25km, but the spatial distribution of pore pressure and low velocity appears to be heterogeneous (Figure 3.24c). [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF] presented the upper and lower bounds of the in situ pore pressure and maximum horizontal stress for the CSL and K 0 stress paths. The minimum principal stress (S 3 = S h ) was obtained from the stepped-rate injection test (SRIT) and Leak-off tests (LOTs) conducted at three distinct depth interval for Site C0002 [START_REF] Strasser | Proceedings of the integrated ocean drilling program[END_REF], [START_REF] Tobin | Proceedings of the integrated ocean drilling program[END_REF]). Unlike in [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] the LOT and SRIT were used as a constraint to estimate the effective mean stress and differential stress by rewritten Equation 3.15 and Equation 3.16) respectively:

p ′ = (1 + k)(S v -P p0 ) + (2 -k)(S h -P p0 ) 3 -(P p -P p0 ) (3.21)
and the differential stress

q = K 2 -k + 1(S v -S h ) (3.22)
.

where P p is pore pressure, P p0 is the seawater pressure at the seafloor or mudline, and

(k = S H -S h
Sv-S h ) is the ratio of maximum differential stress in horizontal and vertical planes. In the Kumano basin, the pore pressure is nearly hydrostatic, but in the accretionary prism it is only moderately overpressured, as shown by the pore pressure results given by [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF] along Hole C0002 (Figure 3.24b). The pore pressure ratios are defined according to [START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF], Yaolin Shi and Chi-Yuen [START_REF] Shi | Generation of high pore pressures in accretionary prisms: inferences from the Barbados Subduction Complex[END_REF]: pore fluid pressure ratio to overburden stress λ = Pp-P p0

Sv-P p0 and excess pore fluid pressure to effective overburden stress λ * = Pp-P h Sv-P h . In the Kumano basin the λ = 0.6 and λ * = 0.25, while in the accretionary prism the λ = 0.58 and λ * = 0.16 and at greater depths near the megathrust fault λ = 0.85 and λ * = 0.75.

Pore pressure and permeability estimation from modular formation dynamics tester (MDT)

The Modular Formation Dynamics Tester is a wireline microfracturing tool that takes precise pressure readings and reliable fluid samples [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF][START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF]. It can be used to analyze both vertical and horizontal permeability (Figure 3.25c), in situ minimum stress magnitude, and pore fluid pressure (Figure 3.25a and b) [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF][START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF], Araki et al. 2009). These measurements require high-resolution gauges that can consistently react to dynamic conditions, as the pressure gauge is prone to large temperature and pressure changes. To measure the formation pressure downhole, an area of interest is isolated with the dual packers.

The parkers are installed and inflated within borehole intervals that are gauged or 0.1 excess of the gauged borehole diameter [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF][START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF]. As the parker is inflated the probes measure the internal pressure that begins to develop and the fluid pressure from the formation between the parkers [START_REF] Carnegie | An advanced method of determining in situ reservoir stresses: Wireline conveyed micro-fracturing[END_REF], Boutt et al. 2012[START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF]. Pressure build-up between the dual parkers is a good sign of seal integrity of the borehole walls [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF], [START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF]. The parkers' excess pore pressure is bled off by pumping fluid from the parkers into the borehole until a stable pressure is reached. Above the packed off zone, borehole pressure was monitored for packer seal integrity. After pressure stabilization, the fluid was withdrawn at a particular maximum flowrate through a procedure call the drawdown test. The effective stress and minimum principal stress within the borehole can be determined [START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF] The Nankai Subduction Zone: A prime in situ laboratory for understanding subduction zone

Geodynamic context

The Nankai trough is formed by the subduction of the Philippine Sea Plate beneath the Eurasia Plate at a convergence rate of 17 mm/yr to 65 mm/yr [START_REF] Seno | A model for the motion of the philippine sea plate consistent with nuvel-1 and geological data[END_REF][START_REF] Seno | The instantaneous rotation vector of the philippine sea plate relative to the eurasian plate[END_REF][START_REF] Karig | Geologic constraints on subduction rates in the nankai trough[END_REF][START_REF] Ranken | Kinematics of the philippine sea plate[END_REF][START_REF] Miyazaki | Crustal velocity field of southwest japan: Subduction and arc-arc collision[END_REF]. The Nankai subduction zone has one of the longest histories of repeated big magnitude earthquakes, spanning around 1,300 years [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF]. It is a region with a significant seismic hazard, as evidenced by the M8+

1944 Tonankai earthquake and the 1946 Nankaido earthquake shown in Figure 4.1 [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF][START_REF] Kanamori | Tectonic implications of the 1943 Tonankai and the 1946 Nankaido earthquakes[END_REF]). It has also been linked as a locus of SSE and VLFE (Araki et At the frontal thrust margin, it is characterised with faults originating from the décollement [START_REF] Moore | Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect[END_REF], Gulick et al. 2004). Within the imbricate thrust zone, thrusting also occurred with minimal frontal accretion [START_REF] Kinoshita | Heat flow estimated from bsr and iodp borehole data: Implication of recent uplift and erosion of the imbricate thrust zone in the nankai trough off kumano[END_REF]. The subducting sediments beneath the active basal décollement are relatively undeformed by thrust faulting [START_REF] Mcneill | Deformation and in situ stress in the nankai accretionary prism from resistivity-at-bit images, odp leg 196[END_REF][START_REF] Tobin | Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone[END_REF], Bangs et al. 2009). The OOST marks the landward boundary of the outer accretionary prism. Mass-transport deposits, seabed erosion and erosional processes may be found at imbricate thrusts as a result of uplift from the OOST fault zone [START_REF] Tuyl | Strain decoupling reveals variable seismogenic risk in se japan (nankai trough)[END_REF]. Evidence from drilling has revealed the accretionary prism's rapid growth 1.95-2.0 Ma [START_REF] Screaton | Interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula, japan: Results from iodp expedition 316 sites c0006 and c0007[END_REF], Strasser et al. 2009[START_REF] Kimura | Spatial and temporal evolution of the megasplay fault in the nankai trough[END_REF]. Rapid growth of the outer wedge's base, marked by a seismical low velocity zone quiet region and slow earthquakes, appears to have been aided by underplating of clay deposits in the Shikoku Basin [START_REF] Park | A low-velocity zone with weak reflectivity along the Nankai subduction zone[END_REF][START_REF] Kamei | On acoustic waveform tomography of wideangle obs data-strategies for pre-conditioning and inversion[END_REF]. Ma overlying sediments, the accretionary prism is ∼7.9-8.1 Ma at the C0009 drill site and at the C0002 drill site it is 5.0 Ma [START_REF] Saffer | Response of submarine hydrologic monitoring instruments to formation pressure changes: Theory and application to Nankai advanced CORKs[END_REF], Moore et al. 2015). The rapid forearc basin development was induced by the quick uplift of the KBEFZ, which was driven by the accretion of trench-filling sediments supplied to the trench [START_REF] Kimura | Transition of accretionary wedge structures around the up-dip limit of the seismogenic subduction zone[END_REF]).

The reflection seismic (Figure 4.2) reveal that the splay fault is a first order, margin dominating structure that spans more than 120 kilometers along strike [START_REF] Moore | Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect[END_REF][START_REF] Kimura | Transition of accretionary wedge structures around the up-dip limit of the seismogenic subduction zone[END_REF][START_REF] Jin-Oh | Splay fault branching along the nankai subduction zone[END_REF], Bangs et al. 2009[START_REF] Tobin | The IODP Nankai trough seismogenic zone experiment[END_REF]. The upper slope of the trench and Kumano Basin, the megasplay fault traverses the older portion of the accretionary prism (Figure 4.2). The megasplay fault can be followed from ∼10 km beneath the accretionary wedge to ∼1 km above the top of the subducting crust (Figure 4.2). It cuts discontinuously through the accretionary wedge along multiple branches [START_REF] Moore | Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect[END_REF][START_REF] Moore | Threedimensional splay faultgeometry and implications fortsunami generation[END_REF]. Megasplay is one of the principal coseismic faults that may have contributed to the occurrence of historically severe earthquakes and tsunamis [START_REF] Tobin | The IODP Nankai trough seismogenic zone experiment[END_REF]. Deep parts of the splay fault are continuous along strike across the whole 12 km 3D survey width, but shallower segments often continue along strike for 3-5 km.

The spectrum of seismic behaviors

The Nankai subduction zone has one of the longest histories of repeated large magnitude subduction earthquakes, spanning around 1,300 years [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF][START_REF] Kanamori | Great earthquakes at island arcs and the lithosphere[END_REF], 1972, Seno et al. 1993). Recurrence intervals for significant earthquakes of historical significance in the Nankai region are typically between 100 and 200 years [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF][START_REF] Shimazaki | Time-predictable recurrence model for large earthquakes[END_REF][START_REF] Ishibashi | Status of historical seismology in japan[END_REF]). The Nankai region is identified as a high seismic risk zone. Here, the seismic activity have been connected to the rupture of the trough in some segments (Ando 1975, Cummins andKaneda 2002). Both the eastern and western segments were ruptured by the two most recent past large magnitude earthquakes: the December 7, 1944 Tonankai earthquake and the December 21, 1946

Nankaido earthquake [START_REF] Kanamori | Tectonic implications of the 1943 Tonankai and the 1946 Nankaido earthquakes[END_REF], 1971, Seno et al. 1993[START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF][START_REF] Dessa | Deep seismic imaging of the eastern nankai trough, japan, from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration[END_REF]. Though the most recent two earthquakes happened within a span of around two years of one another, the rupture segments of the two events are quite distinct from one another, as noted by [START_REF] Ando | Source mechanisms and tectonic significance of historical earthquakes along the nankai trough, japan[END_REF] and [START_REF] Murotani | Rupture process of the 1946 nankai earthquake estimated using seismic waveforms and geodetic data[END_REF]. Seismic activity with such destructive potential originates in the seismogenic zone but ruptures along the plate boundary and along the megathrust fault to the seafloor, generating large Tsunami.

There hasn't been a megathrust earthquake in the Nankai trough of this magnitude in over seven decades. However, whether or not another major earthquake will occur is still uncertain.

However, in Nankai subduction zone, shallow zone seismic activity such as the SSEs, VLFEs, and ETS (Figure 4.3) have been recognized as having significant tsunamigenic potential [START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF][START_REF] Obara | Connecting slow earthquakes to huge earthquakes[END_REF][START_REF] Takemura | Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes[END_REF], 2022). They occur at the deep downdip or shallow updip portion of the seismogenic zone [START_REF] Obara | Connecting slow earthquakes to huge earthquakes[END_REF][START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF][START_REF] Yokota | Shallow slow slip events along the nankai trough detected by gnss-a[END_REF]. In 2011, the Japan area experienced a large magnitude earthquake of more than +9 MW, which was caused by a large slip that occurred at the shallowest portion of the subduction interface [START_REF] Lay | Great Earthquakes on Plate Boundaries[END_REF]). Hence, it has become important to understand the heterogeneous patches at the limit between the seismic and aseismic zones of the Nankai subduction zone. creted trench-fill turbidite sediment, and is characterized by multiple imbricate thrust faults and folds [START_REF] Moore | Structure of the nankai trough accretionary zone from multichannel seismic reflection data[END_REF]. At deeper depths above the décollement within the accretionary prism, a low velocity zone (LVZ) characterized by poor reflectivity and V p between 2.7 and 3.2 km/s [START_REF] Park | A low-velocity zone with weak reflectivity along the Nankai subduction zone[END_REF]). The décollement is the reflective surface with a high horizontal amplitude that is interpreted as the plate boundary fault [START_REF] Park | A low-velocity zone with weak reflectivity along the Nankai subduction zone[END_REF][START_REF] Park | A deep strong reflector in the nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release[END_REF]. The underthrust sediments are located between the décollement and the oceanic crust (downgoing plate). It has a weak reflectivity and a velocity between 3.5 and 4 km/s. In the Nankai subduction zone, the underthrust is interpreted as hemipelagic sediment from the Shikoku Basin.

Recent

Lithostratigraphy of Nankai Accretionary Prism

The lithostratigraphy units in the Nankai accretionary prism is broadly classified into three units: The Shikoku Basin is the primary sedimentary basin on the subducting Philippine Sea plate's northern margin. During the late Oligocene to late Miocene (15 -25 Ma), the basin was produced by rifting and back-arc spreading behind the Izu-Bonin arc system [START_REF] Okino | Evolution of the shikoku basin[END_REF][START_REF] Kobayashi | Shikoku Basin and Its Margins[END_REF]. Regional and local differences in basement relief, sediment thickness, and sedimentary facies characterize the Shikoku Basin. Under the Nankai forearc, seamounts on the subducting plate have been imaged [START_REF] Kodaira | Subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake[END_REF]. Subduction of basement highs can result in substantial deformation of the upper plate and a shift in the stratigraphic location of the plate interface [START_REF] Kimura | Spatial and temporal evolution of the megasplay fault in the nankai trough[END_REF]). The Northern Shikoku Basin is comprised of three primary seismic sequences, which are distinguished as three distinct stratigraphic units. The first sequence is the Upper Shikoku Basin sequence, which has an average decrease in facies in the Kumano basin in the north. The second sequence is the lower Shikoku Basin sequence, which is characterized by sandy-package facies and complex basement-related profiles. [START_REF] Moore | New insights into deformation and fluid f low processes in the nankai trough accretionary prism: Results of ocean drilling program leg 190 geochemistry geophysics geosystems[END_REF][START_REF] Mazzotti | Full interseismic locking of the nankai and japan-west kurile subduction zones: An analysis of uniform elastic strain accumulation in japan constrained by permanent gps[END_REF] describe the Quaternary turbiditic sequence as the third sequence.

These sediments will enter the subduction zone and become accreted on the prism or advected with the Philippine plate as it subducts.

c. Accretionary Prism:

The accretionary prism is composed of the inner and outer accretionary wedges. The inner wedge is composed of the accretionary prism and overlying strata. The outer wedge of the accretionary prism is composed of Shikoku Basin sediments and earlier turbiditic strata deposited at the trench [START_REF] Saffer | Response of submarine hydrologic monitoring instruments to formation pressure changes: Theory and application to Nankai advanced CORKs[END_REF]. The thrust fault is aligned with the updip part of the splay by the outer arc high of the accretionary wedge. The bottom portion of the forearc slope is composed of thrust faults that have significantly diminished the sediments of the wedge [START_REF] Moore | Evolution of tectono-sedimentary systems in the kumano basin, nankai trough forearc[END_REF]. A hypothesis proposed by [START_REF] Wang | Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge[END_REF] claimed that the megasplay fault is the structural interface between the inner and outer wedges, as well as the aseismic and seismogenic fault activity.

Nankai Trough Seismogenic Zone Experiments (NanTroSEIZE)

The Nankai subduction zone is among the most studied seismogenic and tsunamigenic plate boundary zones globally (Tobin andKinoshita 2007a, Tobin et al. 2009)

Methodology

The methodology used for this thesis combines drilling engineering methods with geophysical approaches.

1. The main contribution of this PhD is the quantitative modeling of the mud pressure data.

To understand this modeling we first introduce the basics of mud circulation during drilling operation, before developing the scheme for retrieving the flow between the borehole and the formation. The key parameters to be determined are highlighted.

2. To support the results, we estimate pore pressure, using either empirical Eaton methods or laboratory consolidation data. They rely on strong assumptions but are useful for checking the self-consistency of our results.

3. Since fluid flow and pore pressure are estimated, rough estimates of permeability can be retrieved.

Introduction Drilling Engineering, Data Acquisition and Processing

Overview of drilling system

Drilling is the process of cutting a circular cross-sectional hole through a solid material. This is intended for the purpose of extracting natural resources (oil, gas, gold, etc.) as well as scientific research into the earth's dynamic processes and structure. Over time, the oil and gas industry has thrived because drilling has become the primary method of extracting resources. Recently drilling has been adopted for academic scientific purposes like the International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP). Such campaigns provide direct evidences and links to understanding earth dynamic processes such as seismicity, environmental hazards, fluid pressures, temperature distribution and the earth structure. Drilling is accomplished by using a complex set of machines known as the drilling rig system (Figure 5.1).

The rig system is made up of six components: the hoisting system, the rotary system, the drill string, the mud circulating system, the power system, and the blowout preventer system (BOP) (Figure 5.1). formation pressure, maintaining wellbore stability, and increase drilling efficiency [START_REF] Bourgoyne | Applied Drilling Engineering[END_REF][START_REF] Ward | Pressure while drilling data improves reservoir drilling performance[END_REF][START_REF] Hutchinson | Using downhole annular pressure measurements to anticipate drilling problems[END_REF][START_REF] Simpson | Well-Bore Construction (Drilling and Completions[END_REF].

The drill mud is mixed with the rock cuttings as the formations are drilled, and transported via Riser drilling and riserless drilling are the two main drilling methods that have been utilized throughout the NanTroSEIZE project (Figure 5.3a). In riser drilling, the drill mud and cuttings are returned to the surface through a special riser pipe that is linked to a seafloor blowout preventer

[BOP] (Figure 5.3a). For a riserless drilling the cuttings are pumped out of the borehole with seawater onto the seafloor; hence, nothing is known about the amount of drilling mud or the cuttings that comes out of the borehole. This method was utilized to drill the sites (C0024, C0006, C0004, etc.) that were used for this thesis. 5.1). This is important because it helps to determine the exact pressure of the seafloor (P sea = ρ w Z w g) where,

P sea is the pressure at the mudline, ρ w is the density of the seawater, Z w is the seawater column, and g is the acceleration due to gravity.

• Accounting for the location of the DHAP sensor: Accounting for the location of the DHAP sensor above the bit from the total depth of the well (Table 5.2) (M Dmbsf = Bit -Depth(m) -Sensor -Depth). where P m is the pressure at the mudline, ρ w is the density of the seawater, z is the seawater column, g is the acceleration due to gravity.

Q f = Q out -Q pump (5.2) Where Q f is

Forward modeling of DHAP to estimate mud pressure anomalies

This considers the contribution of static pressure and dynamic hydraulic frictional pressure losses induced during pumping by fluid circulation. Any anomalies not accounted for by static and dynamic pressure contributions are attributable to fluid influx (Q f ) between the borehole and the formation. We assume that swabs and surges are negligible because the interpretation of DHAP is restricted to the data set corresponding to the times corresponding to actual drilling. The forward modeling involves:

a. Contribution of cuttings to drilling mud

The weight of the returned mud surpasses the weight of the mud that was first injected because the mud returning out of the borehole through the annulus conveys rock cuttings. However, because Holes C0024A, C0004B, and C0006A were drilled with a riserless system, we do not have direct information about the contribution of the cuttings to the drilling mud because it was lost to the seafloor. Therefore, the principle of mass balance is used to estimate the effective density of the mud (i.e., the combination of the clean density of the mud and the cuttings). For the sake of this calculation, the following assumptions were made:

1. Within the borehole is a homogeneous mud with an effective density (ρ eff ) that is assumed to be independent of temperature, pressure, and therefore depth.

2. The amount of mud that returns to the seafloor is equal to the amount of mud that leaves the pumps (i.e., no mud loss , neither storage within the pipes and annulus). We used the Caliper-log values in our calculation to obtain the upper estimate of the contribution of the cuttings. Caliper data for hole C0024A, and C0004B were provided (Figure 5.13), while the caliper data for Hole C0006A was not acquired completely during the expedition. Therefore to compensate for this error, a new caliper log was derived from the relationship between gammaray correction factor, mud weight and caliper data from Hole C0006B (Details can be found in the result section of Hole C0006A). Initial mass at t0 :

M 0 = ρ ef f * V bor + [ρ grain (1 -ϕ) + ρ w ϕ] dV (5.3)
Final mass at t 0 + dt (borehole of volume V bor + dV full of mud) :

M (t 0 + dt) = ρ ef f * (V bor + dV ) (5.4)
Hence, subtracting equation 1.2 from equation 1.3:

dM = M (t0 + dt) -M 0 = [ρ ef f -ρ grain (1 -ϕ) -ρ w ϕ] dV (5.5)
But also, using the flow in (fresh and clean mud with flow Q i n = Q pump ) and the flow out.

Mud entering the pump:

(ρ M W Q in = rho M W * Q pump ).
Mud leaving the annulus at the seafloor:

(ρ M W Q out = rho ef f * Q out ).
Hence, calculating the mud difference in and out of the borehole (positive in the case of influx to the well, negative in the case of outflow) contribute to the flow returning to the surface Q out . Fluid flow into the well is assumed to come from a section between the DHAP sensor and the drill bit. Above, an impermeable mud cake is supposed to be fully developed.

annulus, by subtracting the two equations above:

dM = M in -M out = ρ M W Q in dt -ρ ef f Q out dt (5.6)
Finally, we have the equation

ρ M W Q in -ρ ef f Q out = [ρ ef f -ρ grain (1 -ϕ) -ρ w ϕ] Q f ormation (5.7) with Q f ormation = π(d bit ) 2 4 ROP ∼ π(d bor ) 2 4 ROP = dV dt (5.8)
We put the ρ ef f terms together

ρ M W Q in + [ρ grain (1 -ϕ) + ρ w ϕ] Q f ormation = ρ ef f [Q f ormation + Q out ]
(5.9)

ρ ef f = ρ M W Q in + ρ grain (1 -ϕ) Q f ormation + ρ w ϕ Q f ormation Q f ormation + Q out (5.10) We assume that Q inf lux ≪ Q pump .
If there were major influx of water (influx):

ρ w (Q inf lux -Q outf lux ) + ρ M W Q in -ρ ef f Q out = [ρ ef f -ρ grain (1 -ϕ) -ρ w ϕ] Q f ormation
(5.11)

ρ ef f = ρ w (Q inf lux -Q outf lux ) + ρ M W Q in + ρ grain (1 -ϕ) Q f ormation + ρ w ϕ Q f ormation Q f ormation + (Q in + (Q inf lux -Q outf lux ))
(5.12)

ρ eff = ρ M W Q pump + ρ r ROP π d 2 b 4 ROP π d 2 b 4 + Q pump (5.13)
Given that the cores could not be recovered for most of the borehole, the bulk density of the formation was estimated as ρ r = ρ g (1 -ϕ) + ρ w ϕ, where ρ g is the grain density determined from the cored section, ρ w is the density of salted water that fills the pores of the rock (assumed to be seawater, so ρ w = 1028 kg/m 3 ) and ϕ is the porosity of the rock, as estimated on board from the resistivity logs [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]).

b. Contribution of dynamic hydraulic losses

Due to the viscosity of the mud, increased mud pressure at the bottom of the hole is required to allow the mud to flow back to the sea via the borehole annulus. Hydraulic resistance will cause a difference in pressure between the annular pressure at the DHAP sensor position and the seafloor, which depends on the flow circulating through the annulus.

As a first approximation, it is assumed that any flow (Q f ) between the formation and the well occurs only below the DHAP sensor and therefore the hydraulic loss along the drilling string can be calculated using the known pumping rate (Q pump ) in equation. 5.2. In addition, hydraulic resistance is a function of the hydrodynamic regime in which it exists. Therefore, the hydrodynamic regime is defined based on the Reynolds number, which is either a laminar (Re ≪ 2000)

or turbulent (Re ≫ 4000) regime. The dimensionless Reynolds number is a critical quantity that influences the choice of frictional pressure loss equations:

Re = ρ eff v d e µ (5.14)
Where Re is the Reynolds number (dimensionless), ρ eff is the effective density (kg/m 3 ) (Equation. 5.13), µ is the dynamic (or absolute) viscosity of fluid (Pa • s), v is the annular average velocity and d e is the hydraulic diameter(m). v is estimated through the mass balance equation, providing a direct relationship with the flow rate and an inverse relationship with the surface area of the drill string:

v = 4 Q out π d b 2 -d p 2 (5.15)
Where d b is the diameter of the borehole and d p is the external diameter of the drill string. The hydraulic diameter (d e ) is defined as the function of the borehole diameter (d b ) and the external diameter of the drill string (d p ) [START_REF] Bourgoyne | Applied Drilling Engineering[END_REF]:

d e = d 2 b + d 2 p - d 2 b -d 2 p ln d b dp (5.16)
The value of the estimated Reynolds number (Re ≫ 50000) indicates a turbulent hydrodynamic flow regime exist within the borehole annulus. Hence, the hydraulic frictional pressure loss within the borehole annulus for the interval borehole length (dz) can be determined through the Fanning equation [START_REF] Bourgoyne | Applied Drilling Engineering[END_REF]):

dp f dz = 2f ρ eff v2 d e
(5.17)

Where dp f is the hydraulic pressure losses (Pa), f is the Fanning friction coefficient. [START_REF] Blasius | Das Ähnlichkeitsgesetz bei Reibungsvorgängen in Flüssigkeiten[END_REF] showed that the Fanning coefficient is related to the Reynolds number in a simple way [START_REF] Bourgoyne | Applied Drilling Engineering[END_REF], provided that we ignore the roughness of the wall of the pipe:

f = B Re 1/4
(5.18)

where, experimentally, B = 0.0791.

Since the Holes C0024A, C0006A, and C0004B are drilled with seawater, a Newtonian fluid type model is considered. Combining Equations 5.17 and 5.18, we get the appropriate pressure loss equation for a Newtonian fluid turbulence model based on the Fanning equations [START_REF] Bourgoyne | Applied Drilling Engineering[END_REF]). This equation expresses the gradient of frictional pressure drop (dp) along a section of the borehole annulus of length (dz):

dp f dz = 2 B ρvde µ 1/4 ρv 2 d e (5.19) which simplifies into dp f dz = 2B ρ ρ 1/4 v 2 v 1/4 1 d 1/4 e d e µ 1/4
(5.20) In Equation 5.22, it is assumed that the fluid is incompressible and that the flow from the formation does not build up pressure, because it just escapes to the surface through the annulus of the borehole. A new equation is derived to calculate hydraulic pressure losses (P a) by combining equations 5.22, 5.15, and 5.16:

dp f dz = 2B ρ 3/4 v 7/
dp f dz = 2 × 4 7/4 π 7/4 × B ρ 3/4 µ 1/4 × Q 7/4 out d 5/4 e × d b 2 -d p 2 7/4
(5.24)

dp f dz = 4 9/4 π 7/4 B ρ 3/4 Q 7/4 out µ 1/4 × 1 d 5/4 e × d b 2 -d p 2 7/4
(5.25) by integrating between 0 and z DHAP .

∆p f = 4 9/4 π 7/4 B ρ 3/4 Q 7/4 out µ 1/4 × z DHAP d 5/4 e × d b 2 -d p 2 7/4
(5.27)

This equation is integrated between the seafloor and the current depth of the DHAP sensor:

∆p(z DHAP ) = 4 9/4 B Q 7/4 out π 7/4 F (z DHAP ) (5.28)
Here, F (z DHAP ) is the bundle term for all the depth-dependent terms. For each depth considered, the actual diameter of the borehole as captured by the caliper log (Figure 6.4e) and the actual configuration of the drill string (Figure 5.3b):

F (z DHAP ) = z DHAP 0 ρ eff (z) 3/4 µ(z) 1/4 d b (z) 2 + d p (z) 2 - d b (z) 2 -dp(z) 2 ln d b (z) dp(z) 5/8 d b (z) 2 -d p (z) 2 7/4 dz (5.29)
Hence, we devised a forward relationship between dynamic pressure loss and flow from the formation (Q out ), with ∆p(z DHAP ) = HL (Q out ), where the function HL is defined as:

HL (x) = 4 9/4 B π 7/4 F (z DHAP ) x 7/4
(5.30)

Inverse model of DHAP to estimate flow between formation and borehole

Any anomaly not captured by the previous forward modeling of DHAP above is attributed to fluid exchange between the well and the surrounding rock formation. Therefore, it is convenient to use equation 5.28 to convert the unexplained DHAP anomaly into an anomaly in the vertical upward flow. Hence, the fluid flow Q f between the borehole and the formation is given as:

Q f = HL -1 (DHAP -ρ eff gz -P sea ) -Q pump (5.31)
where HL is the hydraulic loss function introduced in equation 5.

Pore Pressure Modeling

The pore pressure modeling was used to validate the fluid flow model. In the subduction zones, pore pressure generation is controlled by the permeability and fluid retention capacity of the rock formation, loading history, under-compaction, tectonic or slope change events [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF][START_REF] Tobin | Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone[END_REF]. The compaction evolution of these sediments in a simple drained diagenetic process supports increase in overburden stress [START_REF] Terzaghi | Soil Mechanics in Engineering Practice[END_REF]). The sediment compaction under hydrostatic pressure is termed the normal compaction trend (NCT). When the pore pressure falls below or exceeds the hydrostatic pressure, this is termed anomalous pressures (i.e. under or overpressured) (Figure 5.17).

Anomalous pore pressures can result when there is partial expulsion of fluid from pores when sediment are loaded rapidly (acceleration of subsidence or tectonic loading) and the permeability of the formation is very low to drain out the overpressure [START_REF] Zhang | Abnormal pore pressure mechanisms[END_REF]). The remaining fluid in the pores will support all or part of the weight of the overburden sediment. This results in less pore compaction, hence higher porosity is observed than in a normal compaction formation [START_REF] Zhang | Abnormal pore pressure mechanisms[END_REF]. This creates an unusually high pore water pressure, which causes the porosity to decrease more slowly than it should with depth, resulting in disequilibrium of the state of compaction or undercompaction. High pore pressures are responsible for serious drilling incidents (kicks and blowouts) [START_REF] Skalle | Trends extracted from 1,200 Gulf coast blowouts during 1960-1996[END_REF]Podio 1998, Holand 2001). [START_REF] Terzaghi | Soil Mechanics in Engineering Practice[END_REF] proposed the empirical relationship to estimate pore pressure from total overburden stress and effective stress (σ e ).

σ e = σv -P f (5.32)

Where the pore pressure (P f ) in formation is a function of the total overburden stress (σv) and the effective stress (σ e ). Here, the overburden stress is estimated from the bulk density and porosity data for each hole. This is further discussed in the subsection 5.3.1.1.

Overburden Stress Estimation

Overburden stress or the vertical stress (σv) at a given depth (Figure 5.17) is the total exerted pressure on a formation by the total weight of the overlying rocks and fluids above. The overburden can be estimated using density logs if available. But unfortunately the density logs have not been acquired when drilling three wells C0024A, C0006A, and C0004B. Alternatively to derive overburden stress, one can use bulk density and porosity information: Here we discuss how (1) porosity and (2) bulk density are calculated for the three wells.

Porosity Estimation

In hole C0024A the porosity onboard was obtained in two different ways and compared with each 1942,1952). This method uses inputs from well log parameters hence, providing quantitative relationship between porosity, resistivity of rock (R t ) and the fluid saturation of rocks. This method was applied to estimate porosity in all the three holes (C0024A, C0006A, and C0004B).

The resulting estimate is very sensitive to the choice of Archie's law constants [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF].

S n w = a.R w ϕ m R t (5.33)
Where S w is the water saturation, a is the tortuosity factor (a = 1), ϕ is the porosity, m is the cementation exponent (m = 2.4), n is the saturation exponent, R w is the brine resistivity, R t is the True Resistivity (assumed to equivalent to a deep resistivity log reading). Porosity is also linked with the formation factor, through the mathematical relationship expressed by Archie:

F = a ϕ m (5.34)
In a rock with non-conducting grains, the bulk conductivity depends linearly on the conductivity of the fluid, and the formation factor F conveniently depend as the ratio of fluid conductivity (cf) to bulk conductivity (c). The formation factor is calculated as

F = R o R f (5.35)
Where is R o =R w is the Resistivity of rock with only water (S w = 1). Here, we assumed that the pore fluid is similar to seawater. The formula used to calculate the resistivity of seawater (R f ) as a function of temperature T (°C) is as follows [START_REF] Shipley | Proc. odp, init. repts[END_REF] :

R f = 1 2.803 + 0.0996T
(5.36)

The temperature profile was calculated at each site considering a conductive heat transfer, the measured surface heat flux, and an estimation of the thermal conductivity based on previous

Bulk density

In C0024A, the matrix density was determined from the moisture and density (MAD) test done on selected core samples obtained from wells C0024 (B, C, D, F, G) (Figure 4.1b). The matrix density is used with the porosity data (calculated from Archie's equations using the resistivity log data) to calculate the bulk density. In holes C0006A and C0004B a constant grain density (ρ r = 2.65 g/cm 3 ) and density of water (ρ w = 1.024 g/cm 3 ) were used. Hence, bulk density is estimated as:

ρ b = ϕρ w + (1 -ϕ) ρ r (5.39)
Where ρ b is the bulk density, ϕ is the porosity, ρ w is the density of water and ρ r is the density of the matrix.

The overburden pressure is estimated by integrating the bulk density from the seafloor, taking into account the seafloor pressure and the true vertical depth. The overburden gradient (σ vg ) can be estimated using the equation below:

σ vg = P sea + Z 0 ρ b (Z) g dZ -P sea Z (5.40) = Z 0 ρ b (Z) dZ Z g
(5.41)

Hydrostatic Pressure Estimation

This is the fluid pressure exerted at a given depth within the fluid, due to gravitational force.

This pressure increases with depth from the seafloor due to increasing weight of fluid exerting downward force from the top. The hydrostatic pressure gradient (P hg ) is calculated using:

P hg = (P sea + ρ w g Z) -P sea Z = ρ w g (5.42)

Pore Pressure Prediction:-Eaton's Method

From the modeling of the DHAP, the observed high fluid influx from the formation into the borehole is driven by a high pore pressure within the formations. Therefore as a support to the fluid flow estimation, we used the empirical Eaton's method using two independent datasets (drilling parameters and sonic compressional velocity) as inputs and (Kitajima andSaffer 2012, Kitajima et al. 2017) method to check for self consistency. The Eaton's approach has been widely applied in petroleum basins, where there is little or no active tectonics. In these basins the normal compaction trend (NCT) is determined by the compaction of shale for example, the Gulf of Mexico.

However, subduction zones are characterized by active tectonics and due to unavailability of direct evidence of hydraulic properties in this hole, we still used this method. To compensate for this limitation, we used two versions of this method using two independent datasets: (a) using drilling parameters [START_REF] Jorden | Application of drilling performance data to overpressure detection[END_REF])(b) using the sonic velocity [START_REF] Eaton | The equation for geopressure prediction from well logs[END_REF]) Eaton methods.

The Eaton empirical technique [START_REF] Eaton | The effect of overburden stress on geopressure prediction from well logs[END_REF][START_REF] Eaton | The equation for geopressure prediction from well logs[END_REF] establishes a relationship between the geopressure gradient, the electrical log properties of the lithologies, the drilling parameters and the normal compaction trend (NCT). This equation depends on the pore pressure gradient, as pressure gradients are more convenient to employ when determining the mud weight in drilling engineering [START_REF] Zhang | Abnormal pore pressure mechanisms[END_REF]. This method presumes that the abnormal variations observed in LWD data during drilling can be related to abnormal pore pressures but also it could be due to lithology change, temperature, salinity, hole size, and mud weight [START_REF] Zhang | Real-time pore pressure detection: Indicators and improved methods[END_REF]. Therefore, it is critical to use several LWD data to delineate abnormal pore pressure and check for self-consistencies.

As noted above, the Eaton method is limited when obtaining the NCT within an active zone, hence the need to compare the pore pressure estimates from Eaton with another empirical method derived from consolidation experiments done on rock sampled during the NanTroSEIZE project (Kitajima andSaffer 2012, Kitajima et al. 2017). This method uses the empirical relationship between P-wave velocity, porosity and effective stress derived from samples representative of the Nankai accretionary prism [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF].

Eaton d-exponent method using drilling data

In drilling engineering the d-exponent (d x ) method delineates the empirical relationship between rock strength, bit size, and formation drillability [START_REF] Bingham | A new approach to interpreting rock drillability[END_REF]. Pore pressure increase is estimated by accounting for the normalisation of the rate of penetration (ROP). In typical normal compacted sediments under hydrostatic conditions the ROP should follow an exponential decay law with depth i.e it becomes difficult to drill through highly compacted sediments. High pore pressure facilitates rock failure and ROP increases rapidly [START_REF] Zhang | Real-time pore pressure detection: Indicators and improved methods[END_REF] or decreases at a slower rate than it would normally decrease in normal compacted materials. During drilling, the penetration rate is influenced by the lithological variation, the weight on the bit (WOB), the pore pressure, the rotation rate of the drill string (RPM), the torque and the type of the bit. [START_REF] Jorden | Application of drilling performance data to overpressure detection[END_REF] noted that under variable drilling conditions, there is a recognizable relationship between d-exponent and differential pressures (the bottom-hole pressure difference between the formation and the mud column): Where P sea is the pressure at the mudline (seafloor), Z is the true vertical depth (TVD) in meter below seafloor (mbsf) and g is acceleration due to gravity (9.81 m/s 2 ). [START_REF] Jorden | Application of drilling performance data to overpressure detection[END_REF] proposed that the pore pressure gradient (P pg ) could be determined from the d-exponent, the overburden gradient (σ vg ) and the hydrostatic pressure gradient (P hg ):

d x = log ROP 60 RPM log 12 WOB
P pg = σ vg -(σ vg -P hg ) d xc d n n
(5.46)

d n = d 0 + d Z (

Eaton method applied to seismic velocity

From the relationship between seismic velocity and effective stress, [START_REF] Bowers | Pore pressure estimation from velocity data : Accounting for overpressure mechanisms besides undercompaction[END_REF] postulated that the pore pressure can be estimated from the ratio between effective stress and the velocity in normally pressured sediments. Compressional velocity depends on the grain type, fluid content, and porosity of the different lithologies [START_REF] Eaton | The effect of overburden stress on geopressure prediction from well logs[END_REF]. The variability of the overburden stress gradients [START_REF] Terzaghi | Soil Mechanics in Engineering Practice[END_REF]) depends on the region of study but is generally a function of the burial depth and the pressure gradients of the pores. In borehole logging, seismic slowness 1/V p is often used instead of velocity. It is expressed in µs/ft and is denoted as ∆t, since it is proportional to the travel time. Abusively, seismic slowness is also named "travel time". Pore pressure gradient can then be estimated considering the seismic slowness of the shale:

P pg = σ vg -(σ vg -P hg ) ∆t n ∆t m (5.49)
Where ∆t is seismic slowness in shales from well logs, ∆t n is seismic slowness in shales (normal pressure condition), m is an exponent (empirically m equals 3). Departure of the sonic slowness away from the NCT to higher values indicates evidence of overpressure but true if within the same lithology. To estimate the NCT of the shale seismic slowness, we first preprocess the sonic seismic slowness log by filtering and smoothing the data. The NCT (∆t n ) was generated by fitting an exponential relationship of the sonic seismic slowness in relation to the drilled depth:

∆t n = ∆t m -(∆t ml -∆t m ) e -cz (5.50)
Where ∆t m is the seismic slowness in the shale matrix, ∆t ml is the seismic slowness at the mudline (Z = 0), Z is the true vertical depth in meters below seafloor (mbsf), and c is the compaction parameter. The pore pressure is estimated using equation 5.48.

5.3.4

Pore pressure prediction using the workflow of [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] An alternate estimate of pore pressure (P f ) was determined using the workflow developed in [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] and [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]. This method uses an empirical relationship between P-wave velocity, porosity, and effective stress derived from samples representative of the Nankai accretionary prism [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF]. The samples were coming from Site C0011, cored in the subducting Philippine sea plate.

The first step of the workflow is to derive the porosity from the P-wave velocity data, using the empirical relationship of [START_REF] Erickson | Velocity-porosity relationships for water-saturated siliciclastic sediments[END_REF]:

V p [km/s] = 1.11 + 0.178 ϕ t + 0.305 (ϕ t + 0.135) 2 + 0 : 0775 + 0.61 (v sh -1) X m (5.51)
where ϕ t is the total porosity, v sh is the shale volume, and X m accounts for the brutal change in behavior at a critical porosity ϕ c = 0.39, above which stress is supported by the fluid rather than by the grain skeleton of the rock:

X m = tanh [20 (ϕ t -ϕ c )] -∥ tanh [20 (ϕ t -ϕ c )] ∥ (5.52)
For consistency with the method of [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF], we will consider the porosity stress(q). p ′ = 10 0.79-e 0.40

(5.61)

q = p ′ (5.62)
With only two equations provided by Equations 5.59 or 5.61, additional assumptions on stress have to be made. A first assumption is that the vertical direction is a principal stress direction.

Another assumption is about the amplitude of minimum horizontal stress S hmin . Contrary to [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF], no leak-off test data is available. Hence, following Zhang and Zhang (2017), we assume the minimum horizontal principal stress is provided by the elastic response to an overburden loading [START_REF] Turcotte | Geodynamics[END_REF].

S ′ hmin = S hmin -p f = (σ v -p f ) ν 1 -ν (5.63)
Poisson ratio ν was obtained from the V p and V s ratio (ν =

V 2 p -V 2 s 2(V 2 p -V 2 s )
). Hence, S hmin could not be retrieved at depths, since V s data are often not recovered, due to low signal-to-noise ratio.

Equations 5.59 or Equations 5.61 are inverted to estimate pore pressure p f and the principal horizontal stress S Hmax in the K0 and CSL conditions, respectively.

A general caveat for applying the independent pore pressure methods is to consider the lithological changes when interpreting the results. This is true because the changes in well log signatures can be influenced by lithological change and not necessarily due to over pressure condition.

Permeability Estimation Using Hvorslev Method

The estimated results of the fluid flow from DHAP modelling and pore pressure were used to quantitatively estimate the permeability by using an empirical relationship called the Hvorslev equation [START_REF] Hvorslev | Time Lag and Soil Permeability in Ground-Water Observations[END_REF]) (Equation 5.71). This equation was derived from the fundamental Darcy's flow rule ⃗ q = -k µ ⃗ ∇p which has been used widely to evaluate the hydraulic head, flow test, and measure in situ hydraulic conductivity [START_REF] Ratnam | Revisiting Hvorslev's intake factors using the finite element method[END_REF]. This relationship is expected to yield approximate result, because lithologies are not infinite in extent and are rarely uniform in character [START_REF] Hvorslev | Time Lag and Soil Permeability in Ground-Water Observations[END_REF]. However, because the hydraulic conductivity characteristics of the lithologies are rarely known precisely in advance, the formula used in this study is often acceptable for preliminary permeability measurement [START_REF] Hvorslev | Time Lag and Soil Permeability in Ground-Water Observations[END_REF]. Few assumptions were made for this study:

∆k =   µ 1 2π L ln Re Rw DHAP -p f   ∆Q +   µ Q 2π L ln Re Rw (DHAP -p f ) 2   ∆p f (5.72)
Where, k s the permeability of the porous medium (m 2 ), µ is the dynamic viscosity (P a • s), Q f is the total discharge rate (m 3 /s), L is the length of the screen (m), r w is the wellbore radius (m), P f is the pore pressure in the undisturbed formation (Pa), calculated by the Eaton method, The radius of influence (r e ) is calculated as r e = 100 r w .

The modified Hvorslev equation (Equation 5.72) is differentiated into two parts (a) the expression in the brackets is the sensitive parameter, and (b) the differential factor as the uncertainty parameter. While the fluid flow is considered as the uncertainty parameter, the pore pressure is kept constant in the first part of the equation, and in the second derivative part of the equation the fluid flow is kept constant while the pore pressure is the uncertainty parameter.

Monte Carlo Simulation

To predict the results of a certain stochastic process, this numerical simulation method employs a combination of several probability distributions. This typically occurs whenever there are uncontrollable factors involved in the procedure. Monte Carlo Simulation (MCS) relies on the idea of repeatedly drawing samples at random from a specified set of probability distributions. There are primarily four ways to implement this method:

• Find the appropriate empirical equation for the modeling. The work uses a variant of the Hvorslev equation (Equation 5.72).

• Find out what goes into the process and how the various inputs interact with one another.

• It is important to specify the distributions of the relevant parameters.

• Take a representative sample from each distribution and combine the means according to (point 1).

• Based on the simulation's sample rate, the process can be repeated indefinitely.

While the fluid flow is assumed to be uniformly distributed, the pore pressure is assumed to be normally distributed (mean and standard deviation values) (minimum and maximum values). For this reason, we run the code through a Monte Carlo simulation with a sample size of 10000 (Figure 5.20). The simulation results are presented in the results section of the manuscript. A full spectrum of a BHA installed with the following sensors, MicroScope, arcVISION, Tele-Scope, SonicScope, and seismicVISION (Figure 5.3) was used to log Hole C0024A [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]). Figure 5.3a and Table 6.1 show the positions of all the sensors used to acquire different lithological and fluid properties downhole. The data recorded by each of this sensor can be seen in Table 6.1. Some of the LWD data types (resistivity, sonic, gamma-ray, P-wave velocity) recorded are shown in Figure 6.4. These data have been used to identify bedding planes, faults, faults dip, breakouts, lithogies, fluids, etc. The quality of the data can be affected by the poor condition of the borehole during logging [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]).

6.1 -CONTEXT 6.2 Hydraulic profiles

Identification of flowing zones within the borehole

The methodology for fluid flow modelling was adopted for each borehole based on the availability of the required input data. All data processing steps for the mudline depth correction (Figure 6.5a and Table 6.2), depth corrections (Figure 6. The modeling was done for the entire time series, but the vertical profiles only show the times related to the actual drilling (Figure 6.6) , when the borehole was extended (Figure 6.9b, for a description of the time-depth conversion).

Contribution of cuttings and hydraulic losses on the DHAP

The contribution of rock cuts to the clean mud during drilling (Equation 5.13) is estimated to be between 1.63 -63 kg/m 3 . This results in a maximum of 6.1% percent increase in the effective density value of mud, which ranges from 1029.63 -1091 kg/m 3 . This suggests that cuttings It was computed for a turbulent hydrodynamic regime in the annulus of the borehole as suggested by the Re ≫ 50000 results. Hence, the frictional hydraulic loss was reasonably calculated with the Fanning equation. The predicted model fits satisfactorily to the mud pressure (DHAP) data, with a difference less than 1 M P a within the accretionary prism until the décollement zone is reached (Figure 6.9c). Then, the pressure anomaly (Figure 6.9c) rises to an excess of 2.5 -5 MPa at the décollement interval (< 813 mbsf). This anomaly in the mud pressure entering the décollement was not fully explained by the hydraulic model.

Figure 6.9d shows the flow between the formation and the borehole. At shallow depths, this flow is negative, meaning mud loss from the borehole to the formation. This is most noticeable between 0 -462.8 mbsf (bottom of lithological subunit 1b) and slightly between 627 -700 mbsf. Below the 468.8 mbsf, mud loss is zero, indicating that there is no flow exchange between the borehole and formation. This inflow tames when entering the upper Shikoku basin (570 -595 mbsf and 700 -770 mbsf), with a flow rate lower than 0.01 m 3 /s. This is a zone where the borehole is gauged and is devoid of fractures.

It can be observed that there is a large flow of fluid from the formation into the borehole below the décollement. The influx rate increases to excess of +0.05 m 3 /s and is most prominent within the two asymmetric damage zones below the two strands of the fault core at a depth of 813 mbsf and 852 mbsf. This large fluid flow (Figure 6.9d) into the borehole accounts fully for the significant mud pressure anomaly observed beneath the décollement (Figure 6.9c).

Parametric study

In Figure 6.9, the fluid injected into the well was assumed to be seawater (ρ M W = 1028 kg/m 3 , µ = 1 mPa • s). This result is quite satisfactory but the mud density used is lower than the one indicated in the daily drilling report with values (ρ M W = 1350 kg/m 3 , µ = 51 mPa • s). Mixing The effect of slightly varying the density or viscosity properties of clean mud in the DHAP model is not easily differentiated from this (Figure 6.10e and f). However, this is already identified in Figure 6.10b and c when compared with the clean mud (Figure 6.10a). The parametric study shows that the model is in good agreement with empirical DHAP data only if the parameters (clean fluid density, viscosity) are close to the seawater data. Therefore, this disputes the mud properties provided by the daily drilling reports that earlier suggested that the mud used for drilling well C00024A is more denser and viscous.

Pore Pressure Prediction Results

Two independent methods were used to estimate the pore pressure along this borehole. The Eaton's method and based on the empirical relationship by [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF], [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]. The estimated pore pressure profile from the two Eaton's methods is dependent on the construction of a reasonable NCT and its relational variation with the trend d xc and ∆t.

All input parameters (Figure 6.11 a-d) were available to estimate the pore pressure using the dexponent method. The sonic travel time data was also avilable for the Eaton sonic travel time method. Trend deviations in d xc and ∆t relative to NCT are indications of abnormal pressure zones. Estimated overpressures are indicated as excess pore pressure (P * = P f -P hydro ) above hydrostatic pressures (Yaolin Shi and Chi-Yuen [START_REF] Shi | Generation of high pore pressures in accretionary prisms: inferences from the Barbados Subduction Complex[END_REF], and the lithostatic load is generally stated in the form of an overpressure ratio λ = P f -P seaf loor P litho -P seaf loor [START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF] and the modified excess pore pressure ratio is λ * = (P f -P hydro ) (P litho -P hydro ) [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]. The value λ * normalizes the excess pore pressure relative to the lithostatic pressure (λ * is 0 at the hydrostatic pressure and 1 at the lithostatic), making it easier to assess the importance of the simulated excess pore pressure. The interpretation of these values was provided for intervals where the pore pressure was above the hydrstatic pressure.

Pore Pressure d xc -exponent

The d xc line does not follow the NCT in Figure 6.12b in the depth range of 0-180mbsf (coincides with part of subunit 1a [Figure 6.12a]). This interval was not considered when constructing the NCT, because these facies are assumed to be characterised by unconsolidated sediments from accreted continental or fluid-rich subducting plate sediments still undergoing possible erosional sediment unloading. Therefore, it is considered a hydrostatically pressured interval. The line d xc follows the NCT line between 180-490mbsf (which comprises part of subunit 1a, 1b and 1c) because it was the primary lithological unit used to construct the NCT line. The d xc trend is consistent with increasing depth and vertical effective stress. With a pore pressure gradient value of 1.0 g/cm 3 (Figure 6.13c) and increasing pressure between 39.37 MPa to 47.8 MPa (Figure 6.12c), this depth range is also considered hydrostatically pressured, as illustrated on Figure 6.12c and f. Overall, the mud pressure is higher than the pore pressure and hydrostatic pressure between 0-490 mbsf (Figure 6.12 f). Therefore, the entire interval is considered normally pressured.

In Figure 6.12b the d xc begins to depart from the NCT to lower values at the depth of 490 mbsf. This depth coincides within subunit 1c (Figure 6.12 a) and marks the top of the geopressure zone. Therefore, the overpressured zone is located between 490 mbsf and the bottom of the borehole. The variation in pore pressure within this depth range is influenced by the changing value of d xc along the trend line. On Figure 6.12b, d xc gradually drops below 1, then gradually increases to a value of 1.06 at a depth of 786.4 mbsf, before decreasing to lower values (0.75) within the décollement interval. The d xc method cannot be applied rigorously below the second strand of the décollement fault core, since the NCT for the sandy lithology of the footwall is not characterized. But a further decrease in d xc shows the existence of higher pore pressure.

The pore pressure gradually rises and at the depth of 510.8 mbsf, a crossover between the pore pressure and the DHAP is observed (Figure 6.12 f). This point marks the onset of higher pore pressure values over the mud pressure and it rises gently to maximum value of 52.6 MPa (Figure 6.12f) with localized pore pressure gradient (Figure 6.13c) rising up to 1.05-1.6 g/cm 3 . This method shows that the excess pore pressure ranges P * ≈ 0.1 -4.79 MPa above the hydrostatic pressure and the lithostatic load (λ ≈ 0.54 -0.66, λ * ≈ 0.1 -0.62), with lower range values within the accreted sediments and maximum values below the décollement and the underthrusting sediments (P * ≈ 2.38 -4.79 MPa, λ ≈ 0.66 -0.8, λ * ≈ 0.28 -0.62). There is localized step in pressure (Figure 6.12c and f) when crossing the fault core of the décollement (813 mbsf and 852 mbsf). ) along the borehole with an observable deviation to lower values of the NCT line (black) at the top of subunit 1c (accreted wedge Facies). This particular depth marks the top of the geopressured interval. (c) The plot of the variable pressure gradients, the hydrostatic pressure gradient in blue coloured line, overburden or lithostatic pressure gradient in green coloured line, averaged pore pressure gradient in red line, raw pore pressure gradient in gray line. (d) The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within normal pressure zone. While within the overpressured zone the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole.

Pore pressure determined from sonic velocity

The sonic velocity follows the NCT (Figure 6.12d) between the depth range of 0 to 580 mbsf. It coincides with Unit 1 (accretionary trench wedge facies) and the upper part of Subunit 2a (upper part of Shikoku basin hemipelagic-pyroclastic facies) (Figure 6.12a). With a pore pressure gradient value of 1.0 g/cm 3 (Figure 6.14c) and increasing pressure between 39.37 MPa to 47.8 MPa (Figure 6.12e), this depth range is also considered hydrostatically pressured, as illustrated in Figure 6.12e). Overall, the mud pressure is above pore pressure (Figure 6.12f) and the hydrostatic pressure between 0-580 mbsf. Therefore, this is considered a normal pressured interval.

The line ∆t departs significantly from the NCT to a higher increasing slowness in this lithology at a depth of 580 mbsf (Figure 6.12e). The depth coincides with the upper part of the Shikoku basin facies (Figure 6.12a) and it marks the top of the geopressurized zone. Therefore, the overpressure zone is defined as the depth range between 580 mbsf and 871 mbsf (bottom of the borehole). The pore pressure gradually increases and at a depth of 611 mbsf, a crossover between the pore pressure and the DHAP is observed in Figure 6.12f. This point marks the onset of higher pore pressure values over the mud pressure and it rises gently to maximum value of 50.83 MPa (Figure 6.12f) with localized pore pressure gradient (Figure 6.14c) rising up to 1.06-1.4 g/cm 3 . This method shows that the excess pore pressure ranges P * ≈ 0.05-3.03 MPa above hydrostatic pressure and lithostatic load (λ ≈ 0.54-0.66, λ * ≈ 0.1-0.41), with the lowest range values within the accreted sediments and maximum values below the décollement and the underthrusting sediments (P * ≈ 2.57 -3.03 MPa, λ ≈ 0.63 -0.72, λ * ≈ 0.34 -0.41). There is localized step in pressure (Figure 6.12e and f) when crossing the fault core of the décollement (813 mbsf) into the first asymmetric damage zones of the footwall as observed using the d-exponent method (Figure 6.12e). The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within normal pressure zone. Within the overpressured zone, the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole. 6.2.4.3 Pore pressure determined from the workflow of [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF][START_REF] Simpson | Well-Bore Construction (Drilling and Completions[END_REF] To implement the workflow of [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF][START_REF] Simpson | Well-Bore Construction (Drilling and Completions[END_REF], the porosity was derived from inverting the Erickson-Jarrard equation (Equation 5.51). To avoid the assumption of the value of Vshale as done in [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]. The emperical relationships (Equations 5.55,5.56,5.58 and 5.57) were applied. To ensure the accuracy of the IGR calculation, individual log units (Figure 6.14a) along Hole C0024A were considered separately. The different equations (Equations 5.55,5.56,5.58 and 5.57) were considered to avoid an overestimation or underestimation of the shale volume for each rock interval (Figure 6.15). The results of IGR and V sh from the different equations were compared in Figure 6.16b. The results obtained from the implementation of Equation 5.55 and Equation 5.56 originate from the zero point, while the results obtained from Equations 5.58 and Equation 5.57 do not originate from zero. [START_REF] Clavier | Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques[END_REF] or [START_REF] Stieber | Pulsed neutron capture log evaluation -louisiana gulf coast. proceedings of fall meeting of the society of petroleum engineers of aime[END_REF] curves are more appropriate if the results are compared with the plots [START_REF] Stieber | Pulsed neutron capture log evaluation -louisiana gulf coast. proceedings of fall meeting of the society of petroleum engineers of aime[END_REF][START_REF] Larionov | Radiometry of boreholes[END_REF].

If the ages of the sediments in the Nankai accretionary prism (the sediments of Quaternary age) are considered, it could explain why the [START_REF] Larionov | Radiometry of boreholes[END_REF] curve is not fitting properly the data. [START_REF] Larionov | Radiometry of boreholes[END_REF] equation is more suitable for sediments of old or tertiary age. For the sake of this calculation, the results of Clavier are used in the Erickson Jarrard equation (Equation 5.51) to predict the porosity.

The predicted porosity is in the same range as the porosity predicted from Archie law from Fig. 6.15 The Python script for the estimation of the Vshale using the [START_REF] Clavier | Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques[END_REF][START_REF] Stieber | Pulsed neutron capture log evaluation -louisiana gulf coast. proceedings of fall meeting of the society of petroleum engineers of aime[END_REF], [START_REF] Larionov | Radiometry of boreholes[END_REF] formula resistivity log and from Moisture and Density (MAD) data acquired onboard during the expedition (Figure 6.16, 6.17a). MAD measure total porosity (connected and unconnected), including those included in potential zeolite deposited with volcanic ashes around 600 mbsf. Electrical resistivity is also sensitive to surface conductivity [START_REF] Waxman | Electrical conductivities in oil-bearing shaly sand[END_REF]Smits 1969, Doan et al. 2011). Hence, local discrepancies are expected. The formulation of Erickson-Jarrard does not account for waterbound resistivity and has been recently revised by Dutilleul et al. (submitted).

In Figure 6.17b, the CSL solution tend to predict large pore pressure all along the borehole, much higher than mud pressure. This contradicts the observation of outflow in the upper section suggested by the DHAP modeling (Figure 6.9c). More significantly, such high pore pressure at shallow depths would have impeded drilling and coring. Since borehole C0024B, C0024D, C0024E and C0024G could be drilled with good core recovery down to 320 mbsf, this is not probable.

Hence, the K 0 end member seems to be more representative of the in-situ conditions. Especially, the pore pressure prediction agrees with the Eaton methods (Figure 6.17c) derived from the two independent prediction using the Eaton method. This supports the pore pressure predicted with these methods is reasonable. Since the K 0 loading conditions are related to the sedimentary history of the sediments, it suggests that tectonic loading did not significantly alter the sediments at the toe of the accretionary prism. This explains why the Eaton equations provide satisfactory results despite its strong underlying assumptions.

Estimation of permeability

The methodology presented in this manuscript enables one to retrieve a continuous in situ profile along the borehole, contrary to the more classical straddle packer method [START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF] or the long-term observatory [START_REF] Kinoshita | Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring[END_REF]. MDT have been used to conduct measurements of stress magnitude, pore pressure, and permeability for Nankai region [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF], Saffer et al. 2013[START_REF] Thu | Operational review of the first wireline in situ stress test in scientific ocean drilling[END_REF]. The permeability range for Site C0024 (10 -8 to 10 -16 m 2 ) is 2-4 orders of magnitude higher than the permeability measurements from MDT-Hydraulic fracturing (1.5 -5.5 × 10 -15 m 2 ) and MDT-Drawdown (3 -9 × 10 -18 m 2 ) as reported by [START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF]. Figure 6.18c overlays the C0024 data with previous studies obtained either with direct pumping tests or by ex situ core measurements [START_REF] Bekins | Episodic and constant flow models for the origin of low-chloride waters in a modern accretionary complex[END_REF][START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF][START_REF] Spinelli | Effects of fluid circulation in subducting crust on Nankai margin seismogenic zone temperatures[END_REF]. The results from the Hvorlev method even though higher than previously reported values (Figure 6.18c), but has a trend with the MDT results. These results could be affected by the scale of measurement, lithologic or structural heterogeneity, and/or the technique of the analysis.

Sensitivity Index

Each of the parameter (Q, P f , DHAP and L) in the permeability Equation 5.71 is solved one-ata-time using Equation 5.73. Flow (Q):

γ Q k = ∂k ∂Q . Q k (6.1)
Differentiating Equation 5.71 with respect to Q:

∆k ∆Q =   η 1 2π L ln Re Rw DHAP -p f   (6.2)
Substituting Equation 6.2 into Equation 6.1:

γ Q k = 1 (6.3)
Pore Pressure (P f ):

γ p f k = ∂k ∂p f p f k (6.4)
Differentiating Equation 5.71 with respect to P f :

∆k ∆p f =   η Q 2π L ln Re Rw (DHAP -p f ) 2   (6.5)
Substituting Equation 6.5 into Equation 6.4:

γ p f k = p f DHAP -p f (6.6) DHAP: γ DHAP k = ∂k ∂DHAP . DHAP k (6.7)
Differentiating Equation 5.71 with respect to P f :

∆k ∆DHAP =   η Q 2π L ln Re Rw (DHAP -p f ) 2   (6.8)
Substituting Equation 6.8 into Equation 6.7:

γ DHAP k = -DHAP DHAP -p f (6.9)
Distance between DHAP sensor and drill bit (L):

γ L k = ∂k ∂L . L k (6.10)
Differentiating Equation 5.71 with respect to L:

∆k ∆L =   η 1 2π L 2 ln Re Rw DHAP -p f   (6.11)
Substituting Equation 6.11 into Equation 6.10:

γ L k = -1 (6.12)
The result estimated in Equation 6.3 interprets that when the fluid flow is increased by factor of 1 then the permeability also changes directly with the same factor of single unit. Hence, the fluid flow highly impacts on the permeability. The higher the permeability the higher the fluid flow volume and vice versa. The result estimated in Equation 6.12 depicts that when the length between the DHAP and drill bit (L) is decreased with a unit factor, the permeability increases. The permeability decreases when L is increased. At the other hand solving Equation 6.9 and Equation 6.6 shows a weird relationship exists between DHAP, Pore pressure and the permeability. The values observed were beyond -1 and +1 factor limit of an ideal sensitivity index. First, both pore pressures determined by the Eaton method converge (Figure 6.20j) and that derived from the empirical methods of [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF], [START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]. Within the resolution limit of the methods (about 2MPa, as seen from the scatter of the pressure determined from the raw data), they both overlay and highlight two features: (1) below 600 mbsf, the pore pressure departs from the hydrostatics, and (2) the pore pressure increases again when crossing the first strand of the décollement. Given that the sonic data was not acquired on the second strand, only the d xc prediction suggests step in pressure on the second fault strand.

Discussion

Secondly within the logging subunit 1a (unconsolidated sediments possibly still undergoing reactivation) and subunit 1b. This loss is consistent with the predictions by Eaton's method showing that the mud pressure is higher than the formation pore pressure (Figure 6.20 j). This is typical of a safe drilling procedure.

The loss of mud pressure to the formation becomes null around 462.8 mbsf. Consistently, at the same depth, the pore pressure increases and becomes equal to the mud pressure. The flow changes to the right (positive) side of the baseline (Figure 6.20i) when the pressure predictions of both Eaton methods converge to a value higher than the mud pressure, around 615 mbsf. This provides a self-consistent picture of the flow. When the mud pressure exceeds the pore pressure, the borehole becomes unstable (Figure 6.20d), as seen by the more infrequent peaks in the mud pressure time series during non-drilling periods, which are attributed to packoffs. This higher pore pressure in the hemipelagites also explains the difficulties met when coring Hole C0024F (Figure 4.1b), which could not exceed 731 mbsf [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]. Other geophysical proxies are consistent with an increase in pore pressure below 490 mbsf. The ratio V P /V S decreases from that depth (Figure 6.20k) suggesting also higher pore pressure. The borehole images (Figure 6.20b) also show a change in the breakout direction from that depth, consistent with a change in effective stress that could be related to a non-hydrostatic pore pressure. The large fluid flow predicted at the base of the borehole is consistent with the sharp increase in the real-time temperature of the mud at the base of the hole (Figure 6.20 l). This is consistent with hot fluids from the formation heating the cold-borehole mud injected from the surface. As the lithology changes below the second strand of the fault, the Eaton methods cannot be rigorously applied to the lower footwall. However, the large drop in d xc suggests a large increase in pressure.

6.3.2 Pore pressure increase in the accretionary prism

The Eaton method shows that the distributions of increased pore pressure in the Nankai subduction zone are not only restricted within the fault zones, but also pervasive at deeper depths a few hundred meters above the décollement within the accretionary prism. Both Eaton's methods converge to an excess of pore pressure in the hemipelagites, suggesting a departure from normal compaction. Either this anomaly existed prior to subduction or is related to the accretion process.

IODP Expedition 322 of the NanTroSEIZE project was dedicated to characterization of subduction inputs, by sampling the sedimentary column entering the accretionary prism at sites C0011 and C0012 (Figure 4.1c). From these samples, [START_REF] Hüpers | Origin of a zone of anomalously high porosity in the subduction inputs to Nankai Trough[END_REF] shows an anomalously high porosity zone in the subduction input. This anomaly was explained by the inclusion of volcanic ashes in the sediment, whose silica strengthened the skeleton and prevented further compaction.

Volcanic ashes can be identified as peaks in gamma ray logs [START_REF] Hüpers | Origin of a zone of anomalously high porosity in the subduction inputs to Nankai Trough[END_REF].

In C0024A, [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF] identify this anomalous high porosity zone from subduction inputs as a change in porosity at 550 mbsf from electrical logs and from the MAD (Moisture And Density) study from the cores of Hole COO24E and related to this zone. This high porosity could affect the sonic log and alter the pore pressure estimation using the Eaton method. This high porosity should also be associated with an increase in permeability, and therefore with an increase in flow from the borehole to the formation if there were no hydraulic anomalies. On the contrary, our analysis of the DHAP shows a decrease in flow and even inflow from the formation below 700 mbsf, which requires the pore pressure to be higher than the mud pressure. Furthermore, the high porosity zone is limited in the upper layer of the Shikoku hemipelagics, being 150m thick in the C0011 hole, while we show that the pore pressure tends to increase steadily with depth over the entire layer of Shikoku hemipelagic clay, even in the zones of low gamma-ray. In summary, the hydraulic anomaly cannot be discarded as an artifact caused by the porosity anomaly within the sedimentary column entering the subduction. Although the anomaly pre-existing in the input sediments affects the occurrence of the hydraulic anomaly, it cannot explain alone the high pressure in the hanging wall.

The accretion of the layers to the prism introduces additional compressional lateral stress onto these formations. Furthermore, the seismic cross section of Figure 4.1b shows that the slope of the prism evolves with time: a splay fault causes the overthrusting of the landward sediments onto the layers on which Hole C0024A is drilled, and the deposition of slope sediment on its footwall.

Our prediction of pore pressure provides an additional constraint for modeling and understanding these processes. According to the seismic cross section of Figure 5.13, Hole C0024A intersects faults at 171 mbsf, 281 mbsf and 441 mbsf. Since transient peaks in flow rate were not considered in our interpretation, local flow along these faults could not be identified. Crossing these faults does not introduce any large-scale change in pressure, contrary to the décollement. Given that these faults were not identified in the borehole image [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]) and Figure 6.20g), these faults can be considered minor, without significant hydraulic influence.

At site C0024, the predicted pore pressure at the toe of the accretionary prism (along the Kumano transect) is higher below the décollement than within the accretionary prism. This contrasts to the observation made along the Muroto transect (Flemings andSaffer 2018, Gamage and[START_REF] Screaton | Excess pore pressures within subducting sediments: Does the proportion of accreted versus subducted sediments matter?[END_REF] at sites 808 and 1174 where the predicted pore pressure is higher within the accreted sediments than below the décollement. This shows that different over-pressurization states exist in these two transects and could further show variability in the property of décollement at a regional scale in the Nankai subduction zone.

6.3.3 Hydraulic structure of the décollement zone at the Frontal thrust region

Our new compiled hydraulic information with a focus on the décollement compared with preexisting information, interpreted by [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]. At site C0024, the décollement zone is associated with a zone of anomalous fluid flow, with indications of fluid exchange from formation into the borehole (Figure 6.21h). The décollement is complex, with two strands at 813 mbsf and 851 mbsf. Each strand is asymmetric, with a fault core near the hanging wall and damage zones a few meters thick (6 -8m) concentrated in the footwall, as observed. However, the second asymmetric zone may be thicker than what is observed from the well logs, assuming that the C0024A well is drilled to much deeper depths. Although no core could be recovered from the décollement, the zone was investigated using a complete suite of geophysical logs (Figure 6.21).

The asymmetric damage zones are characterized as conductive zones, as seen in the electrical borehole imaging from deep resistivity (Figure 6 The new hydraulic information is shown within the blue frames of Figure 6.21. Each strand is associated with an increase in pore pressure (6.21i), but the increase in flow is more localized (Figure 6.21h), and related to large-scale fractures visible in the image logs (Figure 6.21b and f) at 813mbsf and 852mbsf. After this last fracture, the flow rate remains at large values. Therefore, the permeability of the damage zone is fracture-supported and not matrix-supported. Our findings are consistent with the hypothesis that impermeable décollement acts as a barrier to upward fluid convection (Gamage andScreaton 2006, Saffer andTobin 2011), which means that there is no hydrological connection between accreted sediments and underthrust below the décollement (that is, they are made up of different hydrogeological systems). The study here examines the toe of the accretionary prism at the Kumano transect; hence the hydrological status may be different in other locations like reported at the Muroto transect. For example, [START_REF] Zhang | Pore Pressure Regime and Fluid Flow Processes in the Shallow Nankai Trough Subduction Zone Based on Experimental and Modeling Results from IODP Site C0023[END_REF]) discovered that at site 1173 and 808, west of Nankai Trough, the accreted and underthrust sediments form a single hydrogeological system and that the décollement does not act as a fluid barrier. With the assumption made by [START_REF] Zhang | Pore Pressure Regime and Fluid Flow Processes in the Shallow Nankai Trough Subduction Zone Based on Experimental and Modeling Results from IODP Site C0023[END_REF] about the hydrological state of the décollement along the Muroto transect, it could further depict why the pressurization state (Flemings andSaffer 2018, Zhang et al. 2021) is different from our observations in the Kumano transect. Our findings are novel, as they are based entirely on data from LWD and MWD tools, which provide high-resolution in situ information on the characteristics of various lithologies along the length of the borehole.

Implication of high pore pressure on seismotectonics at the Trench

The location of the largest fault slips along subduction megathrust and possible occurrence of devastating tsunami is largely influenced by pore fluid pressure and its variation with depth [START_REF] Madden | The state of pore fluid pressure and 3-d megathrust earthquake dynamics[END_REF]. High fluid pressure maintains low effective stress on the fault zone [START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF]. In Hole C0024A, fluid flow and high pore pressure are localized on the fault zone, mainly below the décollement (Figure 6.21j). Site C0024 is about 3 km away from the trench [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]). This has two implications: 1. About the extension of the high pore pressure patch below the slope of the Nankai accretionary prism, given that Site C0024 provides additional boundary conditions 2. about the potential of tsunami generation in the Nankai subduction zone.

(Kitajima and Saffer 2012) provided pressure ratios within the low-velocity zone under the accretionary slope, but their prediction of pore pressure stopped 13 km away from the trench with lithostatic load (λ = 0.51-0.77) and supports (λ * = 0.45-0.91) of the overburden stress. However, recent observations show that VLFE extends quite shallow [START_REF] Takemura | Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes[END_REF], beyond the limits of [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF]. Also, [START_REF] Edgington | Migration of Shallow Slow Slip Events to Trench: Evidence from borehole observatories in the Nankai Trough[END_REF], [START_REF] Ariyoshi | Precise monitoring of pore pressure at boreholes around nankai trough toward early detecting crustal deformation[END_REF] showed that the SSE of March-May 2020 was shallow enough to induce significant change at the CORK observatory installed in Site C0006, which is located 2 km away from the trench [START_REF] Kinoshita | Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring[END_REF]. The high pore pressure seen at Site C0024 (λ = 0.54-0.8 and λ * = 0.28-0.62) suggests that the high pore pressure patch identified by [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] could extend almost up to the trench, encompassing the shallow locations of SSE [START_REF] Edgington | Migration of Shallow Slow Slip Events to Trench: Evidence from borehole observatories in the Nankai Trough[END_REF][START_REF] Ariyoshi | Precise monitoring of pore pressure at boreholes around nankai trough toward early detecting crustal deformation[END_REF]) and VLFE [START_REF] Takemura | Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes[END_REF]. Diffusion of pore pressure from low velocity zones is possible along the damage zone of the décollement in a up-dip direction [START_REF] Bourlange | Numerical model of fluid pressure solitary wave propagation along the décollement of an accretionary wedge: Application to the nankai wedge[END_REF]Henry 2007, Saffer andTobin 2011).

Fig. 6.22 Correlations of the published pore pressure ratio derived by [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] with the results of the pore pressure ratio results estimated at the frontal thrust of the accretionary prism.

Our results also have tsunamigenic potential at the trench. [START_REF] Madden | The state of pore fluid pressure and 3-d megathrust earthquake dynamics[END_REF] made a numerical simulation of the dynamic rupture along the Sumatra subduction zone, with different scenarios of pore pressure and pore pressure gradients. The pore pressure at site C0024 is about 62% of the lithostatic load at the trench, which is consistent with the third scenario of earthquake rupture proposed by [START_REF] Madden | The state of pore fluid pressure and 3-d megathrust earthquake dynamics[END_REF], in which a tsunami can occur. Therefore, any earthquake nucleating in the seismogenic zone might propagate in an up-dip direction, inducing large slip at the trench, since the stress change necessary to produce slip is reduced when effective normal stress is lower. The hydraulic state of the décollement found along the Kumano transect differs from the Muroto transect, 150 km to the west, with a slightly lower pore pressure ratio (λ = 0.71)

, but with a different distribution of pore pressure (Flemings andSaffer 2018, Zhang et al. 2021).

In the Muroto transect, it is the hanging wall that is the most overpressurized, whereas the footwall is not overpressurized. Therefore, the incoming sediments may be drier, inducing a reduced pore pressure downdip. [START_REF] Takemura | Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes[END_REF][START_REF] Takemura | Spatiotemporal variations of shallow very low frequency earthquake activity southeast off the kii peninsula, along the nankai trough, japan[END_REF] has shown that there are more SSE and VLFE occurrences along the Kumano transect than in the Muroto transect. This suggests that differences in hydraulic structures at the toe of the accretionary prism reflect different pore pressure distribution downdip, and different seismotectonic behaviors along the subduction transect.

CHAPTER 7

Frontal Thrust: Site C0006

7.1 Context 7.1.1 Expedition 314: Site C0006

The main objective of Expedition 314 (Figure 7.1) was to use a complete LWD/MWD suite to obtained information about the different lithologies, rock deformation, stress state, and physical properties of the hanging wall rocks and frontal thrust fault zone [START_REF] Kinoshita | Expedition 316 scientists[END_REF]). Site C0006 is also located at the frontal thrust of the Nankai accretionary prism near the trench axis, which is several kilometers from Site C0024 [START_REF] Kinoshita | Expedition 316 scientists[END_REF], Tobin et al. 2009). The well was drilled to a total depth of 885 mbsf [START_REF] Kinoshita | Expedition 316 scientists[END_REF]) crossing possibly through the main frontal thrust at the depth of about 711 mbsf. Unfortunately, core samples have not been acquired for this depth due to bad drilling conditions. Other candidate fault zones were encountered along the borehole length.

Hydraulic profiles

The methodology for fluid flow modelling was adopted for each borehole based on the availability of the required input data. Modeling the fluid flow in this borehole was achieved by following the step by step guide in Figure 6.8. All data processing steps for the mudline depth correction (Table 7.1), depth corrections, and DHAP first time drilling (Figure 7.5) as discussed in the data QC/Processing subsection of the methodology section were implemented in detail. Key input with y = 0.0887x + 0.675, where y is the correction factor and x is the borehole diameter.

• A simple k-nearest neighbors (KNN) supervised machine learning algorithm was used to effectively get the final caliper log data (Figure 7.10b). The KNN algorithm is a simple and straightforward supervised machine learning algorithm that may be applied to both classification and regression tasks. The models were trained with inputs from the gammaray log and the correction factor derived from the linear function.

• Three scenarios were considered for each of the models with test data, training data, and predicted model as shown in Figure 7.8b. The third scenario has the highest performance with R 2 = 0.8 while the other two R 2 = 0.62 and 0.57. The predicted value correction factor is the value of (y) in the linear equation above.

• As a final step, the value of y obtained in the previous step was substituted in the linear between 0-500 mbsf, the observed fluid flow within this interval is attributed to the poorly coarse data fit of the derived caliper log at the top of the well. It will be unreasonable to assume that the large flow observed in the shallow zone (Figure 7.11) is accurate. If we assume such value exists, then there must be a large pore pressure driving the fluid flow at the top of the borehole, but as we'll see in the next section (pore pressure modeling section), that's not the case.

Our result is more reasonable from a depth below 500 mbsf to the bottom of the well. Between 500 -657 mbsf there is minimal or no mud loss, because the plot aligns on the zero line, indicating that there is limited fluid flow exchange between the well and the formation. The

Pore Pressure estimates

The Eaton methods were used to estimate the pore pressure for this hole. All input parameters (Figure 7.13 a-c) were available to estimate the pore pressure using the d-exponent method, while the sonic velocity was not acquired for Hole C0006A due to the loss of MWD communication from 274 to 885.5 mbsf [START_REF] Kinoshita | Expedition 316 scientists[END_REF]). Therefore, V p was derived using the Erickson-Jarrard relationship (Equation 5.51). The pore pressure calculations were executed using the Eaton pore pressure workflow discussed in the methodology section of the manuscript.

Estimated overpressures are indicated as excess pore pressure (P * = P f -P hydro ) above hydrostatic pressures (Yaolin Shi and Chi-Yuen Wang 1988), and the lithostatic load is generally stated in the form of an overpressure ratio λ = P f -P seaf loor P litho -P seaf loor [START_REF] Rubey | Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis[END_REF] and the modified excess pore pressure ratio is λ * = (P f -P hydro ) (P litho -P hydro ) [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF]. The value λ * normalizes the excess pore pressure relative to the lithostatic pressure (λ * is 0 at the hydrostatic pore pressure and 1 at the lithostatic), making it easier to assess the importance of the simulated excess pore pressure. The pore pressure gradually rises and at the depth of 711 mbsf, a crossover between the pore pressure and the DHAP is observed (Figure 7.14c). This point marks the onset of higher pore pressure values over the mud pressure, and it rises gently to a maximum value of 52.6 MPa

Pore pressure determined from sonic velocity

As mentioned earlier, V p was derived from the Erickson-Jarrard relationship (Equation 5.51). To ensure the accuracy of the IGR calculation, individual logging units (Figure 7.14a) along the hole C0006A were considered separately. The shale volume was calculated using Equations 5.55, 5.56, 5.58 and 5.57. This was done for comparison and to avoid the possibility of overestimation or underestimation of the shale volume for each rock interval. The results of IGR and V sh from the different equations were compared in Figure 7.15 a. The results of [START_REF] Stieber | Pulsed neutron capture log evaluation -louisiana gulf coast. proceedings of fall meeting of the society of petroleum engineers of aime[END_REF][START_REF] Clavier | Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques[END_REF] originate at zero, whereas the results of [START_REF] Larionov | Radiometry of boreholes[END_REF] for old rocks or tertiary rocks originate slightly above zero (Figure 7.15a). The different values of IGR were substituted in Equation 5.51 to obtain the log information V p (Figure 7.15 a). On average, from Figure 7.15 b the results overlap each other reasonably well without a significant difference in the spread of the data points. To be more pessimistic, the value of V sh from [START_REF] Clavier | Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques[END_REF] was used as input in Erickson-Jarrard relationship (Equation 5.51) to estimate V p . This inturn is used to estimate the pore pressure using Eaton sonic travel time.

The ∆t follows the NCT follows the NCT line between (Figure 7.16b) between the depth A slight increase in pore pressure above hydrostatic pressure can be observed below within unit 3a, and within unit 3b in Figure 7.16c). In general, the pressure of the mud is above the pore pressure (Figure 7.16 c) and the hydrostatic pressure between 0-711 mbsf. Therefore, this is considered a normally pressurized interval.

The pore pressure gradually rises and at the depth of 711 mbsf, a crossover between the pore pressure and the DHAP is observed (Figure 7.16d). This point marks the onset of higher pore pressure values over the mud pressure, and it rises gently to maximum value of 49 MPa (Figure 7.16c). Below the main frontal thrust fault the excess pore pressure ranges P * ≈ 0.5 -1.85 MPa above the hydrostatic pressure and the lithostatic load (λ ≈ 0.56 -0.67, λ * ≈ 0.1 -0.28). The pore pressure within the accretionary prism is slightly hydrostatic, but higher below the main frontal thrust fault within the underthrusted sediments. Localized step in pressure (Figure 7.16c) when crossing the fault core of the main frontal thrust fault is observed.

Estimation of permeability

On Figure 7.17a-ii the logarithm of permeability decays with depth along the borehole. Generally, the order of magnitude of permeability for the 4 main lithological units varies between 10 -8 to 10 -14 m 2 (Figure 7.17Ia, Table 7.2). Meanwhile, the porosity ranges from 46 to 35 %. Figure 7.17 b shows that permeability tends to increase with porosity. The results of the permeability still high when compared with other results in the study area (Figure 7.17c). Additionally, the results from the sensitivity analysis shows that low permeability values beyond 10 -14 m 2 (Table 7.3) cannot be achieved. Therefore, the observed results of high pore pressure obtained from the d xc method did not accurately represent the pore pressure state along the hole. At the other hand the results Eaton ∆t method might be more representative of the pore pressure magnitude below the main frontal thrust fault.

On the seismic image the borehole cross-cuts 9 faults at interval depths within the accretionary prism (Figure 7.18a) indicating that the site has undergone or is still undergoing compressive stress conditions. [START_REF] Kitajima | Consolidation state of incoming sediments to the nankai trough subduction zone: Implications for sediment deformation and properties[END_REF], [START_REF] Lin | Distribution of stress state in the nankai subduction zone, southwest japan and a comparison with japan trench[END_REF] have shown that core samples have experienced thrust fault or strike-slip fault stress with principal stress. [START_REF] Byrne | Anelastic strain recovery reveals extension across sw japan subduction zone[END_REF][START_REF] Wu | Stress field observation and modeling from the nantroseize scientific drillings in the nankai trough system, sw japan[END_REF], [START_REF] Lin | Present-day principal horizontal stress orientations in the kumano forearc basin of the southwest japan subduction zone determined from iodp nantroseize drilling site c0009[END_REF] showed the stress change along the borehole with evidences from the borehole breakouts. These borehole breakouts have been identified all through the borehole but weakly developed with 060 • [START_REF] Kinoshita | Expedition 316 scientists[END_REF]. Therefore, the Eaton method might not be the best method to be used in this location, since it predicts the pore pressure condition in a normal sediment compaction scenario. The results from the Eaton method needs to be carefully compared with other methods as discussed in the state of art, since the drill site is a strongly deformed fold and thrust belt [START_REF] Screaton | Interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula, japan: Results from iodp expedition 316 sites c0006 and c0007[END_REF], Moore et al. 2009). The results of the pore pressure are compared with other geophysical proxies to check for consistency (Figure 7.18).

Below the main frontal thrust fault the V P decreases to low values ( Megasplay is one of the principal coseismic faults that may have contributed to the occurrence of historical severe earthquakes and tsunamis [START_REF] Tobin | The IODP Nankai trough seismogenic zone experiment[END_REF]. Their mechanical stability is influenced by elevated pore pressure [START_REF] Scholtz | The mechanics of earthquake and faulting[END_REF], hence reducing the effective normal stress so that the fault could easily slip.

Context

8.1.1 Expedition 314: Site C0004 Site C0004 is located at the seaward margin of the Kumano Basin uplift, in which the megasplay fault system branches and approaches the surface [START_REF] Kinoshita | Expedition 316 scientists[END_REF]). The main objective of Site C0004 (Figure 8.1) is to drill through the shallow portion of the megasplay fault system, thrust sheets and the thin overlying slope sediment cover sequence [START_REF] Kinoshita | Expedition 316 scientists[END_REF]). This also attempts to drill and log across the fault zone, the associated hanging wall and footwall structures. The well was drilled to a total depth of around 390 mbsf. The lithologies in the site were divided into three units (Figure 8.2 based on visual inspection of the gamma-ray (GR

[gAPI]) log, sonic velocity, resistivity (ohm.m), and caliper [inches] [START_REF] Kinoshita | Expedition 316 scientists[END_REF]). (a)

Unit I are identified as slope sediments between 0-67.9 mbsf. (b) Unit II lies between the depth of 67.9-323.8 mbsf. Furthermore, it is subdivided into Subunit IIA (67.9-96. The indicators used are negative-polarity, and high-amplitude fault zone reflections (Cochrane et al. 1994a,b, Park et al. 2002, Tsuru et al. 2002, Bangs et al. 2004, Park et al. 2010, Bangs et al. 2009). These observations are in agreement with the interpretation fluid flow from the formation into the borehole (Figure 8.9h).

When a seismogenic rupture propagates the trough axis, the slope of the seafloor with high steepness tends to slide towards the sea which can potentially cause huge tsunamis, as it happened in the 2011 Tohoku earthquake [START_REF] Chester | Structure and composition of the plate-boundary slip zone for the 2011 tohoku-oki earthquake[END_REF]. The persistently high pore pressure underneath the mega-splay fault suggests a still-active alimentation of high-pore pressure fluid from the base of the megasplay fault, in the low-velocity zone of Figure 8.10. [START_REF] Orange | The effects of fluid escape on accretionary wedges 2. seepage force, slope failure, headless submarine canyons, and vents[END_REF] have shown that 80 -95 % of ejected fluids escape through high-permeability zones in accretionary prisms. So, It is not suprising the magnitude of fluid flow and elevated pore pressure observed along this borehole and within the footwall most especially. kilometers from each other [START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]).

• The décollement is impermeable at the site C0024, hence the fault core is acting as a barrier to upward fluid convection (Gamage andScreaton 2006, Saffer andTobin 2011), which means that there is no hydrological connection between accreted sediments and underthrust below the décollement. The fault structure support localised step in pore pressure and large fluid flow within the permeable and porous damage zones. This is contrast with the fault structure at the site C0006, where the fault is permeable, thereby, supporting vertical flow of fluids from the understhrusted sediments into the accretionary prism. Hence, the main frontal thrust and the thrust faults within the accretionary prism serve as conduits or paths for fluid flow and high pore pressure dissipation at site C0006.

• [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] provided pressure ratios within the low-velocity zone under the accretionary slope (Figure 9.2), but their prediction of pore pressure stopped 13 km away from the trench with lithostatic load (λ = 0.51-0.77) and supports (λ * = 0.45-0.91) of the overburden stress. In this manuscript, we have shown evidence of high pore pressure (λ = 0.9 -0.96 and λ * = 0.62) at site C0024 (Figure 9.1, 9.2) suggesting that the high pore pressure patch identified by [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] could extend almost up to the trench at site C0024 location. But, at site C0006 there is no evidence of high pore pressure (λ = 0.87 -0.91 and λ * = 0.28) and large fluid flow (Figure 9.1. This means the high pore pressure patch identified by [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] extended to the trench to the cite C0024 but not up to location of hole C0006A (Figure 9.2, 9.3a). This implies that when there is any seismic activity nucleating in the seismogenic zone or low volocity zones, the released stress might propagate in an up-dip direction along the damage zone of the décollement to the trench at site C0024, but at site C0006 it gets further distributed partly along the main frontal thrust and partly into the accretionary prism. At a regional scale the hydraulic state of the décollement found along the Kumano transect differs from the Muroto transect, 150 km to the west (Figure 9.3b), with a slightly lower pore pressure ratio (λ = 0.71) , but with a different distribution of pore pressure (Flemings andSaffer 2018, Zhang et al. 2021). In the Muroto transect, it is the hanging wall that is the most overpressurized, whereas the footwall is not overpressurized. Therefore, the incoming sediments may be drier, inducing a reduced pore pressure downdip. [START_REF] Araki | Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust[END_REF], [START_REF] Takemura | Structural Characteristics of the Nankai Trough Shallow Plate Boundary Inferred From Shallow Very Low Frequency Earthquakes[END_REF][START_REF] Takemura | Spatiotemporal variations of shallow very low frequency earthquake activity southeast off the kii peninsula, along the nankai trough, japan[END_REF] has shown that there are more SSE and VLFE occurrences along the Kumano transect than in the Muroto transect (Figure 9.3b). This suggests that differences in hydraulic structures at the toe of the accretionary prism reflect different pore pressure distribution downdip, and different seismotectonic behaviors along the subduction transect. 9.2 A fluid-rich and complex megasplay fault At site C004B the large fluid flow is also predominant below the fault within the footwall. The high pore pressure at the megasplay fault implies that localized fluid flow through permeable faults is a significant component of the fluid budget, transporting solutes and heat over long distances [START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF]. The megasplay fault at hole C0004B is a basal detachment fault which extends to the seafloor ( Figure 8.10). Hence when a seismogenic rupture propagates the trough When the fluid flow and pore pressure along the megasplay fault is compared with that along the décollement at the toe of the accretionary prim (Figure 9.1). The megasplay region host the largest fluid flow and by extension the high pore pressure along the fault zone. On the basis of their tsunamigenic potential, the megapslay region tends to be more prone and destructive than along the décollement at the toe of the accretionary prism. In the case of coseismic slip, the rupture propagates seaward, leading to large tsunamis in the megasplay axis than in the toe of the accrteionary prism [START_REF] Park | A low-velocity zone with weak reflectivity along the Nankai subduction zone[END_REF], Tsuji et al. 2014).

-AN HETEREGENEOUS D ÉCOLLEMENT NEAR THE TOE OF THE BOREHOLE

Part E CONCLUSION CHAPTER 10

Conclusion and Future Work

Conclusion

In this manuscript, a methodology was developed to characterize hydraulic profiles along several boreholes drilled across major tectonic structures of the Nankai subduction zone.

• It provides consistent results suggesting that overpressure develops in the shallow part (at the megasplay fault region and fronntal thrust region) of the accretionary prism, which enhances the tsunamigenic risk.

• Holes C0024A and C0006A show contrasting hydraulic behavior, highlighting the sharp heterogeneity along the plate boundary interface.

• Fluid flow could quantify the extent of the damage zone of the décollement and the megasplay fault. Despite these promising results, some quantification is still required to recover a proper order of magnitude for permeability, maybe by leveraging the strong assumptions made on the flowing intervals of the boreholes.

The methodology we develop could be replicated for any borehole drilling in other contexts, as long as the mud pressure at the drillbit is recorded. For instance, several sites of the Hikurangi subduction zone, offshore New Zealand, were drilled with the LWD technology within the framework of the IODP consortium, and mud pressure data are available to characterize further the hydrogeology of this area. The field of application could extend to characterize oil and gas fields, as well as geothermal reservoirs, for which knowing hydraulic properties is critical.

Future Work

There are key outstanding questions yet to be resolved around understanding the the hydrogeology of the subduction zone. (a) Understanding the spatial variability of fault hydraulic property and provide a better estimate of the permeability at regional scale. Most studies are done along the borehole, apart from numerical models that tend to provide the hydrogeological properties at a regional scale. At the same time it ignores the complexities of the building of the accretionary prism. For example the highly complex and heteregenous plate boundary and megasplay faults.

Hence, I will like to combine different geological dataset from micro to macro scale coupled with machine learning algorithms to provide more insights to the hydrogeology and the mechanical 10.2 -FUTURE WORK properties of rocks in the subduction zone. I will also extend the work done in the PhD to model the fluid flow by providing different empirical relationship that accounts for fluid flow at the fault and above the fault. This is important because the faults could act as a hydraulic anaomalous interval thereby supplying a fluid into the borehole.

  Nankai est l'un des sites d'étude les plus propices : outre le risque sismique que représentent les mégaséismes (M w > 8) pour les côtes japonaises densément peuplées, c'est également le lieu de glissements lents. C'est ce qui a motivé le projet NanTroSEIZE, un projet phare du programme IODP (International Ocean Discovery Program), concrétisé par treize expéditions de 2004 à 2019. Une compréhension détaillée du régime hydraulique est nécessaire pour comprendre la diversité de comportement à Nankai. Des preuves indirectes suggèrent que cette zone de subduction est particulière riche en fluide. Pour affiner ces conclusions, nous intégrons de manière innovante les données de forage et de diagraphie acquises dans le cadre du projet NanTroSEIZE, dans la région frontale (sites C0024 et C0006) et dans la région de la faille "mégasplay" (sites C0004). Nous obtenons un profil des propriétés hydrauliques à haute résolution le long des forages NanTro-SEIZE, pour caractériser finement l'architecture des failles dans la zone de subduction de Nankai. Ceci aidera à estimer : (a) des profils de pression interstitielle à travers le prisme d'accrétion (b) des profils hydrauliques à haute résolution pour identifier la zone endommagée autour de la faille de décollement, et vérifier si elle est un chemin de drainage significatif. (c) l'évolution de la perméabilité. Ceci permet d'inférer l'origine des fluides (diffusion le long du décollement ou émission locale de fluide suite à la compaction de la pile sédimentaire), et d'extrapoler les données près de la fosse vers des zones plus profondes sismiquement actives. La ré-analyse de la pression de boue permet d'estimer l'afflux de fluides de la formation vers le forage. Pour vérifier ses prédictions, 2 méthodes ont été utilisées pour prédire la pression de pore : (a) des méthodes de type Eaton basées sur des données de foration ou des données sismiques (b) des relations empiriques entre pression de pore, porosité et vitesse des ondes P, obtenues à partir d'expériences de consolidation. On peut calculer des perméabilités en combinant estimations de flux et de pression. Sur le Site C0024, un afflux de fluides (> 0.05m 3 /s) est localisé dans les zones endommagées du décollement, à 813 mbsf. Elle est associée à une surpression importante (P * ≈2.38 -4.79M P a) sous la frontière de plaque. Le décollement agit comme une barrière hydraulique verticale, mais comme un drain longitudinal qui diffuse des surpressions depuis le site où se produisent les séismes et les glissements lents Au contraire, au Site C0006, les afflux de fluide restent faibles, et localisés juste sous le chevauchement frontal, à 711 mbsf. Il n'y a pas i d'augmentation apprente de la pression interstitielle à l'intérieur du prisme d'accrétion, peut-être car une faille entre C0024 et C0006 draine les fluides surpressurisés. Dans la région de la faille mégasplay, Sur le Site C0004B, un important écoulement de fluide (0.07m 3 /s) et une pression interstitielle élevée sont également observés dans la zone de faille, vers 310 m de profondeur. L'important débit de fluide témoigne d'une pression interstitielle élevée, facilitant ainsi la propagation du glissement cosismique vers le plancher océanique. La région de mégasplay est alors plus propice que la région du pied du prisme pour générer des séismes tsunamigéniques. Mega-Splay Fault: Site C0004 8.1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 Expedition 314: Site C0004 . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Hydraulic profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Fluid flow from mud pressure . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Self-consistency in the prediction of hydraulic quantities . . . . . . . . . . 8.3.2 Megasplay fault controls the hydrology of hole C0004B . . . . . . . . . . heteregeneous décollement near the toe of the borehole . . . . . . . . . . . . . 9.2 A fluid-rich and complex megasplay fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii LIST OF FIGURES 2.1 A typical diagram illustrating the Subduction zone with all resulting geological setting and processes. The figure shows the megathrust faults, coseismic slip and afterslip zones (Weiss et al. 2019). . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The figure shows distribution of active subduction zones around the world with different magnitude of historic earthquakes between 1900 -June, 2012. MWmagnitude of earthquakes, Numbers signify the MW values (Schellart and Rawlinson 2013). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Schematic diagram of seismogenic zone and aseismic zones (a) Seismogenic zone: an area prone strong tremors and earthquakes. ; (b) Subduction of the oceanic crust beneath continents, characterized by the presence of island arcs, and the assumption that the serpentinized forearc mantle forms the main downdip limit of the seismogenic zone (Hyndman 2007). . . . . . . . . . . . . . . . . . . 2.4 (a)Schematic cross-section of the seismogenic zone, and the aseismic zone with descriptive extends of where large earthquakes and SSEs, LFEs can initiate. (b) General perspective of the seismogenic zone and aseismic zones. The seismogenic zone can be identified by the following: mechanically, by large regions or asperities that, when they fail, result in a frictionally unstable or velocityweakening situation manner. The aseismic area updip and downdip of the seismogenic zone or within the frictionally stable or velocity-strenghtening manner (Lay 2016). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 (a) Schematic diagram of updip and downdip aseismic zone (a) Subduction thrust faults updip aseismic zone towards the trench. which host large magnitude earthquakes. (b) Subduction thrust faults downdip aseismic zone. (c) As the updip

  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii LIST OF FIGURES 2.7 Processes of fault slip and hydrology diagram. High pore pressure, represented by blue shading, can weaken fractures and sediments. The arrows depict the flow of fluids and are qualitatively scaled to indicate flow rates. Sediments from an underthrust that have been drained could mix with the fault's movement. Throughout the igneous ocean crust, fluid flow has the potential to advect heat and/or solutes (not shown). Around the updip and downdip limits of interseismic locking and coseismic slip, VLFE, ETS occur, and excess pore pressure may cause low effective stress. Concentration of flow and pressure translation in fractured fault zones and permeable sands are shown in the insets (Saffer and Tobin 2011)). . . . . . . 3.1 (a) Sketch of Oedometer (Wang and Abriak 2015) . . . . . . . . . . . . . . . . . 3.2 (a) Constant rate of strain apparatus (b) an enlarged diagram of the consolidometer (ASTM 2012) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Schematic illustration of a material wedge nearing Coulomb failure as a result of horizontal compression (Modified fromDavis et al. (1983)). . . . . . . . . . . . . 3.4 Geometry of Coulomb fractures rising from the basal décollement and Mohr diagram illustrating the basal state of (Modified fromDahlen et al. (1984)) . . . . . . 3.5 (a) Numerical model domain, boundary conditions, and grid. The grid has 30rows and 1499 columns, and includes 44,970 elemments (46,500 nodes). The prism has thirteen rows, the underthrust has eleven, the décollement has six closely spaced elements with a higher permeability. Also tightly spaced are the three parts directly above and below the décollement. The boundary conditions are also adopted (Modified fromSaffer andBekins (2006)). . . . . . . . . . . . . . . 3.6 Plot of laboratory-derived permeability versus porosity for a variety of natural argillaceous media. Permeability is shown along the lower horizontal scale: the corresponding hydraulic conductivity to water at room temperature is shown along the upper horizontal scale.[START_REF] Neuzil | How permeable are clays and shales?[END_REF] . . . . . . . . . . . . . . . . . . . . . . 3.7 Constant-flow test system.[START_REF] Ediments | of N Ankai a Ccretionary P Rism[END_REF]. . . . . . . . . . . . . . . . . . 3.8 a) Excess pore pressure (P*) and (b) effective stress (sz0) at the décollement as a function of distance from the trench for the best fit model runs. The shaded area shows the range of shear strength (t) at the décollement for all three cases assuming mb = 0.3-0.4[START_REF] Skarbek | Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering[END_REF] . . . . . . . . . . . . . . . . . 3.9 Simulated porosity and pore pressure ratio (λ * ) results profile for Sites 1173, 1174 and 808[START_REF] Gamage | Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one-dimensional modeling[END_REF] . . . . . . . . . . . . . . . . . . . . . . . 3.10 (a) Consolidation sample test example (b) The arrows show the direction of the stress applied during the experiment (Saffer 2003) (b) This is a schematic depicting the geometry and boundary conditions of the model (Screaton 2006). . . . . . 3.11 Engineering schematic of (a) original CORK for monitoring and sample of single hole intervals and (b) CORK-II design used for multiple interval sampling and monitoring (Modified fromKastner et al. (2006)) . . . . . . . . . . . . . . . . . 3.12 Schematic diagram of the (a) SmartPlug device and the bridge plug (Modified fromHammerschmidt, Davis and Kopf (2013a), Toczko et al. (2012)) (b) Installed CORK system inside the borehole, at specific interval of interest (modfied Kopf et al. (2011), Hammerschmidt, Davis and Kopf (2013a)) . . . . . . . . . . . . . 3.13 Schematic diagram of the (a) GeniusPlug/Smartplug device (b) Installed Genius-Plug CORK system inside the Hole C0010A, at specific interval of interest (Modified fromSaffer et al. (2017)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Installed CORK device in wells along the Muruto (Site 1173) at the interest depths most especially at the fault region. (Modified fromMikada et al. (2002)) . . . . . 3.15 Installed CORK device in boreholes along the Kumano transects at different interest depths most especially at the fault region (a) Hole C0002G (b) seimic structural section with well location at the mega-splay region (c) Hole C0010 at the left hand is the Smartplug configuration and the picture to the right is the GeniuPlug configuration (Modified fromKopf et al. (2011) . . . . . . . . . . . . . . . . . . 3.16 Processed pressure data recorded by the installed CORK device in wells along the Kumano transects (C0010, C0006, C0002). From Dutilleul and Kinoshita (2022) 3.17 Example of tidal and oceanographic signals in the observed pressures. (ai) Observed time series of pressures from 1 January to 1 February 2012. The blue and red lines show formation and reference pressures, respectively. (aii) sample of a 2-day time window of loading efficiency with both diurnal and semidiurnal components. Loading efficiency is indicated by the slope of the relationship between formation and reference pressures. (b)Time series of pressure and (c) Time series of loading efficiency. (Kinoshita et al. 2018). . . . . . . . . . . . . . . . . . . . 3.18 (a) Semblance contours (b) Interval velocity derived from stacked velocity (c) CMP plot showing the coherent reflections (Cochrane et al. 1994b). . . . . . . . x 3.19 (a) Seismic reflection image in three dimensions (3D), with P-wave velocity (in color), derived from prestack depth migration (3D PSDM), and interpreted as a low-velocity zone (LVZ). The outer wedge's LVZ (unit B) is denoted by a thick, light blue region with dots. There seems to be no clear evidence that the LVZ extends into the inner wedge Park et al. (2010) . . . . . . . . . . . . . . . . . . . 3.20 3-D seismic volume showing high-amplitude, reversed-polarity seismic reflection from deep splay fault (red areas). Black lines are faults, accretionary wedge thrusts are the Blue lines Bangs et al. (2009) . . . . . . . . . . . . . . . . . . . . 3.21 3-D seismic volume showing high-amplitude, reversed-polarity seismic reflection from deep splay fault (red areas). Black lines are faults, accretionary wedge thrusts are the Blue lines Bangs et al. (2009) . . . . . . . . . . . . . . . . . . . . 3.22 (a) PSDM 2-D reflection line 215 (b) Line 215's P-wave velocity model with variable low seismic velocity relative to overlying wedge. V.E.-vertical exaggeration. c) On track one: Porosity is the blue curve and the red curve is the PSDM interval velocity. Middle track: Computed pore pressure is the red curve, hydrostatic pressure is the blue curve, and total overburden stress is the green curve. Sediments without excess fluid pressure would plot on hydrostatic line. Bottom track: Vertical effective stress diverges from normal consolidation with trench distance. With = 0.4 and no cohesion, shear stress is defined. Upper and lower curve boundaries are propagated errors. . . . . . . . . . . . . . . . . . . . 3.23 (a) Excess pore pressure above hydrostatic pressure (b) Results of pore pressure ratios (c) Normalized pore pressure ratio at the mega-splay fault hanging wall. (d) Mega-splayfault footwall normalized pore pressure ratio Tsuji et al. (2014). . . . 3.24 (a) P * in the LVZ as a function of distance from the trench, estimated using Vp derived from 3-D seismic data. The minimum and maximum values within the LVZ at each distance from the trench are represented by the vertical bars. (b) Estimated in situ stress levels (S H ¿ S h ¿S v ) and pore pressure (Pp) at Site C0002 for the optimal scenario. SH (red) and Pp (blue) are estimated based on a K0 loading assumption in the Kumano Basin and a CSL assumption in the accretionary prism. (c) Distribution of lambda * in the LVZ under the condition that S H ¿ S h ¿ S v . The beach balls symbolize moment tensor VLFE. (Modified from(Kitajima and Saffer 2012, Kitajima et al. 2017)) . . . . . . . . . . . . . . . 3.25 (a and b) Illustration of MDT dual packer tool string used for Expedition 319 (Modified fromAraki et al. (2009), Thu et al. (2012). . . . . . . . . . . . . . . . xi 3.26 (a) Cross-plot of pore fluid pressure and pumping rate for the MDT -drawdown test at 1539.7 mbsf in Site C0009. (b) Crossplot of pore fluid pressure and pumping rate (right axis) for the MDT -hydraulic fracture test at 1532.7 mbsf. (c) Permeability Vs porosity for derived laboratory test from Costa Rica, Peru, Barbados and Nankai accretionary prisms, MDT-drawdown and MDT-hydraulic fracturing. (Modified fromBoutt et al. (2012) . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 (a) Nankai Map: Index map of the Nankai Trough with the PSP subducting under the Eurasian plate. KBEFZ: Kumano Basin Edge Fault Zone (Tsuji et al. 2015a). 4.2 (a) Kumano transect line indicated as the IODKM-M line on Figure 4.1 showing drill sites of the NanTroSEIZE with few representative faults. Dashed lines = less certain fault locations. Morphotectonic zones are shown between the two sections. KBEFZ = Kumano Basin edge fault zone (modified Park et al. (2008)). 4.3 (a) Regional map of the Nankai subduction zone. Filled blue circles and open light blue circles and are of SVLFEs epicentral locations Asano et al. (2008), The records of SVLFEs offshore are from 2015 and the fault areas hosting SSEs from 2011-2016 (circled in purple) (Araki et al. 2017), deep VLFEs and SSEs are represented with gray rectangle and dots. Modified after Takemura et al. (2019). . 4.4 Seismic reflection image in three dimensions (3D), with P-wave velocity (in color), derived from prestack depth migration (3D PSDM), and interpreted as a lowvelocity zone (LVZ). The outer wedge's LVZ (unit B) is denoted by a thick, light blue region with dots. There seems to be no clear evidence that the LVZ extends into the inner wedge (Park et al. 2010). . . . . . . . . . . . . . . . . . . . . . . . 4.5 (a) Seismic section without interpretation, processed with pre-stack depth migration. [B] Interpreted 3D seismic section in Figure 4.5a: faults indicated as dashed and solid lines. KBEFZ = Kumano Basin edge fault zone; PTZ = Protothrust zone; BSR=Bottom simulating reflection; VE = Vertical exaggeration (modified from Tobin et al. (2020)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 A typical drilling rig system (Bak 2014) . . . . . . . . . . . . . . . . . . . . . . 5.2 (a) Schematic of the drilling mud circulation system (Bourgoyne et al. 1986) . . xii 5.3 (a) The two types of drilling used in the NanTroSEIZE project, tools and DHAP data: (a) The different drilling procedure employed for the riser and riserless drilling types. In the riser: drill mud and cuttings are returned back to the ship platform through a special riser pipe that is linked to a seafloor blowout prevention (highlighted by a red box). Riserles: rock cuttings are pumped out of the borehole with seawater onto the seafloor. The highlighted red box in the riserless drilling shows the Bottom Hole Assembly (BHA) (b) The BHA system used to drill Hole C0024A. Main sensors used for the Logging while drilling (LWD) measurements. The red circle shows the DHAP sensor used to record changes in mud pressure during drilling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Example of python implementation of a median rolling average on a sonic seismic slowness data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 (a) A typical DataFrame from Hole C0024A . . . . . . . . . . . . . . . . . . . . 5.6 (a) A typical box plot from well C0024A with outliers data showing in the temperature log data, and DHAP data without outliers. . . . . . . . . . . . . . . . . 5.7 (a) C0024A mudline check with resistivity and weight on bit logs . . . . . . . . 5.8 (a) C0004B mudline check with gamma-ray and resistivity logs . . . . . . . . . . 5.9 Mudline pressure data for the three holes (C0024A, C0006A and C0004B). The colored line is the first time drilling data for a sampled lithological interval, and the gray vertical line marks the mud pressure baseline. . . . . . . . . . . . . . . 5.10 Time series of drilling data for all the holes. Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. . . . . . . . . . . . . . . . . . . . . . 5.11 The python script implementing the first time of drilling . . . . . . . . . . . . . . 5.12 Systematic workflow for modeling flow from DHAP data . . . . . . . . . . . . . 5.13 DHAP drilling data overlapped on the seismic section for Holes C0024A and C0006A at the frontal region of the Nankai accretionary prism. Large peaks are predominant when crossing the décollement as seen in hole C0024A, while there are no noticeable peaks in hole C0006A. Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. Red dotted line: Décollement, thick red line on seismic section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii 5.14 DHAP drilling data overlapped on the seismic section for Hole C0004B at the megasplay region of the Nankai accretionary prism. Large peaks are predominant when crossing the a faults as seen on the Hole C0004B. Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. Red dotted line: Décollement, thick red line on seismic section. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.15 Schematics of the Eulerian system on which mass balance was conducted (delimited by red dashed lines). The volume drilled between t 0 and t 0 + dt is shown with purple stripes. The arrows show also the fluid flows considered. Both pump flow Q pump and formation flow Q f (positive in the case of influx to the well, negative in the case of outflow) contribute to the flow returning to the surface Q out . Fluid flow into the well is assumed to come from a section between the DHAP sensor and the drill bit. Above, an impermeable mud cake is supposed to be fully developed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.16 Different types of fluids found in drilling operations (modified from Guo and Liu (2011).) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.17 Figure shows a pressure vs depth plot illustrating common pressure terminology along the borehole (Goodwyne 2012). . . . . . . . . . . . . . . . . . . . . . . . 5.18 (a) Porosity (resistivity derived and from moisture density) and Bulk Density for Hole C0024A (b) Porosity and Bulk Density for Hole C0006A (c) Density, Porosity and Bulk Density for Hole C0004B . . . . . . . . . . . . . . . . . . . . . . 5.19 (a) General NCT and abnormal pressure caused by undercompaction (a) D-exponent (b) seismic slowness and (c) Pore pressure (modified from Zhang and Yin (2017)) 5.20 Probability distribution with Pore pressure normal distributed and flow uniformly distributed for Monte Carlo Simulation. This is performed for 10000 simulations 5.21 Implementation of Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . 6.1 (a) Enlarged location of Site C0024 and Site C0006 indicated by the red triangle in figure 4.5b. Seismic IL 2437 section interpretation. mbsl = meters below sea level, XL is the cross-line, seismic stratigraphic packages represented as shaded colours, faults (bold red line), Site C0024, C0006 and C0007 location indicated with yellow vertical lines, USB =upper Shikoku basin. (Modified from Tobin et al. (2020)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 (a) Core based lithological classification at Site C0024. (Modified from Tobin et al. (2020)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv 6.3 (a) The BHA system used to drill Hole C0024A. Main sensors used for the Logging while drilling (LWD) measurements. The red circle shows the DHAP sensor used to record changes in mud pressure during drilling (Tobin et al. 2020). . . . . 6.4 Summary of the geophysical data acquired in Hole C0024A. The plate boundary, also called the décollement, has two strands highlighted by green lines at depths of 813 and 852 mbsf. (a) Lithological units identified onboard by the science party (Tobin et al. 2020) (b) Natural gamma-ray log (GR, API units) from ar-cVISION (c) Electrical resistivity log (with shallow, medium and deep depths of investigation) in Ω • m (d) Sonic slowness (δt, in µs/ft) from SoniScope (Figure 5.3b). (e) borehole diameter (e-caliper) derived from arcVISION and MicroScope tool (Figure 5.3b) (f) Electrical borehole image from deep resistivity with visible breakouts (g) Picked fractures and faults from the borehole image by the science party onboard (Tobin et al. 2020) . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 (a) C0024A mudline check with resistivity and weight on bit logs (b) The plot with the conversion function from measured depth to true vertical depth (c) The measured depth is exactly BIT DEP T H + of f set(7.516m) . . . . . . . . . . . . 6.6 Time series of drilling data, especially bit depth (part A) and DHAP (part B). DHAP and bit depth tend to increase with time. Analysis is restricted to DHAP data when each depth was first reached by the drill bit (black data). DHAP increases with the true vertical depth of the DHAP sensor (part C), with a linear baseline (in gray) corresponding to an equivalent mud density of 1098 kg/m 3 . Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. . . . . . 6.7 Input data for DHAP modeling in hole C00024A (a) Caliper or borehole diameter (b) Flow rate or pumping rate (c) ROP . . . . . . . . . . . . . . . . . . . . . . . 6.8 Workflow adopted for modeling fluid flow in hole C0024A. The highlighted writings in red are the unknown input information. . . . . . . . . . . . . . . . . . . . xv 6.9 Results derived from the DHAP modeling of hole C0024A. (a) Lithological column, same as in Figure 6.4a. (b) Predicted profiles of the mud pressure at various stages of the modeling: with only the contribution of the clean injected fluid (orange dots), with the additional contribution of the weight of the cuttings (red dots) and the complete model, with the hydraulic losses of the flowing mud (blue dots). For all models, the mud is assumed to be seawater (ρ M W = 1028 kg/m 3 , µ = 1 mPa • s). The DHAP data corresponding to actual drilling times (gray dots) are well fitted by the latter model, except below the 2 décollement zones (dashed green lines). (c) Plot of the difference between the DHAP data (gray dots of in graph (a)) and the prediction from the full DHAP modeling (red dots). The null value, where the model exactly fits the data, is highlighted by a thick vertical line. (d) Flow rate between the formation and the hole. Negative value (to the left of the thick vertical line) corresponds to a flow from the hole to the formation, as expected in normal drilling conditions. . . . . . . . . . . . . . . . . . . . . . . . 6.10 Parametric study of the DHAP modeling for variable mud densities and viscosity values (a) Prediction for clean mud with density 1028 kg/m 3 and viscosity 1 mPa • s. (b) Same as (a) but with a slight change in viscosity 2 mPa • s. (c) Same as (a) but with a slight change in density of 1050 kg/m 3 and fixed viscosity 1 mPa • s. (d) Same as (a) with the properties of the mud incorrectly stated in the drilling report with density of 1350 kg/m 3 and viscosity of 51 mPa • s. (e) Normalized error L 2 of the DHAP prediction above décollement, for a range of different mud properties. The colored dots correspond to the profiles illustrated below: clean mud as water (red), clean mud with varied density (yellow),clean mud with varying viscosity (orange), and drilling report mud properties (pink). (f) Normalized L 2 error for the DHAP prediction for entire borehole length with varying mud properties indicated with coloured dots. Clean mud (red), clean mud with varying density (yellow), clean mud with varied viscosity (orange), and drilling report mud properties (pink). . . . . . . . . . . . . . . . . . . . . . . . . 6.11 The input data and the calculated d xc exponent for hole C0024A (a) rate of penetration (ROP) (b) weight on bit (WOB) (c) revolution per minute (RPM) (d) The equivalent circulating density (ECD). . . . . . . . . . . . . . . . . . . . . . . . . xvi 6.12 Pore pressure predictions from the d xc exponent (red circled plots [b and c]) and ∆t sonic (blue circled plots [d and e]) and Kitajima and Saffer (2012), Kitajima et al. (2017) method. (a) Logging units (b) Profile of the Eaton d xc coefficient (raw d xc [gray] and d xc averaged sampled for 500 points[red]) along the borehole with an observable deviation to lower values from the NCT line (black) at the top of the subunit 1c (accreted wedge Facies). This particular depth marks the top of the geopressured interval. (c) The pore pressure profile (red line) follows hydrostatic pressure (blue line) within the normal pressured zone and rises significantly above the hydrostatic pressure within the overpressured zone. (d) Profile of the Eaton coefficient ∆ t (raw ∆ t [gray] and sampled ∆t [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (e) The pore pressure profile (red line) follows hydrostatic pressure (blue line) within the normal pressured zone and rises significantly above the hydrostatic pressure within the overpressured zone. (f) Overlay comparing both pore pressure results from Eaton's methods and Kitajima and Saffer (2012), Kitajima et al. (2017) method. The two pressure profiles ( d xc in gray and ∆t in black) almost overlap near the décollement with a localized increase in the pore pressure across the strands while K 0 uniaxial loading pore pressure result in blue color. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 6.13 Pore pressure prediction from d-exponent method. (a) Logging units. (b) Profile of the Eaton d xc coefficient (raw d xc [gray] and sampled d xc [red]) along the borehole with an observable deviation to lower values of the NCT line (black) at the top of subunit 1c (accreted wedge Facies). This particular depth marks the top of the geopressured interval. (c) The plot of the variable pressure gradients, the hydrostatic pressure gradient in blue coloured line, overburden or lithostatic pressure gradient in green coloured line, averaged pore pressure gradient in red line, raw pore pressure gradient in gray line. (d) The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within normal pressure zone. While within the overpressured zone the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 xvii 6.14 Pore pressure prediction from Eaton modeling based on ∆t sonic method (a) Logging units (b) Eaton ∆t coefficient profile (raw ∆t [gray] and average sampled ∆t [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (c) The plot of the variable pressure gradients, the hydrostatic pressure gradient in blue coloured line, overburden or lithostatic pressure gradient in green coloured line, averaged sampled pore pressure gradient in red line, raw pore pressure gradient in gray coloured line. (d) The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within normal pressure zone. Within the overpressured zone, the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole. . . . . . . . . . . . . . . . . . . . . 6.15 The Python script for the estimation of the Vshale using the Clavier et al. (1971), Stieber (1970), Larionov (1969) formula . . . . . . . . . . . . . . . . . . . . . . 6.16 (a) Porosity prediction from the inversion of the Erickson-Jarrard equation (Equation 5.51) in blue, compared with the MAD (Moisture and Density) data obtained from core samples during expedition 358 (black dots) and with the porosity derived from Archie's equation applied on the resistivity logs (red) (b) Cross-plot of V shale and IGR, and the derived V p with depth along the C00024A borehole length. 6.17 Results from the workflow of Kitajima et al. (2017). (a) Porosity prediction from the inversion of the Erickson-Jarrard equation (Equation 5.51) in blue, compared with the MAD (Moisture and Density) data obtained from core samples during expedition 358 (black dots) and with the porosity derived from Archie's equation applied on the resistivity logs (red) (b) Pore pressure prediction assuming Critical State Loading (CSL) conditions (blue). The maximum horizontal stress (red) is also inverted. The hydrostatic (thin blue) and lithostatic (brown) profiles are also shown. The prediction from Eaton methods are displayed in black and gray, for the version using P-wave velocity and d xc coefficients, respectively. (c) Same as in (b) but for K 0 uniaxial loading. The latter pore pressure prediction agrees better with the predictions from Eaton methods. . . . . . . . . . . . . . . . . . . xviii 6.18 (a) Permeability decaying with depth, with observe-able step in permeability when crossing the fault core of the décollement.(b) Plot of estimated permeability versus porosity from well C0024a considering the 6 identified logging subunits (c) Plot of estimated permeability versus porosity from well C0024a considering the 6 identified logging subunits with an overlayed of the permeability results from Hvorslev method and the results of permeability from Boutt et al. (2012) using MDT-HF tool at a metric scale. . . . . . . . . . . . . . . . . . . . . . . . . 6.19 (a) Permeability from Hvorslev method (b) Permeability after partial differentiation of variables. (c) Permeability after MCS. . . . . . . . . . . . . . . . . . . . 6.20 Summary of the hydraulic properties determined along the whole Hole C0024A. (a) seismic section (b) Electrical borehole imaging from deep resistivity (c) Gammaray (d) caliper log (e) P-wave velocity (f) deep resistivity (g) tadpoles of the main structures picked on the electrical image (h) raw DHAP data, modeled DHAP, effective mud density, and tank mud density (i) Fluid flow exchange between formation and the borehole (j) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure (k) ratio of P-wave velocity and shear velocity (l) mud fluid temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.21 Summary of the hydraulic properties determined along Hole C0024A, with focus on the décollement zone, below 780 mbsf. (a) Seismic section (b) Electrical borehole imaging from deep resistivity (c) caliper log (d) P-wave velocity, (e) deep resistivity, (f) tadpole diagram of the major structures picked on the electrical image (g) raw DHAP data, modeled DHAP, effective mud density and tank mud density (h) fluid flow exchange between the formation and the borehole (i) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure (j) Estimated effective stress change with depth. . . . . . . . . . . . . . 6.22 Correlations of the published pore pressure ratio derived by Kitajima and Saffer (2012) with the results of the pore pressure ratio results estimated at the frontal thrust of the accretionary prism. . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 (a) Enlarged location of Site C0024 and Site C0006 indicated by the red triangle in figure 4.5b. Seismic IL 2437 section interpretation. mbsl = meters below sea level, XL is the cross-line, seismic stratigraphic packages represented as shaded colours, faults (bold red line), Site C0024, C0006 and C0007 location indicated with yellow vertical lines, USB =upper Shikoku basin. (Modified from Tobin et al. (2020)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix 7.2 Logs are superimposed on check shot-corrected seismic profile. (a) logging units (b) GR log: LSF = LWD depth below seafloor. VE = vertical exaggeration (Modified from Kinoshita et al. (2009d)). . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Logs are superimposed on check shot-corrected seismic profile. (a) Caliper log (b) Ring resistivity: LSF = LWD depth below seafloor. VE = vertical exaggeration (Modified from Kinoshita et al. (2009d)). . . . . . . . . . . . . . . . . . . . . . 7.4 Summary of the geophysical data of Hole C0006A. (a) Electrical borehole image from deep resistivity (b) Seismic section along the borehole (c) Lithological units (d) Natural gamma-ray log (GR, API units) from arcVISION (e) Electrical ring resistivity log in Ω • m (f) Electrical resistivity log (with shallow, medium and deep depths of investigation) in Ω • m (g) Resistivity derived porosity (h) Picked fractures and faults from the borehole images (i) Sonic velocity (m/s) (modified (Kinoshita et al. 2009d)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Time series of drilling data, especially bit depth (part A) and DHAP (part B). DHAP and bit depth tend to increase with time. Analysis is restricted to DHAP data when each depth was first reached by the drill bit (black data). DHAP increases with the true vertical depth of the DHAP sensor (part C), with a linear baseline (in gray) corresponding to an equivalent mud density of 1146 kg/m 3 . Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. . . . . . 7.6 (a) Input data critical for DHAP dynamic modeling in hole C0006A (a) pumping rate (b) ROP data essential for estimating cuttings contribution . . . . . . . . . . 7.7 (a) Gamma-ray log from Holes C0006A and C0006B (b) The charts shows the relationship of the correction factor (ratio of the original acquired gamma-ray and the corrected gamma-ray) on the x-axis and the mud weight on the y-axis of the plot. While the caliper log data is on the z-axis. . . . . . . . . . . . . . . . . . . 7.8 (a) Cross-plot of the gamma-ray correction factor and the mud weight with values of 8.76 ppg at the mudline. (b) The plot of the GR correction and borehole diameter with the five data points derived originally from Figure 7.8a. . . . . . . 7.9 Cross-plot of gamma-ray correction with gamma-ray with the test, trained, and predicted model for the three different scenarios . . . . . . . . . . . . . . . . . 7.10 (a) Logging units (b) the derived caliper log in inches (c) gamma ray log (gAPI). xx 7.11 Results derived from the DHAP modeling of Hole C0006A. (a) Lithological Units. (b) Predicted profiles of the mud pressure at various stages of the modeling: with only the contribution of the clean injected fluid (orange dots), with the additional contribution of the weight of the cuttings (red dots) and the complete model, with the hydraulic losses of the flowing mud (blue dots). The DHAP data corresponding to actual drilling times (gray dots) are well fitted by the latter model. (c) Plot of the difference between the DHAP data (gray dots of in graph (a)) and the prediction from the full DHAP modeling (red dots). (d) Flow rate between the formation and the hole. Negative value (to the left of the vertical base line) corresponds to a flow from the hole to the formation, as expected in normal drilling conditions. Positive values denote fluid flow from the formation into the borehole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.12 (a) Logging units (b) the derived caliper log in inches (c) Flow rate between the formation and the hole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.13 The input data and the calculated d xc exponent for hole C0006A (a) rate of penetration (ROP) (b) weight on bit (WOB) (c) revolution per minute (RPM) (d) Profile of the Eaton coefficient d xc (raw d xc [gray] and d xc averaged sampled for 500 points [red]) with overlay of logging units along the borehole. . . . . . . . . 7.14 Prediction of pore pressure from Eaton modeling based on d xc sonic method (a) Logging units (b) Eaton coefficient profile d xc (raw d xc [gray] and average sampled d xc [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (c) The pore pressure profile follows hydrostatic pressure (black line) and less than the mud pressure (orange line) within normal pressure zone. Within the overpressured zone, the pore pressure rises above the mud pressure and hydrostatic pressure. . . . . . . . 7.15 (a) Cross-plot of V s h and IGR, and (b) the derived profile of V p along the borehole. 7.16 Pore pressure prediction from Eaton modeling based on ∆t sonic method (a) Logging units (b) Eaton ∆t coefficient profile (raw ∆t [gray] and average sampled ∆t [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (c) The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within the normal pressure zone. Below the main frontal thrust fault (green dashed line), the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole. . . . . . . . . . . . . . . . . . . . . . . . . . xxi 7.17 (a) Permeability decaying with depth (b) Plot of estimated permeability versus porosity from well C0006A considering the 5 identified logging subunits (c) Plot of estimated permeability versus porosity from well C0006A considering the 5 identified logging subunits with an overlayed of the permeability results from Hvorslev method and the results of permeability from Boutt et al. (2012) using MDT-HF and MDT-DD tool and cores at a metric scale. . . . . . . . . . . . . . . 7.18 Summary of the hydraulic properties determined along Hole C0006A, with focus on the main frontal thrust fault zone, below 550 mbsf. (a) Seismic section (b) logging subunits (c) caliper log (d) Gamma ray log (e) P-wave velocity, (f) deep resistivity (g) fluid flow exchange between the formation and the borehole (h) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure. (i) permeability decay with depth (j) estimated effective stress . . . . . 7.19 Summary of the hydraulic properties determined along Hole C0006A, with focus on the main frontal thrust fault zone, below 500 mbsf. (a) Seismic section (b) Electrical borehole imaging from deep resistivity (c) caliper log (d) P-wave velocity, (e) deep resistivity, (f) tadpole diagram of the major structures picked on the electrical image (g) raw DHAP data, modeled DHAP, effective mud density and tank mud density (h) fluid flow exchange between the formation and the borehole (i) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure. (j) Estimated effective stress change with depth . . . . . . . 8.1 (a) Enlarged location of Site C0004: 3D seismic profile. VE = vertical (Modified from Kinoshita et al. (2009b)) . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 lithological classification at Site C0004 based on core. CSF = core depth below seafloor. (Modified from Kinoshita et al. (2009b)) . . . . . . . . . . . . . . . . . 8.3 (a) The BHA system used to drill Hole C0004B. Main sensors used for the Logging while drilling (LWD) measurements. The red circle shows the DHAP sensor used to record changes in mud pressure during drilling (Kinoshita et al. 2009b). . 8.4 Summary of the geophysical data of Hole C0004B. (a) Electrical borehole image from deep resistivity with visible breakouts. (b) inline seismic section (c) Lithological units identified onboard by the science party (Kinoshita et al. 2009b) (d) Natural gamma-ray log (GR, API units) (e) Ring and deep resistivity log in Ω • m (f) ultrasonic caliper in inches (blue curve) (g) Sonic velocity (m/s) from SoniScope (h) Porosity derived from porosity (i) Picked fracture dips (red tadpoles) and black tadpoles as the bedding from resistivity at bit image (Kinoshita et al. 2009b). xxii 8.5 (a) C0004B mudline check with gamma-ray and resistivity logs . . . . . . . . . . 8.6 Time series of drilling data, especially bit depth (part A) and DHAP (part B). DHAP and bit depth tend to increase with time. Analysis is restricted to DHAP data when each depth was first reached by the drill bit (black data). DHAP increases with the true vertical depth of the DHAP sensor (part C), with a linear baseline (in gray) corresponding to an equivalent mud density of 1051 kg/m 3 . Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. . . . . . 8.7 Input data for DHAP modeling in hole C0004B (a) Caliper or borehole diameter (b) Flow rate or pumping rate (c) ROP . . . . . . . . . . . . . . . . . . . . . . . 8.8 Results derived from the DHAP modeling of Hole C0004B. (a) Lithological column, same as in Figure 6.4a. (b) Predicted profiles of the mud pressure at various stages of the modeling: with only the contribution of the clean injected fluid (orange dots), with the additional contribution of the weight of the cuttings (red dots) and the complete model, with the hydraulic losses of the flowing mud (blue dots). The DHAP data corresponding to actual drilling times (gray dots) are well fitted by the latter model. (c) Plot of the difference between the DHAP data (gray dots of in graph (a)) and the prediction from the full DHAP modeling (red dots). The null value, where the model exactly fits the data, is highlighted by a thick vertical line. (d) Flow rate between the formation and the hole. Negative value (to the left of the vertical base line) corresponds to a flow from the hole to the formation, as expected in normal drilling conditions. Positive value depicts fluid flow from the formation into the borehole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Summary of the hydraulic properties determined along Hole C0004B, with focus on the megasplay zone. (a) Electrical borehole imaging from deep resistivity (b) Logging units (c) caliper log (d) Gamma ray log (e) P-wave velocity, (f) deep resistivity, (g) fluid flow exchange between the formation and the borehole . . . . 8.10 Illustration of areas with high pore pressure situated around the megasplay fault by Tsuji et al. (2014), this can imply high pore pressure at the splay fault is connected to the megasplay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Summary of the fluid flow result at the three drill site location (Site C0024, C0006, C0004). The plot shows the fluid flow profile along the boreholes at megasplay faulting and the toe of the Nankai accretionary prism. . . . . . . . . . . . . . . xxiii 9.2 Correlations of the published pore pressure ratio derived by Kitajima and Saffer

Fig. 2

 2 Fig. 2.1 A typical diagram illustrating the Subduction zone with all resulting geological setting and processes. The figure shows the megathrust faults, coseismic slip and afterslip zones (Weiss et al. 2019).

Fig. 2 . 2

 22 Fig. 2.2 The figure shows distribution of active subduction zones around the world with different magnitude of historic earthquakes between 1900 -June, 2012. MW -magnitude of earthquakes, Numbers signify the MW values (Schellart and Rawlinson 2013).

Fig. 2 . 3

 23 Fig. 2.3 Schematic diagram of seismogenic zone and aseismic zones (a) Seismogenic zone: an area prone strong tremors and earthquakes. ; (b) Subduction of the oceanic crust beneath continents, characterized by the presence of island arcs, and the assumption that the serpentinized forearc mantle forms the main downdip limit of the seismogenic zone (Hyndman 2007).

Fig. 2

 2 Fig.2.4 (a)Schematic cross-section of the seismogenic zone, and the aseismic zone with descriptive extends of where large earthquakes and SSEs, LFEs can initiate. (b) General perspective of the seismogenic zone and aseismic zones. The seismogenic zone can be identified by the following: mechanically, by large regions or asperities that, when they fail, result in a frictionally unstable or velocity-weakening situation manner. The aseismic area updip and downdip of the seismogenic zone or within the frictionally stable or velocity-strenghtening manner[START_REF] Lay | Great Earthquakes on Plate Boundaries[END_REF]).

Fig. 2

 2 Fig. 2.5 (a) Schematic diagram of updip and downdip aseismic zone (a) Subduction thrust faults updip aseismic zone towards the trench. which host large magnitude earthquakes. (b) Subduction thrust faults downdip aseismic zone. (c) As the updip limit is below 150°C and the downdip limit is above 350°C (Hyndman 2007).

and

  Wang 1993) summarized three possible reasons that have been identified to influence weaknesses in large fault. (a) elevated pore fluid pressures (Magee and Zoback 1993); (b) low frictional

Fig. 2 . 7

 27 Fig. 2.7 Processes of fault slip and hydrology diagram. High pore pressure, represented by blue shading, can weaken fractures and sediments. The arrows depict the flow of fluids and are qualitatively scaled to indicate flow rates. Sediments from an underthrust that have been drained could mix with the fault's movement. Throughout the igneous ocean crust, fluid flow has the potential to advect heat and/or solutes (not shown). Around the updip and downdip limits of interseismic locking and coseismic slip, VLFE, ETS occur, and excess pore pressure may cause low effective stress. Concentration of flow and pressure translation in fractured fault zones and permeable sands are shown in the insets (Saffer and Tobin 2011)).

Fig. 3

 3 Fig. 3.1 (a) Sketch of Oedometer (Wang and Abriak 2015)

Fig. 3

 3 Fig. 3.2 (a) Constant rate of strain apparatus (b) an enlarged diagram of the consolidometer (ASTM 2012)

Fig. 3 . 3

 33 Fig. 3.3 Schematic illustration of a material wedge nearing Coulomb failure as a result of horizontal compression (Modified fromDavis et al. (1983)).

Fig. 3 . 4

 34 Fig. 3.4 Geometry of Coulomb fractures rising from the basal décollement and Mohr diagram illustrating the basal state of (Modified fromDahlen et al. (1984))

  Fig. 3.5 (a) Numerical model domain, boundary conditions, and grid. The grid has 30 rows and 1499 columns, and includes 44,970 elemments (46,500 nodes). The prism has thirteen rows, the underthrust has eleven, the décollement has six closely spaced elements with a higher permeability. Also tightly spaced are the three parts directly above and below the décollement. The boundary conditions are also adopted (Modified fromSaffer andBekins (2006)).

Fig. 3 . 6

 36 Fig. 3.6 Plot of laboratory-derived permeability versus porosity for a variety of natural argillaceous media. Permeability is shown along the lower horizontal scale: the corresponding hydraulic conductivity to water at room temperature is shown along the upper horizontal scale.[START_REF] Neuzil | How permeable are clays and shales?[END_REF]

Fig. 3

 3 Fig. 3.8 a) Excess pore pressure (P*) and (b) effective stress (sz0) at the décollement as a function of distance from the trench for the best fit model runs. The shaded area shows the range of shear strength (t) at the décollement for all three cases assuming mb = 0.3-0.4 (Skarbek and Saffer 2009)

Fig. 3 .

 3 Fig. 3.10 (a) Consolidation sample test example (b) The arrows show the direction of the stress applied during the experiment (Saffer 2003) (b) This is a schematic depicting the geometry and boundary conditions of the model (Screaton 2006).

  , Becker and Davis 2005, Tobin and Kinoshita 2007b). It is installed in the borehole within an area of interest, normally the fault zones. It has been used extensively within the different accretionary prism. Over the years there has been modification and advancement on the design of the tool, from the first generation of the (a) CORK I [Figure 3.11a] (Kastner et al. 2006, Becker and Davis 2005) (b) Advanced CORK or ACORK (c) Wireline CORK (d) CORK-II [Figure 3.11b] (Kastneret al. 2006[START_REF] Jannasch | CORK-II: Long-Term Monitoring of Fluid Chemistry, Fluxes, and Hydrology in Instrumented Boreholes at the Costa Rica Subduction Zone[END_REF]) (e) Smartplug (Figure3.12) (f) GeniusPlug (Figure3.13). The device is made up of a combo of: (1) A pressure unit at the CORK head and a multiline hydraulic umbilical running into the hole and terminated at different depths. (2) a data logger having a recording capacity for up to 2-7 yr of monitoring, (3) a downhole string of 10 thermistors, (4) strainmeter and seismometer (5) a pressure sensor situated below the reentry cone seal, and (6) a means by which fluids can pass from the formation, through the seal, and be sampled by a submersible or a remotely operated vehicle.The CORK has been used in the Nankai accretionary prism (Kumano transects [sites C0010, C0006, C0002] and Muruto transect [sites 1173, 808]) with the a primary objective of understanding and measuring in situ pore fluid pressure, temperature, strain and seismic ground motion conditions within the frontal thrust region, megasplay faults, décollement and the Kumano forearc basin region(Tobin and Saffer 2009, Kinoshita et al. 2018). For example, at Site C0010 the tool recorded pressure and temperature changes along the splay faults within an isolated depth between 364 -410 mbsfKopf et al. (2011) (figure 3.13b, 3.15b). At Site C0002 the system measures pressure at four depth levels consisting three different lithofacies in the Kumano forearc basin and between ∼920 to 936 mbsf (a transition zone to the underlying accreted strata)Kopf et al. (2011) (Figure 3.15a). At site 1173, the bridge plug was mistakenly positioned at around 466mbsf in order to measure 5 isolated screen intervalsMikada et al. (2002) (figure 3.14).

Fig. 3 .

 3 Fig. 3.11 Engineering schematic of (a) original CORK for monitoring and sample of single hole intervals and (b) CORK-II design used for multiple interval sampling and monitoring (Modified fromKastner et al. (2006))

Fig. 3 .

 3 Fig.3.16 Processed pressure data recorded by the installed CORK device in wells along the Kumano transects (C0010, C0006, C0002). From[START_REF] Dutilleul | Tidal analysis of the nantroseize long term borehole monitoring system (ltbms) pore pressure records at the nankai margin, sw japan[END_REF]

Fig. 3 .

 3 Fig. 3.18 (a) Semblance contours (b) Interval velocity derived from stacked velocity (c) CMP plot showing the coherent reflections (Cochrane et al. 1994b).

Fig. 3

 3 Fig. 3.19 (a) Seismic reflection image in three dimensions (3D), with P-wave velocity (in color), derived from prestack depth migration (3D PSDM), and interpreted as a low-velocity zone (LVZ). The outer wedge's LVZ (unit B) is denoted by a thick, light blue region with dots. There seems to be no clear evidence that the LVZ extends into the inner wedge[START_REF] Park | A low-velocity zone with weak reflectivity along the Nankai subduction zone[END_REF]

Fig. 3 .

 3 Fig. 3.20 3-D seismic volume showing high-amplitude, reversed-polarity seismic reflection from deep splay fault (red areas). Black lines are faults, accretionary wedge thrusts are the Blue lines Bangs et al. (2009)

Fig. 3 .

 3 Fig. 3.21 3-D seismic volume showing high-amplitude, reversed-polarity seismic reflection from deep splay fault (red areas). Black lines are faults, accretionary wedge thrusts are the Blue lines Bangs et al. (2009)

Fig. 3 .

 3 Fig. 3.22 (a) PSDM 2-D reflection line 215 (b) Line 215's P-wave velocity model with variable low seismic velocity relative to overlying wedge. V.E.-vertical exaggeration. c) On track one: Porosity is the blue curve and the red curve is the PSDM interval velocity. Middle track: Computed pore pressure is the red curve, hydrostatic pressure is the blue curve, and total overburden stress is the green curve. Sediments without excess fluid pressure would plot on hydrostatic line. Bottom track: Vertical effective stress diverges from normal consolidation with trench distance. With = 0.4 and no cohesion, shear stress is defined. Upper and lower curve boundaries are propagated errors.

Fig. 3 .

 3 Fig. 3.23 (a) Excess pore pressure above hydrostatic pressure (b) Results of pore pressure ratios (c) Normalized pore pressure ratio at the mega-splay fault hanging wall. (d) Mega-splayfault footwall normalized pore pressure ratio Tsuji et al. (2014).

Fig. 3 .

 3 Fig. 3.24 (a) P * in the LVZ as a function of distance from the trench, estimated using Vp derived from 3-D seismic data. The minimum and maximum values within the LVZ at each distance from the trench are represented by the vertical bars. (b) Estimated in situ stress levels (S H ¿ S h ¿S v ) and pore pressure (Pp) at Site C0002 for the optimal scenario. SH (red) and Pp (blue) are estimated based on a K0 loading assumption in the Kumano Basin and a CSL assumption in the accretionary prism. (c) Distribution of lambda * in the LVZ under the condition that S H ¿ S h ¿ S v . The beach balls symbolize moment tensor VLFE. (Modified from(Kitajima and Saffer 2012, Kitajima et al. 2017))

  al. 2012). Other details of how injection experiments are used to measure in situ stress and permeability through hydraulic fracturing experiments have been discussed by Boutt et al. (2012), Thu et al. (2012).In the drawdown test as the pumping rate is reduced, the pressure decreases (Figure3.25a), while for the hydraulic fracturing the pressure increases during injection as effective stress also decreases (Figure3.25b). At Site C0009, the results of permeability from the MDT-drawdown and MDT-Hydraulic fracturing (Figure3.25c) were compared to other permeability obtained from core measurements in other subduction zones such as the Costa Rica, Peru, Barbados, and Nankai accretionary prism[START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF]. The results of permeability from the MDT-DD test (9E-17 -3E-17 m 2 ) and MDT-HF test (1.5-5.5 E-15 m 2 ) are 2-4 orders of magnitude higher than permeability measurements from the cores (2.5-2.7 E-19 m 2 )[START_REF] Boutt | Scale dependence of in-situ permeability measurements in the nankai accretionary prism: The role of fractures[END_REF] (Figure 3.25c).

Fig. 3 .

 3 Fig. 3.25 (a and b) Illustration of MDT dual packer tool string used for Expedition 319 (Modified fromAraki et al. (2009), Thu et al. (2012).

Fig. 3 .

 3 Fig. 3.26 (a) Cross-plot of pore fluid pressure and pumping rate for the MDT -drawdown test at 1539.7 mbsf in Site C0009. (b) Crossplot of pore fluid pressure and pumping rate (right axis) for the MDT -hydraulic fracture test at 1532.7 mbsf. (c) Permeability Vs porosity for derived laboratory test from Costa Rica, Peru, Barbados and Nankai accretionary prisms, MDT-drawdown and MDT-hydraulic fracturing. (Modified fromBoutt et al. (2012)

  al. 2017) with identified predominant frequency of 0.1 Hz near the trench axis of the Nankai Trough (Obara and Ito 2005). Other prominent basins along strike in the Nankai subduction zone are Tosa, Muroto and Kumano forearc basins (Figure 4.1).The Nankai forearc is subdivided into three tectonic domains: the inner wedge, the outer wedge and the transitional thrust zone of the outer ridge or upper part of the trench slope[START_REF] Kimura | Transition of accretionary wedge structures around the up-dip limit of the seismogenic subduction zone[END_REF]), called the Kumano Basin Edge Fault Zone (KBEFZ) that may (Figure4.1 and 4.2) have a combination of normal and strike-slip faults[START_REF] Martin | Possible strain partitioning structure between the kumano fore-arc basin and the slope of the nankai trough accretionary prism[END_REF]. Trench-filling turbidites and Shikoku Basin sediments form the outer wedge of the accretionary (Figure4.2) of the Philippine Sea Plate[START_REF] Demets | Geologically current plate motions[END_REF]. From the seaward to the landward direction, the outer prism of Nankai Trough is divided into three separate parts: the frontal thrust zone, the imbricate thrust zone, and the OOST zone[START_REF] Moore | Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect[END_REF][START_REF] Kimura | Spatial and temporal evolution of the megasplay fault in the nankai trough[END_REF]).

Fig. 4

 4 Fig. 4.1 (a) Nankai Map: Index map of the Nankai Trough with the PSP subducting under the Eurasian plate. KBEFZ: Kumano Basin Edge Fault Zone (Tsuji et al. 2015a).

Fig. 4

 4 Fig. 4.2 (a) Kumano transect line indicated as the IODKM-M line on Figure 4.1 showing drill sites of the NanTroSEIZE with few representative faults. Dashed lines = less certain fault locations. Morphotectonic zones are shown between the two sections. KBEFZ = Kumano Basin edge fault zone (modified Park et al. (2008)).

  Fig. 4.3 (a) Regional map of the Nankai subduction zone. Filled blue circles and open light blue circles and are of SVLFEs epicentral locations Asano et al. (2008), The records of SVLFEs offshore are from 2015 and the fault areas hosting SSEs from 2011-2016 (circled in purple) (Araki et al. 2017), deep VLFEs and SSEs are represented with gray rectangle and dots. Modified after Takemura et al. (2019).

4. 3

 3 .1.2 b. Kumano Basin:Kumano Basin is a forearc basin located landward of the OOST fault zone and overlying the thrusts and slope fill of the inner prism's eastern portion[START_REF] Moore | Evolution of tectono-sedimentary systems in the kumano basin, nankai trough forearc[END_REF]. The distal region of the Kumano Basin is generally characterized by landward-tilted horizons[START_REF] Moore | Threedimensional splay faultgeometry and implications fortsunami generation[END_REF], Gulick et al. 2010) and various sets of normal fault populations[START_REF] Sacks | Analysis of normal fault populations in the kumano forearc basin, nankai trough, japan: 2. principal axes of stress and strain from inversion of fault orientations[END_REF]. Kumano fore-arc basin evolutionn is closely intertwined with formation and the uplift of the OOST, developing as a result of tectonics and sedimentation[START_REF] Moore | Evolution of tectono-sedimentary systems in the kumano basin, nankai trough forearc[END_REF]. The Kumano basin is 2 km thick and it is comprised of hemipelagic sediments, fine-grained turbiditic sediments, slope basin sediments, and older Shikoku Basin sediments[START_REF] Gulick | Rapid forearc basin uplift and megasplay fault development from 3d seismic images of nankai margin off kii peninsula, japan[END_REF][START_REF] Strasser | Proceedings of the integrated ocean drilling program[END_REF][START_REF] Tobin | Proceedings of the integrated ocean drilling program[END_REF][START_REF] Kinoshita | Expedition 316 scientists[END_REF][START_REF] Ramirez | Early sedimentation and deformation in the kumano forearc basin linked with nankai accretionary prism evolution, southwest japan[END_REF][START_REF] Taira | Recent scientific and operational achievements of d/v chikyu[END_REF], Saffer et al. 2010[START_REF] Underwood | Evolution of sedimentary environments in the subduction zone of southwest japan: recent results from the nantroseize kumano transect[END_REF].

Fig. 4 . 4

 44 Fig.4.4 Seismic reflection image in three dimensions (3D), with P-wave velocity (in color), derived from prestack depth migration (3D PSDM), and interpreted as a low-velocity zone (LVZ). The outer wedge's LVZ (unit B) is denoted by a thick, light blue region with dots. There seems to be no clear evidence that the LVZ extends into the inner wedge[START_REF] Park | A low-velocity zone with weak reflectivity along the Nankai subduction zone[END_REF]).

  . As discussed in[START_REF] Park | A deep strong reflector in the nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release[END_REF],[START_REF] Moore | Threedimensional splay faultgeometry and implications fortsunami generation[END_REF], the offshore Nankai Trough region is characterised by a welldeveloped forearc basin (Kumano basin) and accretionary prism extending seaward, as well as a major thrust splay fault called the Megathrust Splay fault that originates from deep down beneath the Kumano basin. The Nankai subduction zone has been the focus of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project, which features 13 expeditions from the International Ocean Discovery Program (IODP)[START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]).This program is a coordinated drilling operation that consists of multiple expeditions with the goal of investigating the mechanics of the active faults and the seismogenesis of the Nankai subduction zone, sample the rock composition of these faults, estimate the fluid composition, the pore pressure and the seismogenesis of the aseismic and seismogenic portions of the Nankai subduction forearc[START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF], Tobin and Kinoshita 2007a, Tobin et al. 2009). The different expeditions hosted in this region targeted different tectonic domains with specific research and drilling objectives. In the thesis we concentrate on two regions of the Nankai Trough: (a) Frontal Thrust Region (Sites C0024 [Expedition 358] and C0006 [Expedition 314 and 316 ]) and (b) Mega-Splay Fault Region (Sites C0004 [Expedition 314]) (Figure 4.5).

Fig. 4

 4 Fig. 4.5 (a) Seismic section without interpretation, processed with pre-stack depth migration. [B] Interpreted 3D seismic section in Figure 4.5a: faults indicated as dashed and solid lines. KBEFZ = Kumano Basin edge fault zone; PTZ = Protothrust zone; BSR=Bottom simulating reflection; VE = Vertical exaggeration (modified from Tobin et al. (2020)).

Fig. 5

 5 Fig. 5.1 A typical drilling rig system[START_REF] Bak | Model based design of electro-hydraulic motion control systems for offshore pipe handling equipment[END_REF]

Fig. 5

 5 Fig. 5.3 (a) The two types of drilling used in the NanTroSEIZE project, tools and DHAP data: (a) The different drilling procedure employed for the riser and riserless drilling types. In the riser: drill mud and cuttings are returned back to the ship platform through a special riser pipe that is linked to a seafloor blowout prevention (highlighted by a red box). Riserles: rock cuttings are pumped out of the borehole with seawater onto the seafloor. The highlighted red box in the riserless drilling shows the Bottom Hole Assembly (BHA) (b) The BHA system used to drill Hole C0024A. Main sensors used for the Logging while drilling (LWD) measurements. The red circle shows the DHAP sensor used to record changes in mud pressure during drilling.

  Fig. 5.4 Example of python implementation of a median rolling average on a sonic seismic slowness data.Seven preliminary corrections made on the data from Holes C0024A, C0004B, and C0006A are highlighted below:

Fig. 5

 5 Fig. 5.5 (a) A typical DataFrame from Hole C0024A

Fig. 5 •

 5 Fig. 5.7 (a) C0024A mudline check with resistivity and weight on bit logs

  interpolated values at the seafloor depth (Figure5.9). (c) Alternatively in the complete absence of DHAP data, the pressure at the mudline can be estimated (P m = ρ w zg

Fig. 5 . 9

 59 Fig. 5.9 Mudline pressure data for the three holes (C0024A, C0006A and C0004B). The colored line is the first time drilling data for a sampled lithological interval, and the gray vertical line marks the mud pressure baseline.

Fig. 5 .

 5 Fig. 5.10 Time series of drilling data for all the holes. Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit.

Fig. 5 .

 5 Fig. 5.11 The python script implementing the first time of drilling

Fig. 5 .

 5 Fig. 5.12 Systematic workflow for modeling flow from DHAP data

  the amount of fluid influx from the formation into the borehole due to the high pore pressure within the formations, Q pump is the volume of drilling fluids pumped into the borehole through the drill pipe by the operators (Figure 5.3a), while, Q out is the total volume of fluids flowing out of the borehole through the annulus to the seafloor (i.e comprises of Q pump , Q f and drill cuttings: Figure 5.3a). DHAP modeling is done in two ways. (a) The forward model estimates the pressure anomalies (Figure 5.13 and 5.14) (b) The inversion model is used to estimate the fluid influx (Q f ).

3.

  For the sake of this estimation, the fluid influx (Qf ) from the formation into/out of the borehole is considered to be minimal in comparison to the pumping flow rate (Qpump: initial mud flow pumped from into the borehole through the drill pipe.Mass balance is made on the Eulerian volume system shown in Figure5.15. In the Eulerian volume, fluid flow parameters of interest are given as fields within a defined control volume. The volume of mud in the borehole is expressed as a function of space and time. This volume encompasses the current borehole volume (V bor : i.e the current) and the volume dV = ROP dt π be drilled between initial drilling time (t 0 ) and total drilled time (t 0 + dt). The latter volume is controlled by the rate of penetration (ROP), and the borehole diameter (d b ), which is constrained between the nominal bit size and the borehole Caliper measured at the time of the passing of the electromagnetic tool, typically several tens of minutes after drilling (Figure5.13).

Fig. 5 .

 5 Fig. 5.13 DHAP drilling data overlapped on the seismic section for Holes C0024A and C0006A at the frontal region of the Nankai accretionary prism. Large peaks are predominant when crossing the décollement as seen in hole C0024A, while there are no noticeable peaks in hole C0006A. Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. Red dotted line: Décollement, thick red line on seismic section.

Fig. 5 .

 5 Fig. 5.14 DHAP drilling data overlapped on the seismic section for Hole C0004B at the megasplay region of the Nankai accretionary prism. Large peaks are predominant when crossing the a faults as seen on the Hole C0004B. Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit. Red dotted line: Décollement, thick red line on seismic section.

Fig. 5 .

 5 Fig. 5.15 Schematics of the Eulerian system on which mass balance was conducted (delimited by red dashed lines). The volume drilled between t 0 and t 0 + dt is shown with purple stripes. The arrows show also the fluid flows considered. Both pump flow Q pump and formation flow Q f(positive in the case of influx to the well, negative in the case of outflow) contribute to the flow returning to the surface Q out . Fluid flow into the well is assumed to come from a section between the DHAP sensor and the drill bit. Above, an impermeable mud cake is supposed to be fully developed.

  d b and d p are constant:

  28, whose fluid parameters were adjusted to fit the baseline of the DHAP profile (Figure 5.13). The intensity of this incoming flow depends on at least two factors: (1) the permeability of the rock formation and (2) the pressure difference between the formation and the borehole. Therefore, an estimate of the pore pressure is necessary to quantitatively analyze the inflow data. It should be noted that the choice of pressure loss equation depends on the type of drilling fluid and its rheological properties. These fluids have been categorised in the industry into Newtonian fluids, Bingham plastic fluid, Power Law fluid, Herschel-Bulkley fluid (Guo and Liu 2011):-(a) Newtonian fluids are depicted by Curve an in Figure 5.16. When the flow deforms, the flow resistance rises in a linear fashion, as it does in fluids like water and oil, when the shear rate is proportional to shear stress. (b) Bingham plastic fluid: depicted by Curve b in Figure 5.16. Except for the low shear rate zone, the relationship between shear rate and shear stress is linear. Even with zero shear rate, the shear stress is not zero. Gel strength refers to this non-zero shear stress. As a result, the fluid needs an initial force to distort and move. (c) Power Law fluid: depicted by Curve c in Figure 5.16. Shows that the shear rate and shear stress don't have a linear relationship. (d) Herschel-Bulkley fluid: depicted by Curve d in Figure 5.16. At zero shear rate, the shear stress is not zero, indicating a nonlinear relationship between the two. The fluid needs an initial push to distort and start moving. Deformation raises flow resistance in a non-linear fashion. (e) Dilatant fluid: depicted by Curve d in the Figure 5.16. demonstrates that the shear stress does not linearly increase with increasing shear rate. When deformed, the flow resistance rises more steeply than linearly. Dilatant fluid, created by combining Newtonian fluids with starchlike molecules, is a special kind of fluid.

Fig. 5 .

 5 Fig. 5.16 Different types of fluids found in drilling operations (modified from[START_REF] Guo | Applied drilling circulation systems : hydraulics, calculations, and models[END_REF]

Fig. 5 .

 5 Fig. 5.17 Figure shows a pressure vs depth plot illustrating common pressure terminology along the borehole (Goodwyne 2012).

  other. (a) From the moisture and density (MAD) test done on selected core samples obtained from wells C0024 (B, C, D, F, G). (b) Estimated using the classical Archie's equation (Archie

5. 3 -

 3 Fig. 5.18 (a) Porosity (resistivity derived and from moisture density) and Bulk Density for Hole C0024A (b) Porosity and Bulk Density for Hole C0006A (c) Density, Porosity and Bulk Density for Hole C0004B

  Fig. 5.19 (a) General NCT and abnormal pressure caused by undercompaction (a) D-exponent (b) seismic slowness and (c) Pore pressure (modified from Zhang and Yin (2017))

(

  5.47) P f = P sea + P pg Z (5.48) Where d n is the normal compaction trend, n is an empirical exponent (normally n = 1.2 Zhang and Yin 2017)), d 0 is the shale d-exponent value at the mudline (seafloor), d is a calibration parameter, Z is the TVD below mudline (m).

  (a) Due to no slug test and piezometer data, the screen is the same length as the distance between the bit and the DHAP sensor. (b) the flow lines are horizontal toward or away from the well. Integrating the Darcy rule over the surface (i.e. if the surface is the wall of a cylinder of radius r) and height L on which the flow is flowing (the flux is computed) with the equation valid for all radius r: ⃗ q = -the variation of the hydraulic properties and the changes with respect to individual contribution to the estimation of the permeability was done by differentiating the Hvorslev equation into two parts and using Monte Carlo Simulation. 5.5.1 Partial Differentiation of Variables in the Hvorslev Equation The Hvorslev Equation was modified by varying parameters that could largely affect the permeability results. Therefore, the fluid flow of the DHAP modeling and the pore pressure results were considered because (a) the estimated fluid flow results could have been impacted by several inputs such as the effective mud density and the frictional pressure losses. (b) Pore pressure has several inputs such as ROP, WOB and RPM which vary with the response of the drilled lithologies and other data.

Fig. 5 .

 5 Fig. 5.20 Probability distribution with Pore pressure normal distributed and flow uniformly distributed for Monte Carlo Simulation. This is performed for 10000 simulations

Fig. 6

 6 Fig. 6.1 (a) Enlarged location of Site C0024 and Site C0006 indicated by the red triangle in figure 4.5b. Seismic IL 2437 section interpretation. mbsl = meters below sea level, XL is the cross-line, seismic stratigraphic packages represented as shaded colours, faults (bold red line), Site C0024, C0006 and C0007 location indicated with yellow vertical lines, USB =upper Shikoku basin. (Modified from Tobin et al. (2020))

Fig. 6

 6 Fig. 6.2 (a) Core based lithological classification at Site C0024. (Modified from Tobin et al. (2020))

Fig. 6

 6 Fig. 6.3 (a) The BHA system used to drill Hole C0024A. Main sensors used for the Logging while drilling (LWD) measurements. The red circle shows the DHAP sensor used to record changes in mud pressure during drilling (Tobin et al. 2020).

Fig. 6 . 4

 64 Fig. 6.4 Summary of the geophysical data acquired in Hole C0024A. The plate boundary, also called the décollement, has two strands highlighted by green lines at depths of 813 and 852 mbsf. (a) Lithological units identified onboard by the science party (Tobin et al. 2020) (b) Natural gamma-ray log (GR, API units) from arcVISION (c) Electrical resistivity log (with shallow, medium and deep depths of investigation) in Ω • m (d) Sonic slowness (δt, in µs/ft) from SoniScope (Figure 5.3b). (e) borehole diameter (e-caliper) derived from arcVISION and MicroScope tool (Figure 5.3b) (f) Electrical borehole image from deep resistivity with visible breakouts (g) Picked fractures and faults from the borehole image by the science party onboard[START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF] .

  5b-c), and DHAP first time drilling (Figure6.6) as discussed in the data QC/Processing subsection of the methodology section were implemented in detail. All the required input data (Figure6.6 and 6.7 ) for modelling the flow in this borehole (C0024A) were available except for the inconsistency between the official injected mud weight and the effective mud weight determined from actual DHAP data (Figure6.8). Hence a parametric studies was performed.6.2.1.1 Parametric StudiesDue to this uncertainty about the actual composition of the borehole fluid, we first forwardestimated the hydrodynamic contribution assuming the fluid was purely Newtonian and testing a wide range of viscosity, with the viscosity of water (10 -3 Pa • s) as the lower bound and the maximum viscosity for the tank mud (52 × 10 -3 Pa • s) as the upper bound. To simplify the inversion, we also assume that the mud viscosity and density is uniform within the borehole, as a reasonable assumption, as the mud is circulating during this drilling, uniforming the mud properties along the hole. The results of the parametric studies is discussed in a later section below.

Fig. 6

 6 Fig. 6.5 (a) C0024A mudline check with resistivity and weight on bit logs (b) The plot with the conversion function from measured depth to true vertical depth (c) The measured depth is exactly BIT DEP T H + of f set(7.516m)

Fig. 6 . 6

 66 Fig. 6.6 Time series of drilling data, especially bit depth (part A) and DHAP (part B). DHAP and bit depth tend to increase with time. Analysis is restricted to DHAP data when each depth was first reached by the drill bit (black data). DHAP increases with the true vertical depth of the DHAP sensor (part C), with a linear baseline (in gray) corresponding to an equivalent mud density of 1098 kg/m 3 . Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit.

Fig. 6 . 7

 67 Fig. 6.7 Input data for DHAP modeling in hole C00024A (a) Caliper or borehole diameter (b) Flow rate or pumping rate (c) ROP

  make a negligible contribution to clean mud weight. The effective density results support the assumptions provided in section 5.2.1 for a Eulerian volume system. The difference between the full static pressure model (with clean mud and cuttings) and the clean mud model is attributed to the cuttings in Figure 6.9b. In Figure 6.9b, the static pressure model (clean mud and cuttings) increases slightly above the clean mud pressure, but in a limited way. The parametric investigations (Figure 6.10b and c) further show that the difference between the static pressure model and the clean mud pressure is minor, despite modifying the mud property. The difference remains the same even when the overall mud pressure has increased for the drilling report, as shown in Figure 6.10d. Because of its small contribution, the production of cuttings by drilling cannot alone explain the DHAP anomaly of Figure 6.9b. The hydraulic loss along the borehole (equation 5.28) explains most of the discrepancy between the predicted model and the actual DHAP data (Figure 6.9b).

Fig. 6 . 9

 69 Fig. 6.9 Results derived from the DHAP modeling of hole C0024A. (a) Lithological column, same as in Figure 6.4a. (b) Predicted profiles of the mud pressure at various stages of the modeling: with only the contribution of the clean injected fluid (orange dots), with the additional contribution of the weight of the cuttings (red dots) and the complete model, with the hydraulic losses of the flowing mud (blue dots). For all models, the mud is assumed to be seawater (ρ M W = 1028 kg/m 3 , µ = 1 mPa • s). The DHAP data corresponding to actual drilling times (gray dots) are well fitted by the latter model, except below the 2 décollement zones (dashed green lines). (c) Plot of the difference between the DHAP data (gray dots of in graph (a)) and the prediction from the full DHAP modeling (red dots). The null value, where the model exactly fits the data, is highlighted by a thick vertical line. (d) Flow rate between the formation and the hole. Negative value (to the left of the thick vertical line) corresponds to a flow from the hole to the formation, as expected in normal drilling conditions.

Fig. 6 .

 6 Fig. 6.10 Parametric study of the DHAP modeling for variable mud densities and viscosity values (a) Prediction for clean mud with density 1028 kg/m 3 and viscosity 1 mPa • s. (b) Same as (a) but with a slight change in viscosity 2 mPa • s. (c) Same as (a) but with a slight change in density of 1050 kg/m 3 and fixed viscosity 1 mPa • s. (d) Same as (a) with the properties of the mud incorrectly stated in the drilling report with density of 1350 kg/m 3 and viscosity of 51 mPa • s. (e) Normalized error L 2 of the DHAP prediction above décollement, for a range of different mud properties. The colored dots correspond to the profiles illustrated below: clean mud as water (red), clean mud with varied density (yellow),clean mud with varying viscosity (orange), and drilling report mud properties (pink). (f) Normalized L 2 error for the DHAP prediction for entire borehole length with varying mud properties indicated with coloured dots. Clean mud (red), clean mud with varying density (yellow), clean mud with varied viscosity (orange), and drilling report mud properties (pink).

Fig. 6 .

 6 Fig. 6.11 The input data and the calculated d xc exponent for hole C0024A (a) rate of penetration (ROP) (b) weight on bit (WOB) (c) revolution per minute (RPM) (d) The equivalent circulating density (ECD).

Fig. 6 .

 6 Fig. 6.12 Pore pressure predictions from the d xc exponent (red circled plots [b and c]) and ∆t sonic (blue circled plots [d and e]) and Kitajima and Saffer (2012), Kitajima et al. (2017) method. (a) Logging units (b) Profile of the Eaton d xc coefficient (raw d xc [gray] and d xc averaged sampled for 500 points[red]) along the borehole with an observable deviation to lower values from the NCT line (black) at the top of the subunit 1c (accreted wedge Facies). This particular depth marks the top of the geopressured interval. (c) The pore pressure profile (red line) follows hydrostatic pressure (blue line) within the normal pressured zone and rises significantly above the hydrostatic pressure within the overpressured zone. (d) Profile of the Eaton coefficient ∆ t (raw ∆ t [gray] and sampled ∆t [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (e) The pore pressure profile (red line) follows hydrostatic pressure (blue line) within the normal pressured zone and rises significantly above the hydrostatic pressure within the overpressured zone. (f) Overlay comparing both pore pressure results from Eaton's methods and Kitajima and Saffer (2012), Kitajima et al. (2017) method. The two pressure profiles ( d xc in gray and ∆t in black) almost overlap near the décollement with a localized increase in the pore pressure across the strands while K 0 uniaxial loading pore pressure result in blue color.

Fig. 6 .

 6 Fig. 6.13 Pore pressure prediction from d-exponent method. (a) Logging units. (b) Profile of the Eaton d xc coefficient (raw d xc [gray] and sampled d xc [red]) along the borehole with an observable deviation to lower values of the NCT line (black) at the top of subunit 1c (accreted wedge Facies). This particular depth marks the top of the geopressured interval. (c) The plot of the variable pressure gradients, the hydrostatic pressure gradient in blue coloured line, overburden or lithostatic pressure gradient in green coloured line, averaged pore pressure gradient in red line, raw pore pressure gradient in gray line. (d) The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within normal pressure zone. While within the overpressured zone the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole.

Fig. 6 .

 6 Fig. 6.14 Pore pressure prediction from Eaton modeling based on ∆t sonic method (a) Logging units (b) Eaton ∆t coefficient profile (raw ∆t [gray] and average sampled ∆t [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (c) The plot of the variable pressure gradients, the hydrostatic pressure gradient in blue coloured line, overburden or lithostatic pressure gradient in green coloured line, averaged sampled pore pressure gradient in red line, raw pore pressure gradient in gray coloured line. (d)The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within normal pressure zone. Within the overpressured zone, the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole.

Fig. 6

 6 Fig. 6.16 (a) Porosity prediction from the inversion of the Erickson-Jarrard equation (Equation 5.51) in blue, compared with the MAD (Moisture and Density) data obtained from core samples during expedition 358 (black dots) and with the porosity derived from Archie's equation applied on the resistivity logs (red) (b) Cross-plot of V shale and IGR, and the derived V p with depth along the C00024A borehole length.

Fig. 6 .

 6 Fig. 6.17 Results from the workflow of Kitajima et al. (2017). (a) Porosity prediction from the inversion of the Erickson-Jarrard equation (Equation 5.51) in blue, compared with the MAD (Moisture and Density) data obtained from core samples during expedition 358 (black dots) and with the porosity derived from Archie's equation applied on the resistivity logs (red) (b) Pore pressure prediction assuming Critical State Loading (CSL) conditions (blue). The maximum horizontal stress (red) is also inverted. The hydrostatic (thin blue) and lithostatic (brown) profiles are also shown. The prediction from Eaton methods are displayed in black and gray, for the version using P-wave velocity and d xc coefficients, respectively. (c) Same as in (b) but for K 0 uniaxial loading. The latter pore pressure prediction agrees better with the predictions from Eaton methods.

Fig. 6 .

 6 Fig. 6.18 (a) Permeability decaying with depth, with observe-able step in permeability when crossing the fault core of the décollement.(b) Plot of estimated permeability versus porosity from well C0024a considering the 6 identified logging subunits (c) Plot of estimated permeability versus porosity from well C0024a considering the 6 identified logging subunits with an overlayed of the permeability results from Hvorslev method and the results of permeability from Boutt et al. (2012) using MDT-HF tool at a metric scale..

Fig. 6 .

 6 Fig. 6.19 (a) Permeability from Hvorslev method (b) Permeability after partial differentiation of variables. (c) Permeability after MCS.

6. 3 . 1

 31 Reliability of flow modeling and pore pressure prediction Three independent methods were applied to the C0024A dataset. First, fluid flow modeling from the mud pressure shows that more fluid is coming from the formation into the borehole at greater depths. Second, Eaton's equations (d xc and ∆t) and lastly[START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] and[START_REF] Kitajima | In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002[END_REF]) predict an increase in pore pressure with depth. Unfortunately, no other hydraulic data was obtained from the C0024A well data set, such as pumping tests, long-term observatories, and cores. To compensate, (1) the self-consistency of the behavior of the flow profile and the pore pressure profiles is examined, and (2) the consistency of the hydraulic predictions is checked against other independent proxies.

  looking closely at the fluid flow modeling. Given some possible packoffs (transient blockage of the annulus by rocks collapsing from the borehole wall along the drillstring), which can introduce transient peaks in the pressure of the mud and, therefore, in the flow calculation, the interpretation will be based on the long-term baseline of the flow prediction of Figure 6.20i.DHAP analysis shows that mud pressure is lost to the formation(Figure 6.20i and Figure ??d)

Fig. 6 .

 6 Fig. 6.20 Summary of the hydraulic properties determined along the whole Hole C0024A. (a) seismic section (b) Electrical borehole imaging from deep resistivity (c) Gamma-ray (d) caliper log (e) P-wave velocity (f) deep resistivity (g) tadpoles of the main structures picked on the electrical image (h) raw DHAP data, modeled DHAP, effective mud density, and tank mud density (i) Fluid flow exchange between formation and the borehole (j) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure (k) ratio of P-wave velocity and shear velocity (l) mud fluid temperature.

  .21 b), mechanically weak zones, as indicated by the larger diameter of the borehole (Figure 6.21c), a steady low velocity interval of the P wave (Figure 6.21 d), a low deep resistivity (Figure 6.21e). The fault core was identified as a sharp decrease in resistivity (Figure 6.21e) and a larger caliper (Figure 6.21c).

Fig. 6 .

 6 Fig. 6.21 Summary of the hydraulic properties determined along Hole C0024A, with focus on the décollement zone, below 780 mbsf. (a) Seismic section (b) Electrical borehole imaging from deep resistivity (c) caliper log (d) P-wave velocity, (e) deep resistivity, (f) tadpole diagram of the major structures picked on the electrical image (g) raw DHAP data, modeled DHAP, effective mud density and tank mud density (h) fluid flow exchange between the formation and the borehole (i) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure (j) Estimated effective stress change with depth.

Fig. 7

 7 Fig. 7.1 (a) Enlarged location of Site C0024 and Site C0006 indicated by the red triangle in figure 4.5b. Seismic IL 2437 section interpretation. mbsl = meters below sea level, XL is the cross-line, seismic stratigraphic packages represented as shaded colours, faults (bold red line), Site C0024, C0006 and C0007 location indicated with yellow vertical lines, USB =upper Shikoku basin. (Modified from Tobin et al. (2020))

Fig. 7 . 2

 72 Fig. 7.2 Logs are superimposed on check shot-corrected seismic profile. (a) logging units (b) GR log: LSF = LWD depth below seafloor. VE = vertical exaggeration (Modified from Kinoshita et al. (2009d)).

Fig. 7 . 3

 73 Fig. 7.3 Logs are superimposed on check shot-corrected seismic profile. (a) Caliper log (b) Ring resistivity: LSF = LWD depth below seafloor. VE = vertical exaggeration (Modified from Kinoshita et al. (2009d)).

Fig. 7 . 4

 74 Fig. 7.4 Summary of the geophysical data of Hole C0006A. (a) Electrical borehole image from deep resistivity (b) Seismic section along the borehole (c) Lithological units (d) Natural gammaray log (GR, API units) from arcVISION (e) Electrical ring resistivity log in Ω • m (f) Electrical resistivity log (with shallow, medium and deep depths of investigation) in Ω • m (g) Resistivity derived porosity (h) Picked fractures and faults from the borehole images (i) Sonic velocity (m/s) (modified (Kinoshita et al. 2009d)).

Fig. 7 . 5

 75 Fig. 7.5 Time series of drilling data, especially bit depth (part A) and DHAP (part B). DHAP and bit depth tend to increase with time. Analysis is restricted to DHAP data when each depth was first reached by the drill bit (black data). DHAP increases with the true vertical depth of the DHAP sensor (part C), with a linear baseline (in gray) corresponding to an equivalent mud density of 1146 kg/m 3 . Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit.

Fig. 7

 7 Fig. 7.6 (a) Input data critical for DHAP dynamic modeling in hole C0006A (a) pumping rate (b) ROP data essential for estimating cuttings contribution

  Fig. 7.7 (a) Gamma-ray log from Holes C0006A and C0006B (b) The charts shows the relationship of the correction factor (ratio of the original acquired gamma-ray and the corrected gammaray) on the x-axis and the mud weight on the y-axis of the plot. While the caliper log data is on the z-axis.

Fig. 7

 7 Fig. 7.8 (a) Cross-plot of the gamma-ray correction factor and the mud weight with values of 8.76 ppg at the mudline. (b) The plot of the GR correction and borehole diameter with the five data points derived originally from Figure 7.8a.

Figure 7 .

 7 Figure 7.11d shows the flow between the formation and the borehole. At shallow depths

  borehole is also gauged(Figure 7.12b) within this interval. Below the fault zone there is slight fluid flow from the formation into the borehole. The borehole is enlarged below the fault (Figure 7.12b). Fluid flow explains the slight anomalies in the mud pressure observed below these fault intervals (Figure 7.12c).

Fig. 7 .

 7 Fig. 7.12 (a) Logging units (b) the derived caliper log in inches (c) Flow rate between the formation and the hole.

  7.3.1 Pore Pressure Results: d xc -exponentThe d xc line follows the compaction trend line(Figure 7.14b) between the depth range of 0 to 711 mbsf. It coincides with Unit 1, 2, 3a and 3b (Figure7.14a). In general, there is gradual increase in pressure between 39.37 MPa to 47.8 MPa (Figure7.14c). Overall, the mud pressure is above pore pressure (Figure7.14c) and the hydrostatic pressure between 0-711 mbsf. Therefore, this is considered a normal pressured interval.

Fig. 7 .

 7 Fig. 7.13 The input data and the calculated d xc exponent for hole C0006A (a) rate of penetration (ROP) (b) weight on bit (WOB) (c) revolution per minute (RPM) (d) Profile of the Eaton coefficient d xc (raw d xc [gray] and d xc averaged sampled for 500 points [red]) with overlay of logging units along the borehole.

Fig. 7 .

 7 Fig. 7.14 Prediction of pore pressure from Eaton modeling based on d xc sonic method (a) Logging units (b) Eaton coefficient profile d xc (raw d xc [gray] and average sampled d xc [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (c) The pore pressure profile follows hydrostatic pressure (black line) and less than the mud pressure (orange line) within normal pressure zone. Within the overpressured zone, the pore pressure rises above the mud pressure and hydrostatic pressure.

Fig. 7 .

 7 Fig. 7.15 (a) Cross-plot of V s h and IGR, and (b) the derived profile of V p along the borehole.

Fig. 7 .

 7 Fig. 7.16 Pore pressure prediction from Eaton modeling based on ∆t sonic method (a) Logging units (b) Eaton ∆t coefficient profile (raw ∆t [gray] and average sampled ∆t [red]) along the borehole with an observable deviation to higher values from the NCT line (black) within the upper Shikoku facies. (c) The pore pressure profile follows hydrostatic pressure (blue line) and less than the mud pressure (orange line) within the normal pressure zone. Below the main frontal thrust fault (green dashed line), the pore pressure rises above the mud pressure and hydrostatic pressure to the bottom of the borehole.

Fig. 7 .

 7 Fig. 7.17 (a) Permeability decaying with depth (b) Plot of estimated permeability versus porosity from well C0006A considering the 5 identified logging subunits (c) Plot of estimated permeability versus porosity from well C0006A considering the 5 identified logging subunits with an overlayed of the permeability results from Hvorslev method and the results of permeability from Boutt et al. (2012) using MDT-HF and MDT-DD tool and cores at a metric scale.

  Two independent methods were applied to the C0006A dataset. First, fluid flow modeling from the mud pressure shows that there is a slight inflow of fluids coming from the formation into the borehole at the main frontal thrust. Second, Eaton's equations (d xc and ∆t) predict an increase in pore pressure with depth. Here we discuss the (a) the self-consistency of the behavior of the flow profile and the pore pressure profiles, and (2) the consistency of the hydraulic predictions is checked against other independent proxies.In Hole C0006A, the flow model derived from mud pressure is to be taken with great care, given the lack of original caliper log data. Especially, at the top part, the borehole might have collapsed through time. If the caliper is larger, the function F (z DHAP ) becomes smaller according to equation 5.29. To compensate, an apparently larger flow Q out is need to explain any observed ∆p(DHAP ). Hence, in zones where caliper is underestimated, the flow coming from the formation tend to be overestimated. This is what is expected to occur in the upper part of Hole C0006A (Figure 7.18g). Therefore, our discussion will focus more on the interval (500-887 mbsf ) where we have a reasonable fit between the predicted model and the mud pressure (DHAP) data. With reference to the zero baseline, slight positive fluid flow peaks are observed along the reference depth along the borehole. Below the the frontal thrust the interval fluid flow peaks are unlike whats seen above the fault. Each of the peaks below the fault is consistent with the localised increase in pore pressure from Eaton ∆t method (Figure 7.18h). Comparing the two pore pressure results along the borehole based on depths above and below the main frontal thrust fault: (i) above 711 mbsf, the pore pressure is generally hydrostatic, even though with intervals that are slightly above hydrostatic pressure and below hydrostatic as shown by d xc exponent results. The pore pressure values within this interval are below the mud pressure. (ii) Below the fault, the two pore pressure results increases above the mud pressure when crossing the main frontal thrust fault. The pore pressure values predicted from d xc are higher than the pore pressure values from the ∆t method (Figure 7.18h) . The quality of the results could be affected by two factors generally (a) The d xc computation could be affected by the quality of the weight of the bit (WOB) (b) The reliability of the ∆t method depends on the accurate derivation of the sonic velocity data with the Errickson-Jarrard method. The lack of the original Vp limited the implementation of the empirical relationship between Vp, porosity, and effective stress method by Kitajima and Saffer (2012), Kitajima et al. (2017).The pore pressure results from the two Eaton methods are compared with the volume of fluid flow along the borehole. The slight flow of fluid into the hole, especially when crossing the main frontal thrust fault, suggests that the magnitude of formation pressure might not be large enough as depicted by the d xc method. Evidences from borehole observatories(Ariyoshi et al. 2021, Dutilleul and[START_REF] Dutilleul | Tidal analysis of the nantroseize long term borehole monitoring system (ltbms) pore pressure records at the nankai margin, sw japan[END_REF]) also showed that there is no over pressure along this borehole.

Fig. 7 .

 7 Fig. 7.18 Summary of the hydraulic properties determined along Hole C0006A, with focus on the main frontal thrust fault zone, below 550 mbsf. (a) Seismic section (b) logging subunits (c) caliper log (d) Gamma ray log (e) P-wave velocity, (f) deep resistivity (g) fluid flow exchange between the formation and the borehole (h) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure. (i) permeability decay with depth (j) estimated effective stress .

  Figure 7.18e) with a low deep resistivity (Figure 7.18e) value which depicts the true formation fluids resistivity. There is no observable change in the temperature gradient (Figure 7.18 j) along this hole. This is suggestive that there might not be any evident of flow into the borehole as observed in hole C0024A when the décollement was crossed. 7.4.2 Hydraulic structure of the Frontal thrust Fault Our new compiled hydraulic information with a focus on the faults is compared with pre-existing information. Site C0006 is structurally complex with several thrust faults have been identified along the borehole. Here, the hydraulic structure of the main frontal thrust fault is closely examined with evidence from hydraulic properties and the geophysical well logs (Figure 7.19). The main frontal thrust fault consist of the fault core (711 mbsf) and the damage zone. The fault core marks the top of the unit IV (trench channel facies) and top of a mechanically weak damage zones (Figure 7.19c). It is identified with the sharp decrease in resistivity (Figure 7.19f) and V P (Figure 7.19e). The damage zone is characterised as mechanically weak zones as indicated by the larger diameter of the borehole, a steady low P-wave velocity interval (Figure 7.19d), a conductive zone with low deep resistivity (Figure 7.19e). The footwall is marked with a step in pore pressure (Figure. 7.19g), directly overlain by a mud dominated facies hanging wall (Figure. 7.19c) and with slight moderate effective stress within the damage zone (Figure. 7.19j) . The footwall is about 173.5 m.

Fig. 7 .

 7 Fig. 7.19 Summary of the hydraulic properties determined along Hole C0006A, with focus on the main frontal thrust fault zone, below 500 mbsf. (a) Seismic section (b) Electrical borehole imaging from deep resistivity (c) caliper log (d) P-wave velocity, (e) deep resistivity, (f) tadpole diagram of the major structures picked on the electrical image (g) raw DHAP data, modeled DHAP, effective mud density and tank mud density (h) fluid flow exchange between the formation and the borehole (i) Pore pressure estimated from the 2 Eaton methods, hydrostatic and overburden pressure. (j) Estimated effective stress change with depth .

  2 mbsf) as mass transport deposits, Subunit IIB (96.2-160.3 mbsf) as thrust sheet and characterised with fractures and shear deformation, Subunit IIC (160.3-236.4 mbsf) as thrust sheet and strongly deformed, while Subunit IID (236.4-323.8 mbsf) is highly deformed and fractured. (c) Unit III is considered as sand layer based on the borehole image. Based on seismic data it correlates with underthrust sediments (Figure 8.2). 8.1.1.1 LWD and BHA Configuration:-Hole C0004B The entire borehole length was logged with a LWD/MWD system. The LWD tools used for this hole include geoVISION, sonicVISION, PowerPulse MWD, seismicVISION and adnVISION

Fig. 8

 8 Fig. 8.1 (a) Enlarged location of Site C0004: 3D seismic profile. VE = vertical (Modified from Kinoshita et al. (2009b))

Fig. 8

 8 Fig. 8.3 (a) The BHA system used to drill Hole C0004B. Main sensors used for the Logging while drilling (LWD) measurements. The red circle shows the DHAP sensor used to record changes in mud pressure during drilling (Kinoshita et al. 2009b).

Fig. 8 . 4

 84 Fig. 8.4 Summary of the geophysical data of Hole C0004B. (a) Electrical borehole image from deep resistivity with visible breakouts. (b) inline seismic section (c) Lithological units identified onboard by the science party (Kinoshita et al. 2009b) (d) Natural gamma-ray log (GR, API units) (e) Ring and deep resistivity log in Ω • m (f) ultrasonic caliper in inches (blue curve) (g) Sonic velocity (m/s) from SoniScope (h) Porosity derived from porosity (i) Picked fracture dips (red tadpoles) and black tadpoles as the bedding from resistivity at bit image (Kinoshita et al. 2009b).

Fig. 8 . 6

 86 Fig. 8.6 Time series of drilling data, especially bit depth (part A) and DHAP (part B). DHAP and bit depth tend to increase with time. Analysis is restricted to DHAP data when each depth was first reached by the drill bit (black data). DHAP increases with the true vertical depth of the DHAP sensor (part C), with a linear baseline (in gray) corresponding to an equivalent mud density of 1051 kg/m 3 . Color-coded: All DHAP data recorded throughout the duration of the drilling. Black data: DHAP data when each depth was first drilled by the drill bit.

Fig. 8 . 7

 87 Fig. 8.7 Input data for DHAP modeling in hole C0004B (a) Caliper or borehole diameter (b) Flow rate or pumping rate (c) ROP

Fig. 8 . 9

 89 Fig. 8.9 Summary of the hydraulic properties determined along Hole C0004B, with focus on the megasplay zone. (a) Electrical borehole imaging from deep resistivity (b) Logging units (c) caliper log (d) Gamma ray log (e) P-wave velocity, (f) deep resistivity, (g) fluid flow exchange between the formation and the borehole .

Fig. 8 .

 8 Fig. 8.10 Illustration of areas with high pore pressure situated around the megasplay fault by Tsuji et al. (2014), this can imply high pore pressure at the splay fault is connected to the megasplay..

Fig. 9 . 1

 91 Fig. 9.1 Summary of the fluid flow result at the three drill site location (Site C0024, C0006, C0004). The plot shows the fluid flow profile along the boreholes at megasplay faulting and the toe of the Nankai accretionary prism.

Fig. 9 . 2

 92 Fig. 9.2 Correlations of the published pore pressure ratio derived by[START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] with the results of the pore pressure ratio results estimated at the frontal thrust region and the megasplay fault region (areas in green rectangles). The results are expressed as pore pressure ratio. Modified after[START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF].

Fig. 9

 9 Fig. 9.3 (a) General perspective of the seismogenic zone and aseismic zones. The seismogenic zone can be identified by the following: mechanically, by large regions or asperities that, when they fail, result in a frictionally unstable or velocity-weakening situation manner. The aseismic area updip and downdip of the seismogenic zone or within the frictionally stable or velocitystrenghtening manner (Lay 2016). (b) Regional map of the Nankai subduction zone. Filled blue circles and open light blue circles and are of SVLFEs epicentral locations Asano et al. (2008), The records of SVLFEs offshore are from 2015 and the fault areas hosting SSEs from 2011-2016 (circled in purple) (Araki et al. 2017). Modified after Takemura et al. (2019).

  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

	Symbol or acronym Meaning
	BHA	BottomHole Assembly (equipment at the base of the drill string)
	d p d b		Diameter of the drill string (pipe or BHA, depending on depth considered) LIST OF TABLES Diameter of the borehole (= caliper) CHAPTER 1
	9.3 (a) General perspective of the seismogenic zone and aseismic zones. The seismo-DHAP DownHole Annular Pressure (Mud Pressure) ECD Equivalent Circulating Density Introduction
	WOB 5.1 Mudline correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weight on Bit downhole genic zone can be identified by the following: mechanically, by large regions ROP Rate of Penetration
	or asperities that, when they fail, result in a frictionally unstable or velocity-weakening situation manner. The aseismic area updip and downdip of the seis-Rotation Per Minute of the drill string 5.2 DHAP depth correction table . . . . . . . . . . . . . . . . . . . . . . . . . . . . RPM Q out Total flow rate flowing upwards in the annulus above DHAP sensor 6.1 MWD/LWD tools, Expedition 358. MWD = measurement while drilling, LWD = HL Function relating hydraulic loss to flow rate) 1.1 Study Context
	mogenic zone or within the frictionally stable or velocity-strenghtening manner (Lay 2016). (b) Regional map of the Nankai subduction zone. Filled blue circles Flow rate of clean mud pumped into the borehole Subduction zones are plate tectonic boundaries where an oceanic plate with more density (the Q pump logging while drilling (modified from Tobin et al. (2020)) . . . . . . . . . . . . . 6.2 Mudline correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q f Additional flow from the formation. P sea Seawater Pressure at the seafloor or mudline subducting plate) thrusts beneath a less dense continental plate (the overriding plate) (Figure 2.1).
	and open light blue circles and are of SVLFEs epicentral locations Asano et al. Porosity of the rock formation 6.3 Permeability statistics in well C0024A . . . . . . . . . . . . . . . . . . . . . . . MW ϕ Mud weight (2008), The records of SVLFEs offshore are from 2015 and the fault areas host-7.1 Mudline correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . mbsf Meters below seafloor
	ing SSEs from 2011-2016 (circled in purple) (Araki et al. 2017). Modified after Reynolds number (dimensionless) 7.2 Permeability statistics in well C0006A . . . . . . . . . . . . . . . . . . . . . . . Re dp f hydraulic pressure losses Takemura et al. (2019). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 7.3 Permeability statistics with sensitivity analysis in well C0006A . . . . . . . . . . HL Hydraulic loss
	B r i	Blasius cofficient 8.1 MWD/LWD tools, Expedition 316. MWD = measurement while drilling, LWD = Radius of influence of a pressure disturbance within the borehole
	v	logging while drilling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Average mud velocity within the borehole annulus
	Z ρ eff 8.2 Mudline correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . True Vertical Depth (TVD) Effective density of the mud, cuttings included
	ρ MW	"Mud Weight", i.e. density of the clean mud, free of cuttings
	ρ g		Density of the rock matrix (=grain density)
	ρ r		rock formation density
	ρ w	Density of the fluid filling the pores of the rock , assumed to be seawater
	ρ dyn+f luid f low	Pressure loss due to dynamic effects and fluid flow
	µ		Dynamic viscosity of the mud
	P f	Pore fluid pressure
	σv	Total overburden stress
	σv g	Overburden gradient
	(P hg )	Hydrostatic pressure gradient
	d x	d-exponent
	d xc	Corrected d-exponent
	σ e		Effective stress
	S w	Water saturation
	m		Cementation exponent
	n		Saturation exponent
	R w	Brine resistivity
	R t	True resistivity
	∆t	Transit time in shales
	∆t n	Transit time in shales (normal pressure condition)
	∆t m	Transit time in the shale matrix
	∆t ml	Transit time at the mudline (Z = 0)
	c		Compaction parameter
			xxiv xxv xxvi

Table 5

 5

			.2 DHAP depth correction table
	Hole	Total borehole length (mbsf) DHAP sensor position (m) borehole length
				from DHAP sensor
	C0024A 871	7.56	863.44
	C0006A 885	10.066	874.934
	C0004B 400	20.

Table 6 .

 6 1 MWD/LWD tools, Expedition 358. MWD = measurement while drilling, LWD = logging while drilling (modified from[START_REF] Tobin | NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust[END_REF]

	MWD/LWD	Measured data and units	Total Sensor length
			on BHA (m)
	MicroScope 675	Natural gamma-ray (gAPI), resistivity (Ωm), 5.89
		resistivity image, caliper (in)
	arcVISION 675	Downhole Annular pressure (MPa),	11.56
		temperature ( • C), resistivity(Ωm),
		gamma-ray (gAPI),
	TeleScope 675	Natural gamma-ray (gAPI), torque (kNm),	19.99
		downhole weight on bit (kN),
		rate of penetration (m)
	SonicScope 675	V p (µs/ft), V s (µs/ft)	29.99
	seismicVISION 675 Seismic velocity, time-depth	34.48
		relationship, corridor stack

Table 6

 6

			.3 Permeability statistics in well C0024A
	Mean	Std	Min	25%	50%	75%	Max
	3.424e-08 9.306e-07 3.627e-16 7.909e-13 3.366e-12 4.556e-11 4.842e-05

Table 7

 7

	.1 Mudline correction

Table 7 .

 7 2 Permeability statistics in well C0006A

	Mean	Std	Min	25%	50%	75%	Max
	3.600e-11 4.545e-10 2.606e-14 4.524e-13 2.274e-12 1.570e-11 2.525e-08

Table 7

 7

	.3 Permeability statistics with sensitivity analysis in well C0006A
	Min	25%	50%	75%
	5.261002e-14 2.450987e-11 2.083763e-10 3.917581e-09

Table 8 .

 8 1 MWD/LWD tools, Expedition 316. MWD = measurement while drilling, LWD = logging while drilling.

	MWD/LWD	Measured data and units	Sensor length
			on BHA (m)
	geoVISION	Natural gamma-ray (gAPI), resistivity (Ωm),	4.6
		resistivity image, caliper (in)
	SonicVISION	V p (µs/ft), V s (µs/ft)	12.22
	PowerPulse MWD Downhole Annular pressure (MPa), direction, inclination 20.77
	seismicVISION	check shot interval velocity Natural gamma-ray	25.42
	adnVISION 675	density, porosity, ultrasonic caliper	31.67

Several factors have been identified to be responsible for elevated pore pressure in subduction zones: mineral dehydration, rapid burial and tectonic loading [START_REF] Moore | Fluids in acretionary prisms, rev[END_REF]. Laboratory frictional experiments have provided evidence that earthquake propagation is more favorable through these mechanically weak zones with low effective normal stress conditions [START_REF] Faulkner | Stuck in the mud? earthquake nucleation and propagation through accretionary forearcs[END_REF] . Seismogenic faulting's upper limit may also be affected by pore pressure, which is thought to control the effective stress and consolidation state. [START_REF] Moore | New insights into deformation and fluid f low processes in the nankai trough accretionary prism: Results of ocean drilling program leg 190 geochemistry geophysics geosystems[END_REF][START_REF] Scholz | Earthquakes and friction laws[END_REF].

inverted from Equation 5.51 to compute the void ratio e: e = ϕ 1 -ϕ

(5.53)

Void ratio e is the volume of voids relative to the volume of solids, whereas porosity ϕ is the volume of voids relative to the total volume.

The volume of shale (V sh ) in Equation 5.51 is computed from the gamma-ray index (IGR):

Where GR LogSignal is the Gamma ray log in the subunit/interval of interest, GR CleanRock is the Gamma ray spectrum of the cleanest formation, GR Shale is the Gamma ray spectrum of the shale interval/zone. Four different methods were used to calculate the V shale .

The [START_REF] Clavier | Quantitative Interpretation of Thermal Neutron Decay Time Logs: Part I. Fundamentals and Techniques[END_REF]:

(5.55) [START_REF] Stieber | Pulsed neutron capture log evaluation -louisiana gulf coast. proceedings of fall meeting of the society of petroleum engineers of aime[END_REF]:

(5.56) [START_REF] Larionov | Radiometry of boreholes[END_REF] for tertiary rocks:

V sh = 0.083(2 (3.7 * IGR) -1) (5.57) [START_REF] Larionov | Radiometry of boreholes[END_REF] for old rocks:

The second step consists using experimental relationships between effective stress and porosity, derived from consolidation experiments [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] under "triaxial" conditions (the principal stresses are such that σ 1 > σ 2 = σ 3 ). In this case, two quantities characterize stress: (1) the effective mean stress p ′ = 1 3 (σ 1 + σ 2 + σ 3 ) -P f and the differential stress:

. Two-end members are considered :

1. vertical consolidation without horizontal strain (K 0 ). This loading is similar to the loading occurring during sediment burying in a sedimentary basin. In these conditions, [START_REF] Kitajima | Elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the Nankai Trough subduction megathrust[END_REF] found the following relationships for samples from the Shimano basin, in the Philippine Sea Plate :

(5.59) q = 0.375 × p ′ (5.60) 2. Critical state loading (CSL) in which the sample is subjected to a maximum differential Integrating over the surface on which the flow is flowing (we compute a flux)

(5.65)

Further integrating Equation 5.66 and applying it for 2 cases;

Hvorslev equation (Equation 5.71) used is obtained by subtracting Equation 5.67 from Equation 5.68.

Re-arranging the terms in the equation by making permeability subject of the equation.

Where, k s the permeability of the porous medium (m 2 ), µ is the dynamic viscosity (P a • s), Q f is the total discharge rate (m 3 /s), L is the length of the screen (m), r w is the wellbore radius (m), P f is the pore pressure in the undisturbed formation (Pa), calculated using the Eaton method,

The radius of influence (r e ) is arbitrarily taken as r e = 200 r w . However, if we took the value as different as r e = 20 r w , the estimated permeability would only be divided by ln(10) = 2.3. This assumption is correct to assess an order of magnitude of permeability.

Consistently, with our assumption that the flow is coming from the space between the drill bit and the DHAP sensor (assuming that a fully-formed mud cake prevents fluid exchange between the borehole and the formation above the DHAP sensor), the distance L is taken as the distance between the drill bit and the DHAP sensor. This distance varies for each hole, but its order of magnitude is about 10 m.

Sensitivity Index

The sensitivity index is used to investigate the relationship between input and output parameters in a simulation or model by defining the sensitivity of each parameter (Andrea et al. 2010, Emanuele and[START_REF] Emanuele | Sensitivity analysis: A review of recent advances[END_REF]. In this work, the sensitive index of each parameter was determined with the permeability equation (Equation 5.71). Parameters that have nonnegative signs are considered extremely and proportionally sensitive to an increase in the value of permeability (k), while parameters that have negative signs are considered to be less susceptible to a decrease in permeability (k), and the remaining category is considered neutrally sensitive (with zero relative sensitivity). The below equation was used to determine the sensitivity indices [START_REF] Andrea | Variance based sensitivity analysis of model output. design and estimator for the total sensitivity index[END_REF], Emanuele and Elmar 2016):

where X i represents permeability (k) Equation 5.71. P i represents variable corresponding parameters in Equation 5.71.

In Hole C0006A, the damage zone of the main frontal thrust fault could serve as a fluid flow path for the fluids from the underthrusting sediments or propagating from down-dip to the trench.

If the fault core of the main frontal thrust fault is not sealed then there might be vertical dewatering of fluids from underthrusting sediments into the accretionary prism, hence contributing to the fluid budget within the accretionary prism. In combination with other fluid sources (Davis et al. 1983, Saffer andTobin 2011) in the accretionary prism, the thrust faults along the borehole C0006A could serve as major fluid pathways and pore pressure dissipation ways. This could be the reason why high pore pressure and large fluid flow was not observed along Site C0006.