Louis Géli
email: louis.geli@ifremer.fr

Pierre Henry

• M Namik Çağatay

A review of 20 years (1999-2019) of Turkish-French collaboration in marine geoscience research in the Sea of Marmara

This paper retraces the history and main achievements of the ongoing Turkish-French collaboration in marine geoscience research in the Sea of Marmara, which was initiated in the aftermath of the 1999, Izmit and Duzce earthquakes. The collaboration resulted in nine large oceanographic cruises along with six recovery operations involving diverse vessels, and in the participation in two major EU-funded programmes (ESONET-NoE and MARSITE) and to one bilateral project, e.g. the MAREGAMI Project, co-

1 Introduction: the situation before the 1999 earthquakes

During the second half of the XXth century, after [START_REF] Ketin | Relations between general tectonic features and the main earthquake regions in Turkey[END_REF] first established the relations between the general tectonic features and earthquake activity in Turkey, the North Anatolian Fault Zone received considerable attention (e.g. Şengör 1979; [START_REF] Barka | Seismo-tectonic aspects of the North Anatolian Fault Zone[END_REF].

The westward migration of earthquake ruptures (starting from the 1939 Erzincan earth-quake in the easternmost end of the NAF) had already been established in the late 1970's (e.g. [START_REF] Töksöz | Space-time migration of earthquakes along the North Anatolian fault zone and seismic gaps[END_REF] and in 1997, when Stein, Barka and Dieterich estimated "a 12% probability for a large event (M > 6.7) to occur south of the major western port city of Izmit in the next 30 years" [START_REF] Stein | Progressive failure on the North Anatolian fault since 1939 by earthquake stress trigger-ing[END_REF], it was nothing but a premonition. What a pos-teriori became an astonishingly successful prediction when the earthquake struck in Izmit 2 years later, [START_REF] Barka | The 17 August 1999 Izmit earthquake[END_REF] was based on the detailed mapping of surface slip and fault geometry of the North Anatolian Fault, that resulted from 3 decades of continuous geophysical and geological onshore investigations (e.g. [START_REF] Şengör | Strike-slip faulting and related basin formation in zones of tectonic escape: Turkey as a case study[END_REF][START_REF] Barka | The North Anatolian Fault Zone[END_REF][START_REF] Barka | Slip distribution along the North Anatolian fault associated with large earthquakes of the period 1939 to 1967[END_REF]; [START_REF] Barka | Strike-slip fault geometry in Tur-key and its infuence on earthquake activity[END_REF], and references therein).

On-land studies also benefited from spectacular advances in space geodesy that revolutionized earth sciences in the 1990's (e.g. Le [START_REF] Pichon | Geodetic determination of the kinematics of central Greece with respect to Europe implications for eastern Mediter-ranean tectonics[END_REF][START_REF] Reilinger | Global Positioning System measurements of present-day crustal movements in the Arabia-Africa-Eurasia plate collision zone[END_REF]. In contrast, by the end of the century, there was a tremendous gap in knowledge between the onshore and offshore domains in the Marmara Region. Consequently, the question of the westward propagation of the North-Anatolian Fault into the Sea of Marmara was entirely open. Indeed, prior to 1995, the knowledge of the Marmara sea-floor was essentially based on: hydrographic maps from the Office of Navigation, Hydrography and Oceanography of Turkey (SHOD); sparse borehole data from the oil industry; and a few, widely spaced seismic reflection lines, acquired by the Turkish Petroleum Corporation (TPAO), by Mara-thon Oil, and by the Institute of Marine Sciences and Tech-nology (IMST) of Dokuz Eyul University (DEU) in Izmir (e.g. [START_REF] Smith | High-resolution seismic profiling in the Sea of Marmara (northwest Turkey): late quaternary sedimen-tation and sea-level changes[END_REF][START_REF] Ergün | Structural relationship between the Sea of Marmara Basin and the North Anatolian Fault Zone[END_REF][START_REF] Özel | Marmara Denizinin neotektonik yapısının jeofisik yöntemlerle incelenmesi[END_REF][START_REF] Wong | The Sea of Marmara: a boundary sea in an escape tectonic regime[END_REF]; see also review in Palabiyik et al. 2020;[START_REF] Görür | Origin of the Sea of Marmara from Neogene to Quaternary paleogeographic Evolution of its frame[END_REF] and references therein). In 1995, the research vessel (R/V) Piri Reis of IMST acquired 3300 km of single channel, seismic profiles and 1000 km of deep-towed boomer profiles. These data were used to draw-up a bathymetric map revealing three deep (> 1000 m) basins "bounded by and internally cut by numerous steeply, dip-ping faults" [START_REF] Aksu | Oscillating Quaternary water levels of the Marmara Sea and vigorous outflow into the Aegean Sea from the Marmara Sea Black Sea drainage corridor[END_REF]. In 1997, (the Scientific and 2 In the aftermath of the 1999 earthquakes:

the NATO conference on May 14-17th, 2000

The catastrophic death toll (> 20,000 victims) of the Izmit and Düzce earthquakes in August and November 1999 created an immense worldwide shock and emotion.

Meanwhile, an earthquake hit Athens on September 7th, 1999, causing 143 deaths. These three events compelled international collaboration for future earthquake risk assessment in the region. A first conference was held at ITU in 1999, during which different strategies were presented to fill the knowl-edge gap in the Sea of Marmara (e.g. Le [START_REF] Pichon | The Marmara Fault and the future Istanbul earthquake[END_REF]. A second conference was held in Istanbul on 14-17 May 2000 as a NATO international Advanced Research Seminar with the primary objective of providing a platform for open discussions and for fostering international collaborative studies. Coordinated by TUBITAK, the NATO seminar was attended by 83 scientists from Turkey, France, Italy, USA, UK, Japan, Germany and Greece [START_REF] Görür | Integration of earth science research on the Turkish and Greek 1999 earthquakes[END_REF].

Earthquakes were known to have occurred in the Sea of Marmara near Istanbul in the past (e.g. [START_REF] Ambraseys | The seismicity of Turkey and adjacent areas; a historical review[END_REF]. The question that was on everyone's mind was: when and where will the next earthquake strike? As a corollary: how will the stress be transferred in the submarine domain, between the 1999 rupture in Izmit and the 1912 rupture in Saros? More generally, from a geodynamic point of view, the major scientific issue was to understand how the North-Anatolian Fault, a major strike-slip plate boundary, propagates into the Aegean extensional system. Different views-or different groups of ideas-having different implications for earthquake hazard assessment were discussed during the NATO seminar. The first group considered the prevailing role of extension [START_REF] Parke | Active faults in the Sea of Marmara, western Turkey, imaged by seismic reflection profiles[END_REF] and viewed the Marmara Trough as a pull-apart basin with oblique extension (e.g. Barka and Kadinsky-Cadé 1988;[START_REF] Armijo | Westwards propagation of the North Anatolian Fault into the NorthAegean Timing and kinematics[END_REF][START_REF] Hubert-Ferrari | Seismic hazard in the Marmara Sea region following the 17 August 1999 Içzmit Earthquake[END_REF]. The second considered the prevailing role of strain localization along a transform, through-going, strike-slip fault following a small circle corresponding to the relative motion between the Eur-asian and Anatolian plates (e.g. Le [START_REF] Pichon | The active main Marmara fault[END_REF]. And the third group considered the Marmara Area as "a negative flower structure controlled by a single master fault", identi-fied as the northern branch of the North Anatolian Fault Zone (e.g. [START_REF] Okay | An active, deep marine strike-slip basin along the North Anatolian Fault in Turkey[END_REF][START_REF] Alpar | Integration of earth science research on the Turkish and Greek 1999 Earth-quakes[END_REF].

After the NATO seminar, a number of research programmes in marine geoscience research were set up, through bi-lateral and tri-lateral collaborations between Turkey and a large number of foreign countries, including the United States, Italy, Germany, Japan and France. The Turkish-French cooperation programme, initiated by Xavier Le Pichon and Çelal Şengör, was coordinated by INSU and TÜBİTAK. It started 3 months later, once Xavier Le Pichon had obtained the funds for the mapping cruise of R/V Le Suroit (12 September-3 October 2000) from the ECHO division of the European Community.

3 Actions under the Turkish-French collaboration in marine research since 1999

The collaboration resulted in nine large oceanographic cruises and six recovery operations involving diverse vessels since 1999. All cruises included scientists from other countries (e.g. Germany, Italy, USA, Spain, China, etc.). The collaboration largely benefited from the assistance of the Turkish Coast-Guard Command (Sahil Güvenlik) and was supported by EU-funded programmes, which in turn allowed for access to marine facilities at different institutions in Europe. Most particularly, the research vessel Urania, from the Italian National Research Council, was used on many occasions within the framework of EU supported projects, e.g. ESONET-NoE and MARSITE (see [START_REF] Gasperini | Fluid flow, deformation rates and the submarine record of major earthquakes in the Sea of Marmara, along the North-Anatolian Fault system[END_REF] and cruise reports in http://www.ismar.cnr.it/prodotti/repor ts-campagne). The collaboration also benefited from the knowledge gained through other bi-lateral programmes, such as between Turkey and the USA. For instance, the TAMAM cruise on R/V Piri Reis in 2010 (co-P.I.s Çifci from Dokuz Eylül University and Steckler, from Lamont Doherty Earth Observatory) offered unique opportunities for scientific exchanges within the marine geoscience community.

The cruises that were conducted on French vessels were given a DOI (Digital Object Identifier), providing immediate access to the associated publications. The full list is detailed in Table 1. A brief, chronological summary is provided here below:

• 2000: The MARMARA cruise (Chief scientists: Le Pichon, Şengör, Demirbağ) on R/V Le Suroît provided the first, detailed integral mapping coverage of the Marmara seafloor. In addition to high-resolution, multibeam bathymetry and imagery data (grid-mesh size of 38 m), deep-towed side scan sonar data were collected along with shallow seismic data using a surface sparker source and a deep-towed seismic source (e.g. [START_REF] Rangin | Marine atlas of the Sea of Marmara (Turkey). Data collected on board R. V Le Suroît[END_REF][START_REF] Pichon | The active main Marmara fault[END_REF]Imren et al. 2001;[START_REF] Demirbağ | Investigation of the tectonics of the Main Marmara Fault, by means of deeptowed seismic data[END_REF]. See more and publication list on: https://doi.org/ 10.17600/20100. • 2001: The SEISMARMARA cruise on R/V Nadir (Chief Scientists Hirn, Singh & Taymaz), was conducted with the support of R/V Simik-1 of MTA and divided into two legs. Leg I (P.I. Hirn & Taymaz) resulted in the acquisition of 4000 km of large penetration, reflection seismic data along with refraction seismic data (OBS and onshore stations), to determine the deep (> 10 km) structure below the Sea of Marmara (e.g. Laigle et al. 2008;[START_REF] Bécel | Moho, crustal architecture and deep deformation under the North Mar-mara Trough from the SEISMARMARA Leg-1 offshoreonshore reflection-refraction survey[END_REF][START_REF] Bécel | North Marmara Trough architecture of basin infill, basement and faults, from PSDM reflection and OBS refraction seismics[END_REF][START_REF] Bayrakci | 3-D sediment-base-ment tomography of the Northern Marmara trough by a dense OBS network at the nodes of a grid of controlled source profiles along the North Anatolian fault[END_REF]. Leg II (P. I.

Singh & Taymaz) provided high-resolution seismic imaging of the Eastern Sea of Marmara (Çınarcık Basin and its margins) along 80, densely spaced (0.5-0.9 km) lines (e.g. [START_REF] Carton | Seismic imaging of the three-dimensional architecture of the Çınarcık Basin alongh the North Anatolian Fault[END_REF]. See more and publication list on: https://doi.org/10.17600/1080050. & Çağatay) on R/V L'Atalante with ROV Victor provided fine-scale mapping of active fault scarps along with 3.5 kHz sediment profiling and long cores for paleo-seismological studies (e.g. [START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF]. ROV dives also provided the first visual observations of cold seeps along the fault (e.g. [START_REF] Zitter | Cold seeps along the main Marmara fault in the Sea of Marmara (Turkey)[END_REF] Henry; Şengör; Çağatay), provided the first visual observations of gas seeps and in situ gas samples, leading to the characterization of fluid emissions (e.g. [START_REF] Tryon | Pore fluid in the North Ana-tolian Fault in the Sea of Marmara: a diversity of sources and processes[END_REF] and to the evidence that the North Anatolian Fault was cutting through a gas and oil system related to the Thrace Basin oil field [START_REF] Bourry | Free gas and gas hydrates from the Sea of Marmara Turkey: chemical and struc-tural characterization[END_REF]. See more in § V.2 and publication list on: https://doi.org/10.17600/7010070. sediment, water and gas samples were taken using ROV Victor-6000 (e.g. Ruffine et al. 2018a, b, c). During leg 3, additional 3.5 kHz and sediment cores were acquired at key sites for paleo-seismological studies. See more in § IV.2 and in § V.4, and publications list on: https://doi.org/ 10.17600/14000500. 4 Contribution to studies on the tectonic evolution of the NAF in the Marmara Region

Seafloor mapping and basins structure

The bathymetric map after the Marmara cruise on R/V Le Suroit in 2000 [START_REF] Rangin | Marine atlas of the Sea of Marmara (Turkey). Data collected on board R. V Le Suroît[END_REF]) revealed the main physiographic and structural features of the Marmara seafloor.

It replaced previous maps based on single-beam data (e.g. [START_REF] Aksu | Anatomy of the North Anatolian Fault Zone in the Marmara Sea, western Turkey: extensional basins above a continental transform[END_REF] and was used as a common reference for all subsequent studies in marine geosciences. In the absence of information on the deep crustal structure, this map gave rise to contrasting interpretations, emphasizing model ele-ments as diverse as the importance of extension (e.g. [START_REF] Parke | Interaction between fault-ing and sedimentation in the Sea of Marmara, western Turkey[END_REF][START_REF] Parke | The Sea of Marmara: a twodimensional seismic reflection profile data archive[END_REF]; the presence of negative flower structure related to a deeply rooted, single master fault (e.g. [START_REF] Aksu | Anatomy of the North Anatolian Fault Zone in the Marmara Sea, western Turkey: extensional basins above a continental transform[END_REF][START_REF] Yaltirak | Tectonic evolution of the Marmara Sea and its surrounding[END_REF]; block or platelike junctions or geometry [START_REF] Okay | Active faults end evolving strike-slip basins in the Marmara Sea, northwest Turkey: a multichannel seismic reflection study[END_REF]; pull-apart or transtensional jog activity at diverse scales [START_REF] Armijo | Slip partitioning in the Sea of Marmara pull-apart: a clue to propagation processes of the North Anatolian Fault[END_REF]; evolution to strike-slip localization (Le [START_REF] Pichon | The active main Marmara fault[END_REF]. Shortly afterwards, based on the modelling of the geodetically observed secular velocity field in northwestern Turkey, [START_REF] Meade | Estimates of seismic poten-tial in the Marmara Sea region from block models of secular deformation constrained by Global Positioning System meas-urements[END_REF] suggested that strain accumulation was highly localized in the Marmara region, confirming "the existence of a Marmara block delimited to the north by the northern branch of the North Anatolian fault (with the Marmara branch close to pure dextral strike slip over its whole length) and to the south by the southern branch" (quote from Le [START_REF] Pichon | The North Anatolian Fault in the Sea of Marmara[END_REF]. Based on a complementary data set, including sparker and deep-towed highdefinition seismics [START_REF] Demirbağ | Investigation of the tectonics of the Main Marmara Fault, by means of deeptowed seismic data[END_REF] and additional multichannel seismic lines from TPAO, [START_REF] Rangin | Strain localiza-tion in the Sea of Marmara: propagation of the North Anatolian Fault in a now inactive pull-apart[END_REF] further proposed that most of the N 120° trending normal faults framing the deep basins are now inactive, discarding the hypothesis that strain was distributed along presently active, extensional blocks. These findings eventually lead Şengör et al. (2005) to propose "a new look" of the North Anatolian Fault, including its westward continuation into the Sea of Marmara.

The interpretation of the deep reflection and wide-angle seismic data collected in 2001 during the SEISMARMARA cruise provided information that lead to embed all the different models into a broader and deeper context, discarding hypotheses involving crustal faults only with predominance given to extension, while supporting large, through-going strike-slip faults, cutting the whole crust down to the Moho, as quoted from Laigle et al. (2008): "Sections across the eastern half of the North Marmara Trough (NMT), crossing the Çınarcık and Imralı basins, revealed several faults hav-ing varying strike and varying proportions of normal and strike-slip displacement, that might be viewed as petals of a large scale negative flowerstructure that spreads over a width of 30 km at surface and is rooted deeper in the litho-sphere. Under the Central Basin a very thick sediment infill is revealed and its extensional bounding faults are active and imaged as much as 8 km apart down to 6 km depth. We interpret them as two deep-rooted faults encompassing a foundering basement block, rather than being merely pulled-apart from a jog in a strike-slip above a décollement".

Further analysis of the dense grid of multichannel seismic profiles along the southwestern margin of the North Marmara Trough reveals a dipping reflector through the upper crust with tilted basement blocks on top, while wide-angle reflection and refraction seismics using OBS reveal a significant and sharp reduction in Moho depth, in the order of 5 km beneath both the eastern and western rims of the North Marmara Trough, with a more progressive crustal thinning from the south [START_REF] Bécel | Moho, crustal architecture and deep deformation under the North Mar-mara Trough from the SEISMARMARA Leg-1 offshoreonshore reflection-refraction survey[END_REF]). More generally, the crys-talline basement exhibits sharp topography across the Cen-tral High, the Kumburgaz Basin and the eastern tip of the Çınarcık-North Imralı Basin in an unexpected way with respect to the sea-bottom depressions [START_REF] Bécel | North Marmara Trough architecture of basin infill, basement and faults, from PSDM reflection and OBS refraction seismics[END_REF]. Several large, tilted, basement blocks separate the deep basins as between the Çınarcık and Imralı basins, suggesting that the process of deformation partitioning may have taken place over one or even two faults across the North Marmara Trough, which may have changed activity in time and space [START_REF] Bécel | North Marmara Trough architecture of basin infill, basement and faults, from PSDM reflection and OBS refraction seismics[END_REF].

These findings were further detailed by the 3-D tomographic inversion of first arrival times of seismic shots recorded by the OBS and land-stations network [START_REF] Bayrakci | 3-D sediment-base-ment tomography of the Northern Marmara trough by a dense OBS network at the nodes of a grid of controlled source profiles along the North Anatolian fault[END_REF]. Similar sedimentary thickness (> 6 km) is observed below the North Imralı Basin (located on the southern continental shelf) than below its northern neighbours. The Tekirdağ and Central basins form a 60-km-long basement depression, as no basement high is found below the Western High (Note: this general picture was recently updated by [START_REF] Tarancıoğlu | Three-dimensional seismic tomographic imaging beneath the Sea of Marmara: evidence for locked and creeping sections of the Main Marmara Fault[END_REF], using additional OBS data collected by TÜBITAK collected in 2006, and enriched by a discussion on the significance of the results for identifying locked and creeping sections along the Main Marmara Fault). Last but not least, gravity data collected during the MARSITECRUISE were merged with a global gravity model derived from satellite altimetry and were inverted, taking into account constraints from seismic refraction and receiver function studies, to quantify the amount of crustal extension and determine the geometry of the Moho over the whole Marmara area [START_REF] Kendé | Moho depth and crustal thinning in the Marmara Sea region from gravity data inversion[END_REF].

Re-evaluation of the seismo-tectonics of the Sea of Marmara within the Marsite Project

The EU-funded Marsite Project (P. I. Nurcan Meral Özel, www.marsite.eu) provided the unique opportunity to reevaluate the seismo-tectonics of the Sea of Marmara, using all available marine data, including those collected during the Turkish-French collaboration programme: seismic lines, [START_REF] Görür | Tectonic events responsible for shaping the Sea of Marmara and its surrounding region[END_REF] high-resolution bathymetry, chirp profiles, stratigraphic data from sediment cores, drill wells, etc.). The GIS fault map provided as Marsite deliverable D7.2 is based primarily on multichannel data and thus represents the fault geometry at a few kilometres depth within the syntectonic basins (Şengör et al. 2014). Faults are catego-rized as deep-rooted faults that also cut through the seafloor, blind faults, and shallow syn-sedimentary faults.

For the purpose of hazard assessment, information from sediment sounder profiles and high-resolution bathymetry is essen-tial to assess fault activity and deformation style, but the geometry to be preferentially considered for the construction of earthquake scenarii is that of the faults at crustal level. Where crustal structure is poorly imaged below the Sea of Marmara Basin, the three-dimensional fault geometry at depths [START_REF] Hergert | Slip-rate variability and distributed deformation in the Marmara Sea fault system[END_REF][START_REF] Hergert | Geomechanical model of the Marmara Sea region-II. 3D contemporary background stress field[END_REF] was largely inferred from seismic imaging in the syntectonic basins [START_REF] Bécel | North Marmara Trough architecture of basin infill, basement and faults, from PSDM reflection and OBS refraction seismics[END_REF][START_REF] Carton | Seismic imaging of the three-dimensional architecture of the Çınarcık Basin alongh the North Anatolian Fault[END_REF]). Fault models were further constrained, by taking relocated seismicity into consideration [START_REF] Bulut | Microseismicity at the North Anatolian Fault in the Sea of Marmara offshore Istanbul, NW Turkey[END_REF][START_REF] Karabulut | Evolution of the seismicity in the eastern Marmara Sea a decade before and after the 17 August 1999 Izmit earthquake[END_REF][START_REF] Schmittbuhl | Seismic-ity distribution and locking depth along the main Marmara fault Turkey[END_REF].

The work conducted within Marsite (e.g. Özel et al. 2017) resulted in the identification of which faults are cutting the seafloor (and thus currently active) and which ones do not (see deliverable D7.1 report in [START_REF] Henry | Marnaut Scientic Party[END_REF]. Several now inactive (or marginally active) fault systems appear to have played a role in the structuring of the basins. This is for example the case for the system of extensional detachments and tilted blocs between the southern shelf and the deep basins [START_REF] Bécel | Moho, crustal architecture and deep deformation under the North Mar-mara Trough from the SEISMARMARA Leg-1 offshoreonshore reflection-refraction survey[END_REF] for the South Marmara Fault, connecting the Gemlik Bay to the Dardanelles (Le Pichon et al. 2003) and for an en-échelon system lying in prolongation of the Izmit fault [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault[END_REF]). On the other hand, several arrays of normal faults observed at a shallow level in the sediment are not deeply rooted, and could result from the interaction of slope instability with underlying crustal deformation [START_REF] Zitter | Distribution, morphology and triggers of submarine mass wasting in the Sea of Marmara[END_REF]).

Current knowledge on the tectonic history of the NAF System in the Marmara Region

After 20 years of continuous marine investigations, the Marmara Sea floor has become one of the most well-known submarine domains on Earth. The currently active fault system only comprises a fraction of the faults that have been active since the initiation of the North Anatolian Fault, and this situation may account for the initial diversity of tectonic interpretations in the Sea of Marmara. Interpretations of deep seismic and bathymetric data have now largely converged to a scheme with a master fault-the Main Marmara Fault, or MMF, connecting the Izmit segment that ruptured in 1999 with the Ganos segment that ruptured in 1912-and subsidiary fault branches (e.g. Le [START_REF] Pichon | The active main Marmara fault[END_REF][START_REF] Armijo | Slip partitioning in the Sea of Marmara pull-apart: a clue to propagation processes of the North Anatolian Fault[END_REF][START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF][START_REF] Demirbağ | Investigation of the tectonics of the Main Marmara Fault, by means of deeptowed seismic data[END_REF][START_REF] Rangin | Strain localiza-tion in the Sea of Marmara: propagation of the North Anatolian Fault in a now inactive pull-apart[END_REF][START_REF] Polonia | Holocene slip rate of the North-Anatolian Fault beneath the Sea of Marmara[END_REF][START_REF] Seeber | Rapid subsidence and sedimentation from oblique slip near a bend on the North Anatolian transform fault in the Mar-mara Sea Turkey[END_REF][START_REF] Meghraoui | Paleoseismology of the North Anatolian Fault at Gü zelköy (Ganos segment, Turkey): Size and recurrence time of earthquake ruptures west of the Sea of Marmara[END_REF]), a view further supported by studies of seismicity (e.g. Sato et al. 2014;[START_REF] Örgülü | Seismicity and source parameters for small-scale earthqukes along the splays of the North Anatolian Fault (NAF) in the Marmara Sea[END_REF]). Remaining differences arise partly from the characteristics of the data sets used, and notably the resolu-tion and depth of investigation of seismic data.

The present-day knowledge on the tectonic evolution of the Marmara Region is summarized in the three following papers, that use all available data listed in § IV. 2014) establish a sequence of faulting events in the northern Marmara shear zone from the Miocene to present, confirming that the shear zone has been evolving and becoming narrower since its inception. While a tendency is observed towards strain localization in a 7-to-10 km swath along the Main Marmara Fault, some deformation is still accommodated over a broader area, and subsidiary faults display a complex history of fault activation/deactivation (Şengör et al. 2014). Using a comparable approach, based on a dataset enriched with seismic lines from the Marmara southern shelf, Le [START_REF] Pichon | The South Marmara Fault[END_REF] completed the picture by mapping the evolution of the recently documented "South-Marmara Fault" (SMF) since the late Pliocene (Fig. 2). 2) The initiation of the North Anatolian Fault in the Sea of Marmara was preceded by deformation affecting the Eocene-Oligocene Thrace basin, which peaked during the Late Miocene to Early Pliocene. "The beginning of the formation of a localized plate boundary occurred between 4.5 and 3.5 Ma at the location of the present Sea of Marmara by the initiation of a shear zone" (quote from Le [START_REF] Pichon | Propagation of a strike-slip plate boundary within an extensional environment: the westward propagation of the North Anatolian Fault[END_REF]. The first part of the formation of the Sea of Marmara was thus purely extensional. With crustal thinning possibly due to ductile lower crustal flow. At the present rate of areal extension derived from GPS data, about 5 Ma would be needed to explain the observed crustal thinning, which is probably slightly older than the onset of deformation related to the North Anatolian Fault. Hence, the areal extension rate was probably not constant, but may have peaked in the early Pliocene, as suggested by Le [START_REF] Pichon | Propagation of a strike-slip plate boundary within an extensional environment: the westward propagation of the North Anatolian Fault[END_REF]. The pre-sent strike-slip system that today cuts across the whole of Sea of Marmara began to develop after 2.5 Ma and evolved since then, more likely through a progressive reorganization related to basin growth than as a sudden event [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault[END_REF][START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault Marmara Sea Turkey[END_REF][START_REF] Akbayram | Evidence for a minimum 52 ± 1km of total offset along the northern branch of the North Anatolian Fault in northwest Turkey[END_REF]. No major change in tectonic regime at the scale of the Sea of Marmara is required over the last 400 ka, although local variations in the rates of subsidence/uplift and in the strain distribution around the main fault are documented [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault[END_REF][START_REF] Grall | Slip rate estimation along the western segment of the Main Marmara Fault over the last 405-490 ka by correlating mass transport deposits[END_REF].

5 Contribution to paleo-oceanography and paleo-seismology

Paleoceanography

The Sea of Marmara is currently connected to the Aegean Sea and Black Sea through the Dardanelles and Bosphorus straits having sill depths of 65 and 35 m, respectively. Its hydrography is characterized by a two-way water exchange between the waters of the adjacent seas, with a permanent pycnocline located at -25 m between the two water masses.

Due to the shallowness of the sills, oceanographic and environmental conditions alternated between marine and lacus-trine phases during the Quaternary glacial and interglacial glacio-eustatic sea level changes. Despite its interesting oceanographic setting, palaeoceanographic studies on the Sea of Marmara were scarce before the international col-laborations, including the Turkish-French programme (e.g., [START_REF] Stanley | Late Quaternary water exchange between the eastern Mediterranean and the Black Sea[END_REF][START_REF] Aksu | Oscillating Quaternary water levels of the Marmara Sea and vigorous outflow into the Aegean Sea from the Marmara Sea Black Sea drainage corridor[END_REF].

Earlier studies hypothesized the late Quaternary palaeoceanographic history mainly on the basis of the global sea level curve rather than proper chronological dating. These studies were carried out on short gravity cores recovered mostly on board Turkish research vessels such as the MTA Sismik-1, Piri Reis and Bilim. A few of these studies, using radiocarbon datings, showed that Sea of Marmara was fresh-brackish lake during the late glacial period and that marine conditions were fully established ~ 12 ka BP. It was also dis-covered that the marine connection was followed by depo-sition of two sapropel units during the Holocene [START_REF] Çağatay | A mid-late Holocene sapropelic sediment unit from the southern Marmara Sea shelf and its palaeoceanographic significance[END_REF], 2000;[START_REF] Aksu | Last glacial-Holocene paleoceanography of the Black Sea and Marmara Sea: stable isotopic, foraminiferal and coccolith evidence[END_REF].

The Turkish-French collaboration, using the French research vessels, recovered giant piston cores that sampled the older strata reaching back to the marine isotope stage (MIS-6). One ~ 29 m long core (MD01-2430) recovered from the Western High extended back to 70 ka, and provided important paleoceanographic, paleoclimatic and tephra records [START_REF] Vidal | Hydrology in the Sea of Marmara during the last 23 ka: Implications for timing of Black Sea connections and sapropel deposition[END_REF][START_REF] Valsecchi | Vegetation dynam-ics in the Northeastern Mediterranean region during the past 23,000 year: insights from a new pollen record from the Sea of Marmara[END_REF]Çağatay et al. 2015a). The records revealed that the Sea of Marmara was fresh to brackish lake during MIS-4 to MIS2 and that a fully marine connection occurred at 12.6 ka BP. Using the robust age model for the core, the Holocene main and the upper sapropel units were dated to 12.3-5.7 ka and 5.4-2.7 ka BP, respectively (Çağatay et al. 2015a). Earlier, however, [START_REF] Vidal | Hydrology in the Sea of Marmara during the last 23 ka: Implications for timing of Black Sea connections and sapropel deposition[END_REF] documented that the initial marine connection took place at 14.7 ka BP, and that the hydrologic change from lacustrine to marine conditions was gradual. These authors also documented an increase in the salinity and sea surface temperature in the Sea of Marmara after the Younger Dryas (12.7-11.5 ka BP).

Palynological analysis of core MD01-2430 by [START_REF] Valsecchi | Vegetation dynam-ics in the Northeastern Mediterranean region during the past 23,000 year: insights from a new pollen record from the Sea of Marmara[END_REF] documented the existence of climate changes during the Last Glacial Maximum (LGM)-Holocene. Cold and dry conditions prevailed during the LGM, the late glacial until the Bølling-Allerød interstadial and during the Younger Dryas, whereas warm and moist climate conditions occurred during the early and middle Holocene. Multi-proxy analyses of the core by Çağatay et al. (2015a) showed a robust correlation with the north Greenland ice core (NGRIP) isotope (NGRIP members, 2004), Tenaghi Philippon pollen (Mül-ler et al. 2011) and Black Sea Ca [START_REF] Nowaczyk | Dynam-ics of the Laschamp geomagnetic excursion from Black Sea sediments[END_REF] records during MIS-4 to MIS-1, indicating strong teleconnections between the NW Anatolia, Eastern Europe and the North Atlantic. Core MD01-2430 also revealed some interesting tephra layers that were matched with the 3.9 ka Avellino eruption of Vesuvius-Somma, the 22 kyr BP Cape Riva eruption of Santorini, and 39.3 ka Campanian Ignimbrite eruption of Campi Flegrei (Napoli) (Çağatay et al. 2015a). Study of piston cores recovered during the 2004 MAR-MARA-VT 67 cruise effectively elucidated the late Quaternary evolution of the northern shelf of the Sea of Marmara, and the water-exchanges via the Bosporus Straits during the Holocene [START_REF] Eriş | The timing and evolution of the post-glacial transgression across the Sea of Marmara shelf south of Istanbul[END_REF]. Core MD-04-2745 on the northern shelf sampled two additional sapropel units depos-ited during the MIS-5. Data from this core, together with other staggered cores, allowed to date seismic stratigraphic units observed in Chirp profiles on the shelf, and determine the lake/sea level changes during the late Quaternary. Core MD04-2750 recovered a submerged channel-levee com-plex succession from the Marmara outlet of the Bosporus channel. Analyses by [START_REF] Eriş | The timing and evolution of the post-glacial transgression across the Sea of Marmara shelf south of Istanbul[END_REF] concluded that the undulating unit in the succession was formed by a Black Sea outflow during the Younger Dryas, and that the radial delta lobe was sourced by a creek east of the Bosporus dur-ing Mid-Holocene rather than by the Black Sea outflow. However, these conclusions were later challenged by His-cott et al. (2008), [START_REF] Eriş | Reply to Comment on "The timing and evolu-tion of the post-glacial transgression across the Sea of Marmara shelf south of Istanbul" by Hiscott et al[END_REF]. Utilizing pore-water isotope and salinity data from MARMARA-VT 67 cruise cores, [START_REF] Aloisi | Freshening of the Marmara Sea prior to its post-glacial reconnection to the Mediterranean Sea[END_REF] carried out modelling studies that showed that the Marmara "Lake" waters prior to the latest marine reconnection were brackish (S ∼ 4‰), and that following MIS-5, the freshening of the Marmara Sea by the Black Sea spill-out started at least 50 ka BP.

Up to 20 m-long piston cores were recovered from different parts the Sea of Marmara during MARSITECRUISE of R/V Pourquoi pas? in 2014. Some cores were specifically located to date some seismic reflector surfaces and study paleoceanographic conditions during MIS-5 and 6 [START_REF] Çağatay | Morphotectonics of the Sea of Marmara: basins and highs on the North Anatolian continental transform plate boundary[END_REF][START_REF] Kendé | Moho depth and crustal thinning in the Marmara Sea region from gravity data inversion[END_REF] PhD thesis), and for palynological analysis (S. Leroy and D. Biltekin, pers. com.). Multi-proxy analyses of two Marsite cores from the İmralı Basin produced important results on stratigraphy and environmental and hydrographic changes during MIS-5 and 6 (Çağatay et al. 2019). The chronology of cores showed that the regional "Red" seismic reflecting surface of [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault Marmara Sea Turkey[END_REF] corresponded to a hiatus at the MIS 5/MIS 4 transition (70-80 ka BP) rather than 110 ka BP, estimated by these authors. The paleontological and geochemical results indicated that fresh-brackish lacustrine conditions existed in the Sea of Marmara during MIS 6 and that the transition to marine conditions during Termination II took place at ~ 134 BP. Marine conditions prevailed during MIS 5, except during MIS-5b stadial and three sapropels were deposited during MIS-5 interstadials a, c, and e. The geochemical proxy records of the MARSITE cores showed good correlation with the NGRIP oxygen isotope and regional terrestrial and marine records, and the strong influence of the North Atlantic climatic events. The core data also suggested that the Dardanelles sill depth was at ~ -75 ± 5 m during the reconnection at Termination II and at -55 ± 5 m during most of MIS 5, while the Bosporus sill depth was more stable at -35 to -40 m, similar to that of the current day depth (Çağatay et al. 2019).

Paleo-seismology

Sub-aqueous paleoseismological studies are important for obtaining recurrence interval of earthquakes over several thousand years. This information is in turn important for probabilistic earthquake risk assessment. Such studies in the Sea of Marmara started after the 1999 İzmit earthquake [START_REF] Polonia | Exploring submarine earthquake geology in the Marmara Sea[END_REF][START_REF] Parsons | Recalculated probability of M_7 earthquakes beneath the Sea of Marmara, Turkey[END_REF][START_REF] Mchugh | Submarine earthquake geology along the North Anatolian Fault in the Marmara Sea, Turkey: a model for transform basin sedimentation[END_REF]), but increased considerably after Turkish-French geoscience collaboration, with the recovery of long piston cores and seafloor observations using ROV Victor and manned submersible Nautile [START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF][START_REF] Beck | Late Quaternary co-seismic sedimentation in the Sea of Mar-mara's deep basins[END_REF][START_REF] Beck | Estimation of successive coseismic vertical offsets using coeval sedimentary events -application to the southwestern limit of the Sea of Marmara's Central Basin (North Anatolian Fault)[END_REF][START_REF] Drab | The earthquake sedimentary record in the western part of the Sea of Marmara Turkey[END_REF][START_REF] Çağatay | Sedimentary earthquake records in the İzmit Gulf Sea of Marmara[END_REF]Eris et al. 2012;[START_REF] Mchugh | Seafloor fault ruptures along the North Anatolia Fault in the Marmara Sea, Turkey: link with the adjacent basin turbidite record[END_REF]. Up to 38 m-long cores allowed to obtain earthquake records extending back to several thousands of years, and to match the more recent ones with the 2500-yearlong historical records of the Marmara Region [START_REF] Ambraseys | The seismicity of Turkey and adjacent areas; a historical review[END_REF][START_REF] Ambraseys | The seismic activity of the Marmara Sea Region over the last 2000 years[END_REF][START_REF] Guidoboni | Catalogue of Ancient Earthquakes in the Mediterranean area up to 10th century[END_REF]Gui-doboni and Comastri 2005).

The sedimentological study of the 2001 MARMACORE cruise giant piston cores revealed that a considerable part of the sedimentary succession in the deep Marmara basins consists of turbidites, most of which were likely triggered by earthquakes. Radiocarbon dating of the cores indicated high sedimentation rates (1-3 m/ka) in the deep Marmara basins, which could allow the individual seismic events to be distinguished in the sedimentary succession (e.g., [START_REF] Beck | Late Quaternary co-seismic sedimentation in the Sea of Mar-mara's deep basins[END_REF][START_REF] Beck | Estimation of successive coseismic vertical offsets using coeval sedimentary events -application to the southwestern limit of the Sea of Marmara's Central Basin (North Anatolian Fault)[END_REF][START_REF] Eriş | Late-Pleistocene to Holocene sedimentary fills of the Çınarcık Basin of the Sea of Marmara[END_REF]. Moreover, [START_REF] Beck | Late Quaternary co-seismic sedimentation in the Sea of Mar-mara's deep basins[END_REF] also discovered a 5-8-m thick, 16-ka-old turbidite-homogenite (T-H) unit of tsunamogenic origin in cores MD01-2431 and MD01-2429 in the Central Basin. This distinct stratigraphic marker was clearly visible in the 3.5 kHz seismic profiles, and used for sedimentation, slip and subsidence rates estimations. For example, Çağatay and Uçarkuş (2018) estimated average sedimentation rates over the last 16 ka in the depocentre and basin margin of the Central Basin at 2.70 mm/year and 1.05 mm/year, respectively, and the sub-sidence rate in the depocentre at 6 mm/year.

The 4-5 m of piston cores and push cores, together with video recordings and micro-bathymetric mapping of fault scarps, obtained during the 2002 Marmarascarps cruise on board R/V L'Atalante with the ROV Victor provided important data on the paleoseismology (e.g. [START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF]. Considering the fresh fault scarps in the Tekirdağ Basin and the Western High, [START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF] concluded that the 1912 Mw = 7.4 Şarköy-Mürefte earthquake rupture extended all the way to the Central Basin. Uçarkuş (2010) used microbathymetry data and 210 Pb/ 137 Cs dating of the disturbances in the push cores to show that the 1912 earthquake rupture extended at least to the Central High. [START_REF] Drab | The earthquake sedimentary record in the western part of the Sea of Marmara Turkey[END_REF] analysed the long cores of the Marmarascarps cruise for turbidite paleoseismology and documented the records of the) 1894, 1509, 1343 and 740 earthquakes in the Çınarcık Basin; 1912 (Şarköy-Mürefte), 1766, 1343 (or 1354), 1063 and 557) earthquakes in the Tekirdağ Basin; and evidence of the 1912 Şarköy-Mürefte earthquake in the Western High and the Central Basin (all dates are Common Era).

The Marnaut Cruise in 2007 provided 9-10 m long piston cores, ~ 1 m long water/sediment interface cores with undisturbed tops for paleoseismological studies as well as direct observations along the NAF. More than 20 MARNAUT piston and interface cores located in different basins and the highs were studied for turbidite paleoseismology using multi-proxy analyses and radionucleide datings by Çağatay et al. (2013) and three piston cores in the Central Basin by [START_REF] Mchugh | Seafloor fault ruptures along the North Anatolia Fault in the Marmara Sea, Turkey: link with the adjacent basin turbidite record[END_REF]. More recently, the MARSITE-CRUISE with R/V Pourquoi pas? recovered ~ 20-m-long cores from the Kumburgaz and İmralı basins. A 21-m-long Marsite core, dating back to 6.1 ka from the Kumburgaz Basin on the Central High segment, included 28 seismo-turbidite units, providing an average recurrence interval of 220 years [START_REF] Yakupoğlu | Sedimentological and geochemical evidence for seismoturbidite generation in the Kumburgaz Basin, Sea of Marmara: Implica-tions for earthquake recurrence along the Central High Segment of the North Anatolian Fault[END_REF]. However, the age-depth model indicated that the latest 1200-year-long record was missing, and thus, the archive for the recent major historical events such as of 1766 CE and 1509 CE could not be ascertained. Surprisingly, the MARSITE cores from the İmralı Basin contained almost no seismoturbidites, possibly suggesting that low magnitude earthquakes in the south did not produce any mass-wasting from the basin's southern margin.

The main conclusions of all the above seismoturbidite studies can be summarized as follows: (i) turbidites triggered by Mw > 6.5 earthquakes commonly consist of a coarse basal turbidite layer and an overlying homogeneous-looking mud layer (homogenite), forming together a turbiditehomogenite (TH) unit. The TH units range from a few to tens of centi-metres in thickness, and usually have a sharp and erosional basal contact; (ii) the average recurrence interval over the last 5-6 ka is 200-300 years, compatible with GPS velocities and geological slip rates. However, the interval between two consecutive events can be highly variable; (iii) the records from different areas appear to be specific to the events on the nearby fault segment; (iv) basin depocentres and perched basins on highs are the best locations to obtain a complete earthquake record.

6 Fluid emissions, faulting and seismicity

Fluid emissions and their origin

Gas emissions in the water column in response to the August 17th, 1999 earthquake were observed by [START_REF] Alpar | Underwater signatures of the Kocaeli earthquake (August 17th 1999)[END_REF] less than 1 month after the main shock, based on highresolution (Sparker) seismic data from the Gulf of Izmit and from the northern escarpment of the Cinarçik Basin. In the deeper basins of the Sea of Marmara, investigations in February 1999 with sediment sampling and a towed camera during the R/V Meteor, cruise 44 (e.g. [START_REF] Halbach | Methane in Sediments of the Deep Marmara Sea and Its Relation to Local Tectonic Structures[END_REF]Kusçu et al. 2008) and in September 2002 with remotely operated vehicle during the MARMARASCARPS cruise on R/V Pourquoi pas? found indications of fluid expulsion along the active faults. These visual observations include: bubbles immediately beneath the seafloor at reduced sediment patches, notably on the Western High; active chimneys and carbonate crusts in the Central Basin and in the Tekirdag Basin-where fluid outflow is vis-ible, due to a contrast in optical indices [START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF]. After the first findings of [START_REF] Halbach | Migration of the sulphate-methane reaction zone in marine sediments of the Sea of Marmara-can this mechanism be tectonically induced?[END_REF], a shallow sulfate-methane reaction zone (SMRZ) was also detected in sediment cores sampled during the MARMARA-VT cruise from the same areas at depths of 3-5 m below the seafloor. Based on these observations, the high-resolution seismic data (3.5 kHz) and deep-towed side-scan profiles collected by Le [START_REF] Pichon | The active main Marmara fault[END_REF], were reprocessed, confirming the prob-able presence of trapped gases within the uppermost sedimentary sequences and gas escapes into the water column (S. Dupré, Pers. Comm.).

After numerous fluid vents and related features had been discovered along the MMF, it became clear that the Sea of Marmara was an ideal site (possibly unique on Earth) to study the coupling between deformation, seismic activity, fluid flow and gas expulsion within an active, submarine transform fault. As a consequence, from 2007 onwards, a massive effort was undertaken with support of the EU, to test the hypotheses that "the fluid properties within the MMF evolve with time throughout an earthquake cycle", and that "some of these changes, or their consequences, can be recorded at the seafloor". In total. three major expeditions were organized with French vessels to study the submarine hydrogeological system and how it was coupled to the tectonic system, through the interaction of fluid pressure and stress state. These expeditions are described here after.

• MarNaut ( 2007): The MARNAUT expedition of R/V L'Atlante in 2007 [START_REF] Henry | Marnaut Scientic Party[END_REF], was mainly dedicated to: (i) in situ deep-sea sampling of fluids (including gas), carbonate crusts and bacterial mats along the NAF submerged system using the Nautile submersible (e.g. [START_REF] Bourry | Free gas and gas hydrates from the Sea of Marmara Turkey: chemical and struc-tural characterization[END_REF][START_REF] Crémière | Methane-derived authigenic carbonates along the North Anatolian fault system in the Sea of Marmara (Turkey)[END_REF][START_REF] Burnard | Constraints on fluid origins and migration velocities along the Marmara Main Fault (Sea of Marmara, Turkey) using helium isotopes[END_REF]); (ii) gas emission mapping using single-beam 38 kHz echo-sounder (e.g. [START_REF] Géli | Gas emissions and active tectonics within the submerged section of the North Ana-tolian fault zone in the Sea of Marmara[END_REF]); (iii) heat flow measurements (e.g. [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault[END_REF]); (iv) sediment coring; and (v) deployment of seafloor instrumentation (e.g. [START_REF] Tary | Microevents produced by gas migration and expulsion at the seabed: a study based on sea bottom recordings from the Sea of Marmara[END_REF][START_REF] Tryon | Quantifying submarine fluid seep activity along the North Anatolian Fault Zone in the Sea of Marmara[END_REF]. Free gases were sampled at three sites, showing different composition and origin. Gases from the Western High-where gas hydrates were found-and from the Central High were predominantly methane gas of thermogenic origin and genetically related to the Thrace hydrocarbon Basin, whereas microbial methane gases were predominant in the easternmost Çınarcık Basin [START_REF] Bourry | Free gas and gas hydrates from the Sea of Marmara Turkey: chemical and struc-tural characterization[END_REF][START_REF] Ruffine | Geochemical dynamics of the natural-gas hydrate system in the Sea of Marmara offshore Turkey[END_REF]. In contrast, dissolved gas extracted from water samples taken near the Tekirdag escarpment exhibit a predominance of methane, an abundance of CO2 and a mantle helium signature, suggesting a high permeability conduit from the mantle to the seafloor [START_REF] Burnard | Constraints on fluid origins and migration velocities along the Marmara Main Fault (Sea of Marmara, Turkey) using helium isotopes[END_REF]. The diversity of pore fluids com-position reflect mixing of fluids from different sources [START_REF] Tryon | Pore fluid in the North Ana-tolian Fault in the Sea of Marmara: a diversity of sources and processes[END_REF][START_REF] Ruffine | Pore water geochemistry at two seismogenic areas in the Sea of Marmara[END_REF] varying with time, based on long-term monitoring of fluid properties [START_REF] Tryon | Quantifying submarine fluid seep activity along the North Anatolian Fault Zone in the Sea of Marmara[END_REF]. Microbial chemosynthetic communities [START_REF] Ritt | Integrative study of a new cold-seep mussel (Mollusca: Bivalvia) associated with chemosynthetic symbionts in the Marmara Sea[END_REF]) and carbonate crusts [START_REF] Crémière | Methane-derived authigenic carbonates along the North Anatolian fault system in the Sea of Marmara (Turkey)[END_REF] were found to develop near venting sites, in relation to the anaerobic oxidation of methane (AOM) and other heavier hydrocarbons. Lipid biomarkers in authigenic carbonates were investigated, providing evidence that AOM mediated by methanotrophic archaea (ANME) and their bacterial partners, is the major process leading to sequestration of methane-derived carbon [START_REF] Chevalier | Authigenic carbonates at cold seeps in the Mar-mara Sea (Turkey): a lipid biomarker and stable carbon and oxy-gen isotope investigation[END_REF][START_REF] Chevalier | Microbial methane turnover at Marmara Sea cold seeps: a combined 16S rRNA and lipid biomarker investigation[END_REF]). • Marmesonet (2009) and PirMarmara (2010): The Marmesonet expeditions (Leg 1 and 2) of R/V Le Suroit in 2009 were dedicated to: (i) the systematic mapping of the water column using the shipborne multibeam echosounder for gas bubble detection (e.g. [START_REF] Dupré | Tectonic and sedimentary controls for widespread gas emissions in the Sea of Marmara, Results from systematic, ship-borne multibeam echosounder water column imageries[END_REF]Grall et al. 2018a); (ii) high-resolution bathymetric mapping (using an Autonomous Underwater Vehicle -AUV-allowing a resolution of 2.5 m) at three potential candidate sites for the deployment of multi-disciplinary observatories [START_REF] Grall | Slip rate estimation along the western segment of the Main Marmara Fault over the last 405-490 ka by correlating mass transport deposits[END_REF]); (iii) the testing of seafloor instrumentation for the monitoring of fluid properties: piezometers, OBSs and acoustic bubble detector (e.g. [START_REF] Tary | Microevents produced by gas migration and expulsion at the seabed: a study based on sea bottom recordings from the Sea of Marmara[END_REF][START_REF] Bayrakci | 3-D sediment-base-ment tomography of the Northern Marmara trough by a dense OBS network at the nodes of a grid of controlled source profiles along the North Anatolian fault[END_REF][START_REF] Tsang-Hin-Sun | Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers[END_REF]); (iv) high resolution, 3D seismic imaging to characterize the plumbing system below the Western High, where gas hydrates were found, in association with a mud volcano (e.g. [START_REF] Thomas | Contribution of high-resolution 3D seismic near-seafloor imaging to reservoir scale studies: application to the active north Anatolian Fault Sea of Marmara[END_REF]). The 3D seismic imaging was complemented by high-resolution 2D profiling using a longer streamer during the PirMarmara Cruise of R/V Piri Reis (e.g. Saritas et al. 2018). These cruises helped identify the complex connections between the fault network and the manifestations of fluid activ-ity at the seabed (gas emissions, cold seeps, pockmarks, mud volcanoes, sediment unstabilities, etc.). It seems one outcome of the Marmesonet cruise is an estimation of the MMF geological slip rate from offset geomorphological features and mass transport deposits [START_REF] Grall | Slip rate estimation along the western segment of the Main Marmara Fault over the last 405-490 ka by correlating mass transport deposits[END_REF]).

• MarsiteCruise (2014): The MarsiteCruise expedition was divided into three legs. The first was devoted to the deployment of submarine instrumentation to collect long period series of geodetic and geophysical data (in col-laboration with GEOMAR) to determine correlations between fault motion, intermittent fluid discharge and micro-seismicity [START_REF] Sakic | No significant steady state surface creep along the north Anatolian fault offshore Istanbul: Results of 6 months of seafloor acoustic ranging[END_REF][START_REF] Lange | Inter-seismic strain build-up on the submarine North Anatolian Fault offshore Istanbul[END_REF]). The second leg was dedicated to ROV-Victor 6000 dives to investigate the geochemical dynamics of selected cold-seep sites (Fig. 4). During the third leg, sediment samples from long-gravity cores were collected to complete dat-ing of seismic reflector surfaces in view of establishing chronostratigraphy for the last hundred thousand years. A Special Issue, entitled "Fluids and processes at the seismically active fault zone in the Sea of Marmara", was published in 2018 in Deep Sea Research, Part II, provid-ing the state of knowledge on the links between fluids, faults, seismicity and the hydrocarbon system in the Sea of Marmara (Ruffine et al. 2018a). A new strategy was deployed during the MarsiteCruise for gas-seep sampling and for developing new proxies to relate fluid properties to fluid migration history. As a result, the gas bubbles emitted at the seafloor were shown to be mixtures of pri-mary and secondary microbial gases with thermogenic and mantle-derived gases with an asymmetric contribu-tion of each source (Ruffine et al. 2018b). In addition, the authigenic carbonate crusts sampled at the seafloor during the cruise were dated less than 9.6 kyr BP and a majority were younger than 1000 years, suggesting that authigenic carbonates could yield archives of fluid emission activity correlatable to historic seismic activity [START_REF] Çağatay | Seafloor authigenic carbonate crusts along the submerged part of the North Anatolian Fault in the Sea of Marmara: Mineralogy, geochemistry, textures and genesis[END_REF][START_REF] Teichert | Sulfate-dependent anaerobic oxidation of methane at a highly dynamicbubbling site in the Eastern Sea of Marmara (Çinarcik Basin). Deep-Sea Res Part II[END_REF]. The geological and biological diversity of fauna was also investigated, showing that seeps sustain typical Mediterranean cold seep fauna, but also unusual communities likely related to the interaction of seeps with hypoxic conditions [START_REF] Ondréas | Geological and biological diversity of seeps in the Sea of Marmara[END_REF]). These results naturally triggered renewed interest for assessing the hydrocarbon potential of the Marmara Sea basin [START_REF] Palabıyık | The safloor indications for hydrocarbon potential of the Marmara Sea Basin (Northwestern Turkey) and proposed exploration zones along with some drilling locations[END_REF]).

Relations with faulting and seismicity: summary of current knowledge

The new knowledge considerably improves our understanding of the fluid system in relation to faulting and seismicity in the Sea of Marmara and more generally in submarine environments. The EM-302 multibeam survey carried on R/V Le Suroit during the MARMESONET cruise ( 2009) provided a nearly continuous and uniform coverage of acoustic anomalies in the water column indicative of gas emissions at the seafloor. The superposition of the acoustic anomaly map on the active fault map (Fig. 3) indicates that (quote from [START_REF] Dupré | Tectonic and sedimentary controls for widespread gas emissions in the Sea of Marmara, Results from systematic, ship-borne multibeam echosounder water column imageries[END_REF]: "gas emissions are spatially controlled by a combination of factors, including fault and fracture networks in connection to the MMF system and inherited faults, the nature and thickness of sediments (e.g., occurrence of imper-meable or gas-bearing sediments and landslides), and the connectivity between the seafloor Dresen and Malin (2017) and gas sources, particularly in relation to the Eocene Thrace Basin. The relationship between seepage and fault activity is not linear, as active faults do not necessarily conduct gas, and scarps corresponding to deactivated fault strands may continue to channel fluids. Within sedimentary basins, gas is not expelled at the seafloor unless faulting, deformation, or erosional processes affect the sediments. On topographic highs, gas flares occur along the main fault scarps but are also associated with sediment deformation." [START_REF] Henry | A statistical approach to relation-ships between fluid emissions and faults: The Sea of Marmara case[END_REF] later applied a statistical approach to characterize quantitatively the relationship between seeps and active fault and found that gas emissions are more abundant within a swath of 900-1000 m (half-width) from an active fault, which corresponds to the width of the deformation zone observed at the seafloor in association with major faults. To explain the remain-ing 20%, Grall et al. (2018b) distinguish gas migration pathways controlled by sedimentary structure from those controlled by faults. A conceptual model is further pro-posed, in which (quote from [START_REF] Henry | A statistical approach to relation-ships between fluid emissions and faults: The Sea of Marmara case[END_REF]) (see also [START_REF] Okay | Control of active faults and sea level changes on the distribution of shallow gas accumulations and gasrelated seismic structures along the central branch of the North Anatolian Fault, southern Marmara shelf Turkey[END_REF]: "Updip migration along perme-able strata will result in gas migration toward the edges of the basins and its accumulation in anticlines and at sedi-mentary discontinuities (sediment-basement contact and ◂Fig. 3 After [START_REF] Dupré | Tectonic and sedimentary controls for widespread gas emissions in the Sea of Marmara, Results from systematic, ship-borne multibeam echosounder water column imageries[END_REF]. Relations between active gas emissions and active faulting. a EM302 water column multibeam track lines during the 2009 Marmesonet expedition. The white buffer along the navigation stands for the insonified zone within which gas bub-bles are detectable. 2009). Each of these acoustic anomalies is displayed as a 150 m radius red dot intra-sediment onlaps). Gas is eventually expelled along anticlines through extensional structures or along basin edges. This pattern will be observed if faults do not have high permeability, which may be the case during the interseismic period if faults are locked. It can be suggested that this is the current state in the Central High zone. After an earthquake, permeability increase in the ruptured fault enables focused fluid migration through the fault zone and a transient redistribution of gas emission sites at the seafloor. If fault creep also enhances permeability, permanent fluid expulsion through creeping fault zones could occur, following a similar pattern as for post-seismic fluid flow. It can be suggested that this is the case for the Western High zone". Eventually, [START_REF] Henry | A statistical approach to relation-ships between fluid emissions and faults: The Sea of Marmara case[END_REF] put this difference in perspective with recent work showing that "creep is occurring on the western segment of the Main Marmara Fault (this also causing micro-seismicity) while the central Istanbul-Silivri segment may have remained locked since the 1766 magnitude 7 + earthquake. This suggests that aseismic slip (and not only earthquake occurrence) effectively maintains high permeability conduits in fault zones in sediments."

Let us now address the question of the relations between gas emissions and micro-seismicity. Géli et al. (2018) note that the occurrence of gas emissions appears to be cor-related with the distribution of micro-seismicity, particu-larly on those segments where creeping is suspected along the MMF. Indeed, events of magnitude greater than ~ 4.2 regularly occur in the Western Sea of Marmara, generat-ing large sequences of aftershocks below sites where the density of gas emissions is at a maximum along the MMF. A large number of the aftershocks that followed the M 5.1 earthquake of 25th July 2011 occurred below the Western High, within a zone where pressurized gas is expected to migrate along the MMF, all the way up through the surface sediment layers. Hence, they concluded that gas-related processes should also be considered for a complete inter-pretation of the micro-seismicity (~ M < 3), particularly at supposedly creeping segments, where high permeabilities within the damage zone are expected to channel gas up to the seafloor.

Implications for earthquake hazard assessment

The geometry of the MMF is now well established. One of the many questions facing the scientific community is how the MMF will rupture? In one single segment? Or in multiple segments? What is more, active faults caus-ing offsets within the recent sediments have been identified, as well as buried faults which may be interpreted as blind faults with uncertain activity (Şengör et al. 2014), suggesting that ruptures may happen outside the MMF fault plane. While predicting earthquakes on out-of-plane faults (secondary or splay) is generally considered to be impossible, active research is presently ongoing to determine the seismological behavior of the different segments along the MMF, which involves 75% of the geodetic strain across the Sea of Marmara (e.g. [START_REF] Meade | Estimates of seismic poten-tial in the Marmara Sea region from block models of secular deformation constrained by Global Positioning System meas-urements[END_REF][START_REF] Hergert | Slip-rate variability and distributed deformation in the Marmara Sea fault system[END_REF][START_REF] Reilinger | Geodetic constraints on the tectonic evolution of the Aegean region and strain accumulation along the Hellenic subduction zone[END_REF][START_REF] Pichon | Propagation of a strike-slip plate boundary within an extensional environment: the westward propagation of the North Anatolian Fault[END_REF][START_REF] Özbey | Kinematics of the Marmara Region: a fusion of continuum and block models[END_REF]Zabci et al. 2019). Numerous recent papers have been dedicated to address-ing which segments are creeping and which are locked along the MMF.

Land-GPS data show a lack of elastic strain accumulation along the central and western part of the MMF [START_REF] Ergintav | Istanbul's earthquake hot spots: geodetic constraints on strain accumulation along faults in the Marmara seismic gap[END_REF]. Inversion of data in 3D finds heterogeneous locking on the MMF [START_REF] Bulut | Magnitudes of future large earthquakes near Istanbul quantified from 1500 years of historical earthquakes, present-day microseismicity and GPS slip rates[END_REF][START_REF] Klein | Aseismic slip and seismogenic cou-pling in the Marmara Sea: what can we learn from onland Geodesy?[END_REF]), but today, land-GPS data alone cannot yet resolve the distribution of interseismic loading rates vs. aseismic slip rates on the central and western part of the offshore fault [START_REF] Klein | Aseismic slip and seismogenic cou-pling in the Marmara Sea: what can we learn from onland Geodesy?[END_REF]. Seismology approaches suggest that the MMF has a deep creep component in the western Sea of Marmara, but not on the Central High nor on the Prince Islands seg-ment [START_REF] Bohnhoff | An earthquake gap south of Istanbul[END_REF](Bohnhoff et al. , 2016[START_REF] Bohnhoff | Repeating Mar-mara Sea earthquakes: indication for fault creep[END_REF][START_REF] Schmittbuhl | Seismic-ity distribution and locking depth along the main Marmara fault Turkey[END_REF][START_REF] Uchida | Repeating earthquakes and interplate coupling along the western part of the North Anatolian Fault[END_REF]. The Western High and the Central Basin segments are characterized by a higher level of back-ground seismicity that at the adjacent segments [START_REF] Schmittbuhl | Longlasting seismic repeaters in the Central Basin of the main Marmara fault[END_REF] and by the common occurrence of repeating small earthquakes [START_REF] Bohnhoff | Repeating Mar-mara Sea earthquakes: indication for fault creep[END_REF][START_REF] Schmittbuhl | Longlasting seismic repeaters in the Central Basin of the main Marmara fault[END_REF]. Furthermore, acoustic ranging experiments indicate an aseismic creep rate at the seafloor of 9-16 mm/year on the Western High [START_REF] Yamamoto | Seafloor geodesy revealed partial creep of the north Anatolian fault submerged in the sea of Marmara[END_REF] and no creep at the Central High and Kumburgaz Basin, where the level of microseismicity is notably low [START_REF] Sakic | No significant steady state surface creep along the north Anatolian fault offshore Istanbul: Results of 6 months of seafloor acoustic ranging[END_REF][START_REF] Lange | Inter-seismic strain build-up on the submarine North Anatolian Fault offshore Istanbul[END_REF]. Joint inversion of land GPS and marine geodesy constraints shows that there is no incompatibility between these two data sets and that the MMF has significant creep west of 28.2° E but is essentially locked eastwards (Özbey et al. 2021). These conclusions are also supported by other approaches, including the analysis of seismic velocity data (e.g. [START_REF] Tarancıoğlu | Three-dimensional seismic tomographic imaging beneath the Sea of Marmara: evidence for locked and creeping sections of the Main Marmara Fault[END_REF]. If we superpose these results-on the distribution of creeping segments-and those on the distribution of the zones where the amount of gas emissions is at a maximum seep emitting oil droplets, creating a cloud of beads rising into the water column (h); seep discharging a shimmering fluid through small carbonate chimneys along with continuously or intermittently gas and oil seeps (i). j-l Dive MRS-DV595, a 2-km length track from east to west along the SE border of Tekirdag Basin. Site named "Jack the Smoker", a chimney covered with microbial mats and discovered during Marnaut cruise [START_REF] Burnard | Constraints on fluid origins and migration velocities along the Marmara Main Fault (Sea of Marmara, Turkey) using helium isotopes[END_REF][START_REF] Tryon | Pore fluid in the North Ana-tolian Fault in the Sea of Marmara: a diversity of sources and processes[END_REF], revisited during MARSITE cruise (j); near-by, active bubbling site, located at a canyon outlet south of Tekirdag basin (k); bubbling site character-ized by corals, anemones and mussels fixed on carbonate crusts (l). m-o Dive MRS-DV596 in the Cinarcik Basin. Large black reduced sediment patches with surface areas of ~ 100-200m2, bacterial mats (orange and white) and carbonate crusts (m); Seeps associated with numerous massive carbonate crusts of few meters scale in dimension and elongated black (n); Diversified fauna observed upon and around carbonate crusts, along the SE border at the foot of the slope: amphipods, anemones, corals and crustaceans (o) [START_REF] Dupré | Spatial distribution of gas emissions from the Sea of Marmara seafloor from 4 November 2009 to 14[END_REF], we note a systematic correlation between the two [START_REF] Henry | A statistical approach to relation-ships between fluid emissions and faults: The Sea of Marmara case[END_REF]. In summary, for the Western High, we have the following sequence: (i) relation seismicity/creep: [START_REF] Schmittbuhl | Seismic-ity distribution and locking depth along the main Marmara fault Turkey[END_REF][START_REF] Schmittbuhl | Longlasting seismic repeaters in the Central Basin of the main Marmara fault[END_REF]; relation seismicity/fluids: (Géli et al. 2018); relation creep/fluids: [START_REF] Henry | A statistical approach to relation-ships between fluid emissions and faults: The Sea of Marmara case[END_REF]; confirmation of creep using acoustic geodesy: [START_REF] Yamamoto | Seafloor geodesy revealed partial creep of the north Anatolian fault submerged in the sea of Marmara[END_REF].

All these findings draw up new research avenues regarding the possible use of gas emissions mapping as a proxy for characterizing the behaviour (locked vs creeping) of the MMF, and more generally, the behavior of submarine transform-fault segments (e.g. [START_REF] Hensen | Marine transform faults and fracture zones: a joint perspective integrat-ing seismicity, fluid flow and life[END_REF].

7 Implementing seafloor observatories in the Sea of Marmara

Present situation

The turn of the century was also a time when (Çağatay et al. 2015b) In this context, the Sea of Marmara Sea was considered as the ideal location for implementing seafloor observatories geared towards hazard assessment, due to (i) the potentially high societal impact, in a region where more than 15 millions people are under the threat of seismogenic hazard in the whole MARNAUT Area; (ii) the logistics, favored by the proximity to the coastlines (only 5-30 km), which make for cost-effective and realistic the establishment of permanent seafloor observatories; iii) the deformation rates (20 mm/year) are very high compared to all other marine sites in Europe, resulting in active sub-marine processes that are measurable on short time scales. The route towards permanent, real-time, multi-disciplinary seafloor observatories is very long (Fig. 4). However, some progress has been made over the last decade: Four optimum sites for future deployments of multiparameter, permanent observatories were identified at, respectively: the Western High, the central basin, the Central High and at the entrance of the Gulf of Izmit. Moreover, the deployment of autonomous, innovative instrumentation provided the expertise and know-how for monitoring pore fluid properties on the seafloor Çağatay et al. (2015a). The instrumentation included: acoustic bubble detectors (e.g. [START_REF] Bayrakci | Acoustic monitoring of gas emissions from the sea floor. Part II: characterization of gas related processes in shallow sediments, based on the combined interpretation of acoustic and seismological datasets[END_REF]; aqueous flux meters for very low to moderate discharge rates [START_REF] Tryon | Quantifying submarine fluid seep activity along the North Anatolian Fault Zone in the Sea of Marmara[END_REF]; piezometers [START_REF] Henry | Marnaut Scientic Party[END_REF]; oceano-graphic sensors (temperature, salinity, current velocity, dissolved oxygen, turbidity) and methane sensors (e.g. Embriaco et al. 2013); and also, extensometers for acous-tic ranging for seafloor geodesy [START_REF] Sakic | No significant steady state surface creep along the north Anatolian fault offshore Istanbul: Results of 6 months of seafloor acoustic ranging[END_REF][START_REF] Lange | Inter-seismic strain build-up on the submarine North Anatolian Fault offshore Istanbul[END_REF]).

Challenges in seafloor geodesy

The relative motion between Anatolia and Europe is constrained by data from space geodesy (e.g. [START_REF] Reilinger | GPS constraints on continental deformation in the Africa-Arabia-Eurasia continental collision zone and implications for the dynamics of plate interactions[END_REF][START_REF] Reilinger | Geodetic constraints on the tectonic evolution of the Aegean region and strain accumulation along the Hellenic subduction zone[END_REF] and amounts to 24 ± 1 mm/year of the geodetic slip rate on the North Anatolian Fault. The North Anatolian fault system divides westward into a northern fault branch crossing the Sea of Marmara between Izmit Gulf and Ganos and a Southern fault system, which itself comprises the Gemlik fault and its offshore prolongation (the Middle Branch) as well as the Southern Branch. The strain partitioning between the two systems is such that the northern fault branch takes up most of the strike-slip component of plate motion while the southern fault system accommodates oblique extension (e.g. [START_REF] Flerit | Slip partitioning in the Sea of Marmara pull-apart determined from GPS velocity vectors[END_REF][START_REF] Reilinger | Geodetic constraints on the tectonic evolution of the Aegean region and strain accumulation along the Hellenic subduction zone[END_REF][START_REF] Pichon | Propagation of a strike-slip plate boundary within an extensional environment: the westward propagation of the North Anatolian Fault[END_REF]. Therefore, a critical issue for seismic hazard assessment is to determine the mechanical behaviour of the different segments along the MMF, and thereby identify those with the highest probability to rupture during the next expected Marmara earthquake. For instance, the low, or absence of, interseismic loading in the Istanbul area at about 10 km distance from the fault is puzzling [START_REF] Ergintav | Istanbul's earthquake hot spots: geodetic constraints on strain accumulation along faults in the Marmara seismic gap[END_REF][START_REF] Pichon | The North Anatolian Fault in the Sea of Marmara[END_REF]. But the presence of the water cover above the fault trace and the absence of islands to the south of the fault limits the use of land-based GPS data in estimating the strain accumulation and slip deficit along each of the segments. The efforts made so far to collect acoustic-based geodetic data are encouraging (e.g. [START_REF] Sakic | No significant steady state surface creep along the north Anatolian fault offshore Istanbul: Results of 6 months of seafloor acoustic ranging[END_REF][START_REF] Lange | Inter-seismic strain build-up on the submarine North Anatolian Fault offshore Istanbul[END_REF][START_REF] Yamamoto | Seafloor geodesy revealed partial creep of the north Anatolian fault submerged in the sea of Marmara[END_REF]). However, acoustic ranging techniques are restricted to very small areas of a few kilometres in length along the fault, which limits their interest for studies at the scale of the entire submarine fault. Moreover, the integration of acoustic ranging data in GPS-based networks with the required accuracy is a challenge that appears to be unreachable today. Technical solu-tions, hereafter called GNSS/A do exist, combining acoustic transmission in the water column and the Global Navigation Satellite System (e.g. [START_REF] Chadwell | Update on GPS-acoustic measurements on the continental slope of the cascadia subduction zone[END_REF]).

In the next section, we propose to include GNSS/A geodesy in the Maregami concept described hereafter. Work is currently in progress for the possible integration of GNNS/A data acquisition in the MAREGAMI concept.

Conclusions and perspectives

Based partly on the several seismic and bathymetric surveys carried out within the framework of Turkish-French collaboration, there is no question today that the western prolongation of the NAF within the Sea of Marmara comprises a through-going dextral strike-slip fault-called the Main Marmara Fault-that enters the sea through the gulf of Izmit and joins the Ganos Fault at its western extremity. Also, the MMF holds about 75% of the geodetic slip rate (Le [START_REF] Pichon | Propagation of a strike-slip plate boundary within an extensional environment: the westward propagation of the North Anatolian Fault[END_REF]Zabci et al. 2019), which suggests that at least 25% of the displacement is taken outside the MMF principal shear zone, on other fault branches (e.g. 3-4 mm/yr on the southern NAF branch in Gemlik Bay [START_REF] Polonia | Holocene slip rate of the North-Anatolian Fault beneath the Sea of Marmara[END_REF][START_REF] Gasperini | How far did the surface rupture of the 1999 Izmit earthquake reach in Sea of Marmara?[END_REF][START_REF] Vardar | High-resolution seismic characterization of shallow gas accumulations in the southern shelf of Marmara Sea. Turkey[END_REF][START_REF] Özbey | Kinematics of the Marmara Region: a fusion of continuum and block models[END_REF] and as distributed deformation. After 20 years of investigations, on-shore and off-shore, we still do not know where and when the next earthquake will strike. The next earthquake of M > 7 may occur along the seismic gap on the MMF or along one less prominent fault of the NAF system with no, or sparce, historical seismicity. After the success of the prediction of [START_REF] Stein | Progressive failure on the North Anatolian fault since 1939 by earthquake stress trigger-ing[END_REF], the common view was that assessing the probability of the next big one (M > 7.4) in the Sea of Marmara, comparable to the 1766 earthquake, was a reachable objective (e.g. Par-sons et al. 2000;[START_REF] Parsons | Recalculated probability of M_7 earthquakes beneath the Sea of Marmara, Turkey[END_REF][START_REF] Pondard | Fault interactions in the Sea of Marmara pull-apart (North Anatolian Fault): earthquake clustering and propagating earthquake sequences[END_REF]. In 2000, for instance, Parsons et al. found "a 62 ± 15% prob-ability (one standard deviation) of strong shaking during the next 30 years and 32 ± 12% during the next decade". Today, the probability of an earthquake of M > 7 occurring along the MMF is estimated to be the highest for the central or east-ern part of the Sea of Marmara (e.g. [START_REF] Bulut | Magnitudes of future large earthquakes near Istanbul quantified from 1500 years of historical earthquakes, present-day microseismicity and GPS slip rates[END_REF], and the "possibility of a westerninitiated, super-shear rupture propagating eastward, the worst scenario for the Istanbul region, cannot be ruled out" (quote from Aochi and Ulrich 2015). When the September, 26th 2019, earthquake of mag-nitude 5.4 occurred in the MMF principal shear zone near the western end of the Istanbul-Sılıvrı segment [START_REF] Karabulut | The moderate size 2019 SeptemberMw 5.8 Silivri earthquake unveils the complexity of the Main Marmara Fault shear zone[END_REF], 20 years after the Izmir earthquake, nobody was able to tell whether this event was the precursor of a larger earthquake of M > 7. The despair that then affected the scientific community reflects that we still have much to learn and many scientific questions to address:

• What is the role of the deep crustal structure on the seismogenic behavior of active faults? • What is the role of asymmetry in crustal elastic properties north and south of the fault? Does it for instance explain the low, or no, interseismic loading in the Istanbul area at about 10 km distance from the fault [START_REF] Ergintav | Istanbul's earthquake hot spots: geodetic constraints on strain accumulation along faults in the Marmara seismic gap[END_REF])? • What is the real significance of "creeping" along active faults? [START_REF] Cetin | Extent and distribution of aseismic slip on the Ismetpaşa segment of the North Anatolian Fault (Turkey) from Persistent Scatterer InSAR[END_REF] • How much stress is released or transferred to adjacent segments by earthquakes of intermediate magnitude?

• What is the contribution to creeping that can be, respec-tively, ascribed to the presence of: (i) serpentinite crustal bodies at depth as below the IsmiztpaŞa (e.g. Cetin et al. 2020)? (ii) soft sediment cover (e.g. [START_REF] Yamamoto | Seafloor geodesy revealed partial creep of the north Anatolian fault submerged in the sea of Marmara[END_REF])? (iii) high fluid pressure (Géli et al. 2018)? • Are there slow-slip events-yet undetected-along the MMF?

The Sea of Marmara has also been shown to be a unique, natural laboratory to improve our understanding of the rela-tionships between fault activity, seismicity, fluid migration and ecosystems at a major, transform plate boundary (e.g. Ruffine et al. 2018a;[START_REF] Hensen | Marine transform faults and fracture zones: a joint perspective integrat-ing seismicity, fluid flow and life[END_REF]):

• Are there changes in the fault-fluid properties during the earthquake cycle that could be detectable from the seafloor? • Can fluids transformations produce archives of past seismicity along the MMF fault segments? Continuous, long-term time series are cruelly miss-ing to address some of the fundamental questions listed above. Implementing multi-parameter seafloor observatories (including seismology, geodesy and sensors on fluid properties) is a long-term enterprise, that demands sustained efforts, adapted facilities, dedicated manpower, which all require political and financial support. We are only in the middle of the way and we must go on. What is the most needed today, is to unite forces, to build a project supported by all institutions in Turkey and by the international scien-tific community.

  2: Şengör et al. (2014) -see Fig. 1; Le Pichon et al. (2014); Le Pichon et al. (2016). Using mainly the data from the deep basins, Şengör et al. (

Fig. 1 Fig. 2

 12 Fig. 1 After Şengör et al. (2014). Synthetic fault map summarizing the current knowledge on the tectonics of the Sea Marmara, super-posed on seafloor bathymetry map for depths > 100 m (Rangin et al. 2001). The black segments are active faults and the grey segments are presumably inactive. The tectonic interpretation is based not only on the data listed in the text, but also on seismic lines indicated in Panel A of Fig. 4 in Şengör et al. (2014), e.g.: MTA lines, Maden Tetkik

Fig. 4

 4 Fig. 4 After Ruffine et al. 2018b. Seafloor observations with ROV Victor reported during the MARSITE (MRS) Cruise on R/V Pour-quoi pas? See also http://dx.doi.org/10.1016/j.dsr2.2018.03.006. a-c Dive MRS-DV592 starting from eastern border of the Kumbur-gaz Basin to the Central High. Black patches associated with bacte-rial mats and fauna (a); In-situ RAMAN spectrometer analyzing gas streams bubbling from sites covered with specific white spaghetti-like bacterial (b); Hemipelagic sediment covered at some seep sites by dense accumulation of bivalve shells, presumably dead (c). d-f Dive MRS-DV593 on the Western High. Areas with reduced black sediment associated with both bacterial mats (narrow or large ones) and carbonate crusts (d); Large areas of massive carbonate crusts, with or without bacterial mats around and discontinuous oil seeps (e); Brown soft sediment present with bioturbation as well as dead or alive sea urchins outside the seepage areas covering carbonate crusts. g-i Dive MRS-DV594 in the Tekirdag Basin. Intensive bubbling site at the foot of the Tekirdag escarpment discovered during MarNautCruise and called "Boris Bubbler" after Henry (2007) (g); Wide spread oil

7. 3 Fig. 5

 35 Fig.5In 2017, ANR and TUBITAK decided to fund the 3 years-long MAREGAMI Project, with the objective to design an opti-mized submarine monitoring observatory system, having the fol-lowing attributes: i) simple; ii) robust; iii) fully integratable in the local monitoring network; iv) densily distributed (spatially) on the Marmara seafloor. To ensure the adequate spatial distribution, the"MAREGAMI observatory concept" was proposed as a versatile and multi-disciplinary alternative to cabled sea-floor observatories. It is based on three components (see lower panel): (i) a sea-surface «buoy» or «platform» for energy production from solar panels and data transmission to land; (ii) three mooring lines and a link from surface to sea-floor, consisting of an electro-mechanical umbilical for energy transfer and data communication; (iii) a central node equipped with multi-parameter instrumentation and connected (via electric cables) to four remote nodes, located 5 km away. Work is presently in progress for integrating the possibility of collecting GNNS/A data in the MAREGAMI concept

Table 1

 1 List of cruises conducted in the Sea of Marmara within the Turkish-French collaboration in marine geoscience since

	1999 Cruise

Table 2

 2 List of Cores used by Çağatay et al. (2013) for paleoseismological studies in the Sea of Marmara

	Location	Core No.	Core type	Water depth (m)	Length (cm)	Long	Lat	Cruise
	İzmit Körfezi Gölcük Basin İzmit Körfezi Karamürsel	M22-2 M21-1 MARM05-112*	W/S interface W/S interface W/S interface	38 38 212	106 112.5	29.88588 29.88459 29.68351	40.72890 40.72574 40.72973	Urania 2010 Urania 2005
	Basin	MARM05-113*	W/S interface	212		29.68326	40.72961	Urania 2005
		MARM05-115	Gravity	199	231	29.66890	40.72464	Urania 2005
		MARM05-117	Gravity	68	251	29.70303	40.74994	Urania 2005
		MARM05-119	Gravity	210		29.68314	40.72962	Urania 2005
	Gemlik	M13-1	W/S interface	113	87.5	28.96716	40.39837	Urania 2010
		ML01_1	Gravity	112.9	338
		MNTKS-34	Gravity	105	940	28.95870	40.39328	L'Atalante
		MNTKI-13	W/S interface	105	87			Marnaut
	Çınarcık	MNTKS-10	Gravity	1265	905	29.17970	40.73390	L'Atalante
		MNTKI-08	W/S interface	1265	83			Marnaut
		MNTKS-07	Gravity	1265	71.7	29.11416	40.71568
		MNTKI-07	W/S interface	1220	81
	Central high	MEI-01	W/S interface	800	130	28.50292	40.87069	Le Suroit 2009
		MEG-02	Gravity	692		28.25500	40.84667
	Central Basin	MNTKS-12	Gravity	1262	960			L'Atalante
		MNTKS-13	Gravity	1248	920	27.99152	40.81068	Marnaut
		MNTKS-18	Gravity	1260	920	28.01385	40.82330
	Western high	MEI-04	W/S interface	741	40.5	27.72211	40.81464	Le Suroit 2009
	Tekirdağ Basin	MNTKS-32	Gravity	1123	945	27.61130	40.82895	L'Atalante
		MNTKI-12	W/S interface	1123	94			Marnaut
		MNTKS-29	Gravity	1117	930	27.62213	40.80853

  TB Tekirdağ Basin, WH Western High, CB Cen-tral Basin, CH Central, KB Kumburgaz Basin, ÇıB Çınarcık Basin, NIB North Imrali Basin. b The 2-D Marmara shaded bathymetry map with gas flare distribution, c fault networks from Şengör et al. [2014] with the offshore extent of the Eocene Thrace Basin from Le Pichon et al. (2014), and d microearthquake epicenters (yellow dots) recorded between 2005 and 2011[START_REF] Şengör | The geometry of the North Anatolian transform fault in the Sea of Marmara and its temporal evolution: implications for the development of intracontinental transform faults[END_REF] H. Karabulut, personal communication, 2013]. White, orange, and redmarks stand for water column acoustic anomalies recorded in2000, 2007, and 2009, respectively. The maps illustrate the 2009 seepage activity over more than 1 month (from 4 November 2009 to 14 December

Acknowledgements

We particularly thank here Celal Şengör and Xavier Le Pichon for initiating the Turkish-French collaboration in marine geoscience and for their scientific mentoring since 1999; Naci Görür, for his decisive role as TUBITAK's general coordina-tor of

funded by TUBITAK and ANR (the Turkish and French national funding agencies for research, respectively). scientific contributions on the tectonic evolution of the North-Anatolian Fault in the Marmara Region; on the relationships between faulting, seismicity, fluids and ecosystems; and on paleo-seismology and paleo-oceanography in the Sea of Marmara.

Technical Research Council of Turkey (TUBİTAK) initiated the Turkish Marine Programme, coordinated by Naci Görur and funded by the State Planning Organization of Turkey (DPT). The programme had access to the research vessels of Mineral Research and Exploration Directorate of Turkey (MTA) and Turkish Naval Forces-Office of Navigation, Hydrography and Oceanography (SHOD). The programme acquired 2200 km of multi-channel seismic reflection lines, with MTA's R/V Sismik-1 (see review in Imren et al. 2001, p. 145). Two different groups, from Istanbul Technical University (ITU) and from the University

marine geological and geophysical studies in Turkish seas; and all PI's of the different cruises conducted within the collaboration: Emin Demirbağ, Alfred Hirn, Satish Singh, Tuncay Taymaz, Rolando Armijo, Jacques Malavieille, Laurent Labeyrie, Bernard Mercier de Lépinay, Gilles Lericolais, Gilles Delaygue, Livio Ruffine, with special thoughts to the late, inspiring, Aykut Barka, who passed away in 2002 -a few months before co-chiefing the MarmaraScarps cruise, along with Rolando and Jacques. We also thank the general commander of SHOD, who offered repeated support during every single cruise conducted in the Sea of Marmara; the scientific attachés at the French embassy in Ankara, -particularly Mrs Bonnafous Boucher for the Marsite Cruise; Nurcan Meral Özel (KOERI), coordinator of the EU-Funded Marsite Programme; Ziya Çakır co-PI of the Maregami Project. Finally, we acknowledge all active contributors, e. g. Stéphanie Dupré, Céline Grall, Sinan Özeren, Gülsen Uçarkuş, Kadir Eriş, Caner Imren, Cengiz Zabcı, Günay Çifci, Hakan Şarıtaş; as well as Cemil Gürbüz, Doğan Kalafat, Mustafa Comoğlu, Hayrullah Karabulut, Mustafa Aktar, Semih Ergintav, for their support; Luca Gasperini, who repeatedly offered opportunities for French and Turkish groups to embark on board the Italian R/V Urania in 2009, 2011 and 2013; Paolo Favali and his group from INGV who helped develop multi-disciplinary sea-floor observatories initiatives in the Sea of Marmara; Loïc Dussud and Jean-Romain Lagadec, contributors of the Maregami concept; and all those people who contributed to the success of the Turkish-French collaboration in marine geosciences, but are too many to be listed here individually.