E Catalán

O D'acremont

J Galindo-Zaldivar

G Ercilla

F Estrada

M Catalán

E D'acremont

O Azzouz

D Casas

M Chourak

J T Vazquez

A Chalouan

C Sanz De Galdeano

M Benmakhlouf

C Gorini

B Alonso

D Palomino

J A Rengel

A J Gil

Jesús Galindo-Zaldívar

Imaging the growth of recent faults: the case of 2016-17 seismic sequence sea bottom deformation in the Alboran Sea (Western Mediterranean)

Keywords: Eurasian-African plate boundary, fault development, seismicity, mass transport deposits, active sea bottom deformations

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Introduction

Continuous plate motion has led to the activity of tectonic structures developed along plate boundaries, including faults with related seismicity.

Seismic or creep behaviour of a fault is constrained by the rheology of the deformed rocks [START_REF] Sibson | Fault rocks and fault mechanisms[END_REF]. Brittle deformation is generally accommodated by previous fractures because the low cohesion with respect to the undeformed host rock causes them to be more easily reactivated [START_REF] Anderson | The dynamics of faulting and dyke formation with applications to Britain[END_REF][START_REF] Bott | The mechanics of oblique slip faulting[END_REF]. When deformation propagates, the growth of fault zones is produced by stress concentrations at the boundaries of previous fault surfaces (Scholz, 1989) and the larger the fault, the higher the magnitude of the related earthquakes (Wells & Coppersmith, 1994). The activity of a fault requires the shear stress on its surface to exceed the values of cohesion and friction [START_REF] Hajiabdolmajid | Modelling brittle failure of rock[END_REF]. On a fault surface, the cohesion is low and needs lower shear stresses than on the new developing fault segments at the edge of the previous fault [START_REF] Hajiabdolmajid | Modelling brittle failure of rock[END_REF]. Thus, the propagation of a fault in unfractured resistant rocks can imply a high accumulation of elastic energy that may generate earthquakes of magnitudes higher than those triggered by a reactivation of previous fractures.

The Eurasian-African plate boundary in the Alboran Sea (westernmost Mediterranean) offers a unique research opportunity in a natural example that can provide insights as to the propagation of fault zones [START_REF] Cowie | Growth of faults by accumulation of seismic slip[END_REF]) (Fig. 1). The Alboran Basin is a Neogene-Quaternary extensional basin located within the Betic (Spain)-Rif (Morocco) alpine cordilleras, connected by the Gibraltar Arc [START_REF] Andrieux | Sur un modèle explicatif de l'Arc de Gibraltar[END_REF]. The major Trans-Alboran Shear zone [START_REF] Larouzière | The Betic segment of the lithospheric Trans-Alboran shear zone during the Late Miocene[END_REF]Frasca et al., 2015) accommodated the westward displacement of the Betic-Rif orogen during the development of the Gibraltar Arc. The Alboran Basin is floored by a thin continental crust made up of the alpine Internal Zone metamorphic complexes, with a Variscan basement located in the southeastern area [START_REF] Ammar | The tectonic structure of the Alboran Margin of Morocco[END_REF] resting above an anomalous mantle [START_REF] Hatzfeld | Deep-structure of Alboran Sea. C. R. hebd[END_REF][START_REF] Comas | Neogene tectonic evolution of the Alboran Sea from MCS data[END_REF]. The sedimentary infill consists of unconformable Miocene to Quaternary deposits [START_REF] Comas | Neogene tectonic evolution of the Alboran Sea from MCS data[END_REF][START_REF] Juan | Seismic evidence of current-controlled sedimentation in the Alboran Sea during the Pliocene and Quaternary: Palaeoceanographic implications[END_REF]. This sedimentary record is mostly deformed by two conjugated sets of dextral WNW-ESE and sinistral NE-SW faults, and folded by ENE-WSW oriented folds (Estrada et al., 2018;[START_REF] Martínez-García | From Extension to Shortening: Tectonic Inversion Distributed in Time and Space in the Alboran Sea, Western Mediterranean[END_REF], the Alboran Ridge and Francesc Pagès seamount, pertaining to the main antiforms (Bourgeois et al., 1992). The growth of faults and folds takes place in the framework of recent NNW-SSE shortening, and regional Eurasian-African plate convergence [START_REF] De Mets | High-resolution Neogene and Quaternary estimates of Nubia-Eurasia-North America Plate motion[END_REF]. The Eurasian-African plate boundary shows N-S to NW-SE convergence at present (Fadil et al., 2006;[START_REF] Koulali | New GPS constraints on active deformation along the Africa-Iberia plate boundary[END_REF][START_REF] Palano | The Diffuse Plate boundary of Nubia and Iberia in the Western Mediterranean: Crustal deformation evidence for viscous coupling and fragmented lithosphere[END_REF], at a rate of 4.93 mm/yr [START_REF] Argus | The angular velocities of the plates and the velocity of Earth's centre from space geodesy[END_REF]. Regional present-day ENE-WSW extensional stress is coeval with orthogonal compression and main stress axes are inclined [START_REF] De Vicente | Inversion of moment tensor focal Mediterranean: Constraints from an offshore-onshore seismological network and swath bathymetry data[END_REF][START_REF] Stich | Moment tensor inversion for Iberia-Maghreb earthquakes 2005-2008[END_REF].

The Betic-Rif Cordillera and Alboran Sea are affected by a 300 km broad and heterogeneous seismicity band related to the Eurasian-African plate boundary (Buforn et al., 1988). Seismicity generally occurs at shallow crustal levels [START_REF] Buforn | Seismotectonics of the Ibero-Maghrebian region[END_REF]. Intermediate seismicity (40 to 120 km deep) is mainly located along a N-S elongated band in the western Alboran Basin that becomes NE-SW northwards [START_REF] Morales | Active continental subduction beneath the Betic Cordillera and the Alboran Sea[END_REF]López-Casado et al., 2001;[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF][START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF]. Deep seismicity (600 to 640 km deep) is scarce but also occurs beneath the central Betic Cordilleras [START_REF] Buforn | Intermediate and deep earthquakes in Spain, in Source Mechanism and Seismotectonics[END_REF][START_REF] Buforn | The 2010 Granada, Spain, deep earthquake[END_REF]. The area has heterogeneous local stresses probably due to fault interaction [START_REF] Stich | Moment tensor inversion for Iberia-Maghreb earthquakes 2005-2008[END_REF].

Several geodynamic models have been proposed for the region, including delamination (e.g. Seber et al., 1996;[START_REF] Lis Mancilla | Delamination in the Betic Range: Deep structure, seismicity, and GPS motion[END_REF], or subduction with or without rollback (e.g. [START_REF] Pedrera | Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field[END_REF]Ruiz-Constán et al., 2011;[START_REF] Gutscher | The Gibraltar subduction: A decade of new geophysical data[END_REF]González-Castillo et al., 2015;[START_REF] Spakman | Puzzling features of western Mediterranean tectonics explained by slab dragging[END_REF]), yet discussion remains alive. Moreover, in the central and eastern Alboran Sea, the recent fault system mainly composed by two conjugate WNW-ESE dextral and NNE-SSW sinistral fault sets evidences the activity of continental indentation tectonics (Estrada et al., 2018). Within this structural framework, a present-day zone of deformation with high seismic activity crossing the Alboran Sea -from Al Hoceima in the Rif to Adra and Cabo de Gata in the Betics-(Fig. 1) oblique to the previous Trans-Alboran Shear zone [START_REF] Larouzière | The Betic segment of the lithospheric Trans-Alboran shear zone during the Late Miocene[END_REF] has been proposed to be a main plate boundary (Fadil et al., 2006;Grevemeyer et al., 2015). However, relationships with the main tectonic structures observed in the seafloor have not yet been analysed in detail. The few available studies [START_REF] Martínez-García | Strike-slip tectonics and basin inversion in the Western Mediterranean: the Post-Messinian evolution of the Alboran Sea[END_REF]2017 and references herein;Estrada et al., 2018) suggest that the NNE-SSW sinistral Al Idrisi Fault is the main structure with recent and present-day activity in the southern Alboran Sea (Fig. 1). This fault is connected onshore with the Trougout Fault in the Al Hoceima region (Morocco margin), which bounds the Nekor Basin (d'Acremont et al., 2014;[START_REF] Lafosse | Evidence of Quaternary transtensional tectonics in the Nekor basin (NE Morocco)[END_REF] and its propagation towards the Rif is discussed by Galindo-Zaldívar et al. (2009;2015a) and[START_REF] Poujol | Active tectonics of the Northern Rif (Morocco) from geomorphic and geochronological data[END_REF]. The Al Hoceima region is deformed mainly by faults that determine a succession of horsts and grabens, probably developed above crustal detachments (Galindo-Zaldívar et al., 2009;2015a). In the northern Alboran Sea, the fault zone extends onshore toward the Campo de Dalías area, connecting with the Balanegra Fault in the boundary of the Betic Cordillera and Alboran Sea [START_REF] Marín-Lechado | Levelling Profiles and a GPS Network to Monitor the Active Folding and Faulting Deformation in the Campo de Dalias (Betic Cordillera, Southeastern Spain)[END_REF]. January 25, 2016, marked the onset of a seismic sequence in the central-southern Alboran Sea (www.ign.es), with a main shock of Mw=6.3 (Figs.

1 and 2), and whose activity continues up to 2017. The area affected extends from the Francesc Pagès seamount and westernmost Alboran Ridge to the Nekor Basin. The main earthquake was felt in several coastal cities of northern Morocco and southern Spain, causing economic losses in both countries (http://www.ign.es/resources/noticias/Terremoto_Alboran.pdf). The earthquakes of this seismic sequence have been analysed in detail by the IGN (www.ign.es), [START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF], [START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF] and [START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF].

They consider different velocity models for epicentre locations, suggesting that activity occurred in the area nearby Al Idrisi Fault; yet they do not compare their results with the more accurate position of this fault obtained by marine geophysical research [START_REF] Martínez-García | Strike-slip tectonics and basin inversion in the Western Mediterranean: the Post-Messinian evolution of the Alboran Sea[END_REF]2017;Estrada et al., 2018;[START_REF] Lafosse | Evidence of Quaternary transtensional tectonics in the Nekor basin (NE Morocco)[END_REF]. The comparison of seismological and marine geophysical researches clearly shows that the epicentres of the earliest stage of the sequence are located to the west of the Al Idrisi Fault. This paper offers a multidisciplinary analysis of the recent and active near-surface tectonic deformations related to the 2016-17 seismic sequence within the greater context of the 1993-94 and 2004 sequences in the central Alboran Sea. Our contribution provides insights into the propagation of recent fault zones and how they are linked to seafloor deformations in addition to their relationships with the former Al Idrisi Fault.

Methodology and data

The combination of different geodetic and geophysical data made it possible to map the area affected by the 2016-17 seismic sequence, from the deep structure to near-surface morphology as well as the overall geodynamic setting.

Regional GPS data

Permanent

Seismicity data

The seismicity database of the Spanish National Geographic Institute (IGN) (www.ign.es) registered the 2016-17 seismic sequence as well as the two previous main seismic series, in 1993-94 and 2004. The 2004 seismic sequence was carefully relocated by [START_REF] Van Der Woerd | The Al Hoceima Mw 6.4 earthquake of 24 February 2004 and its aftershocks sequence[END_REF]. As the precise location of seismicity is sensitive to velocity models and to the distance of the seismic stations [START_REF] Michelini | The effect of velocity structure errors on double-difference earthquake location[END_REF], literature [START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF][START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF][START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF] shows a non-coincident location for the 2016-17 seismic sequence´ epicentres. The 1993-94 and 2016-17 seismic sequences´ epicentre and hypocentre locations were calculated through a standard procedure considering the first arrivals of P and S waves and a standard velocity model [START_REF] Carreño-Herrero | Sismicidad de la Península Ibérica en el periodo instrumental: 1985-2011[END_REF]. A careful relocation of the main events was provided by [START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF] and the IGN [START_REF] Ign | Informe de la actividad sísmica en el Mar de Alborán 2016[END_REF]www.ign.es) in light of the standard and recent velocity model (El Moudnib et al., 2015). Earthquake focal mechanisms were also obtained from the IGN database (www.ign.es), established from first arrival Pwave polarity. The present-day stress tensor was determined from seismicity using the method by [START_REF] Michael | Determination of stress from slip data: faults and folds[END_REF], improved by Vavryčuk (2014).

Marine geophysics

The area affected by the 2016-17 seismic series was surveyed during the INCRISIS cruise on board the R/V Hesperides in May, 2016. A dense grid of 31 survey lines, with a total length of about 900 km, was designed taking into account the regional water depths and the general orientation of the morphological and structural features (Fig. 1b). Data positioning was determined via a Global Positioning System (GPS). The studied area was covered by a multibeam SIMRAD EM120 echosounder (frequency 12 kHz) that enabled us to record high resolution bathymetry. Vertical resolution was approximately 0.025% of water depth. CARIS Hips software was used for multibeam data processing. This bathymetry was integrated with previous datasets in the area (http://gma.icm.csic.es/sites/default/files/geowebs/OLsurveys/index.htm) and gridded at 25 m.

Simultaneously with multibeam bathymetry, very high resolution seismic profiles were acquired with the SIMRAD TOPAS PS18 system (frequencies of 18 kHz -1 to 6 kHz). In addition to this information, multi-and single-channel seismic records from the Instituto de Ciencias del Mar-CSIC database (http://gma.icm.csic.es/sites/default/files/geowebs/OLsurveys/index.htm) were considered (Fig. 1c). All seismic profiles were integrated into a Kingdom Suite project (IHS Kingdom) for their correlation and interpretation. Likewise, gravity data were obtained during the INCRISIS cruise using a Lockheed Martin BMG3 marine gravimeter with a precision of 0.7 mGal. Gravity Free air and Complete Bouguer anomalies were determined considering a standard density of 2.67 g/cm 3 . To extend the gravity anomalies to the shoreline we used the global free air dataset from [START_REF] Sandwell | New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure[END_REF]. Directional filters (horizontal gravity gradient) were applied in order to analyse the main tectonic structures.

Results

The area affected by the 2016-17 seismic sequence is located in the southern sector of the Alboran Sea central part (from 125 to 1450 m water depth), from the Nekor Basin to the vicinity of Alboran Ridge and Francesc

Pagès seamount (Figs. 1 and2). The northern steep side of the Alboran Ridge gives way abruptly to the Alboran Channel, whereas that of the Francesc Pagès seamount evolves to flat-lying seafloor of the deep basin through a terracedshaped sector that parallels the seamount.

3.1. Present-day Alboran Sea shortening from GPS data and plate deformation Deformation within the Alboran Sea is driven in part by the NW-SE regional convergence of the Eurasian and African plates [START_REF] De Mets | High-resolution Neogene and Quaternary estimates of Nubia-Eurasia-North America Plate motion[END_REF].

The west to west-southwestward motion of the Betic-Rif Alboran block [START_REF] Koulali | New GPS constraints on active deformation along the Africa-Iberia plate boundary[END_REF][START_REF] Palano | The Diffuse Plate boundary of Nubia and Iberia in the Western Mediterranean: Crustal deformation evidence for viscous coupling and fragmented lithosphere[END_REF] may also be the result of ongoing slab rollback towards the west [START_REF] Pedrera | Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field[END_REF][START_REF] Ruiz-Constán | Stress distribution at the transition from subduction to continental collision (northwestern and central Betic Cordillera)[END_REF][START_REF] Gutscher | The Gibraltar subduction: A decade of new geophysical data[END_REF]González-Castillo et al., 2015;[START_REF] Spakman | Puzzling features of western Mediterranean tectonics explained by slab dragging[END_REF], which would explain the E-W to ENE-WSW extensional focal mechanisms observed in the West Alboran Sea [START_REF] Stich | Moment tensor inversion for Iberia-Maghreb earthquakes 2005-2008[END_REF]. At present, the GEODVEL plate model [START_REF] Argus | The angular velocities of the plates and the velocity of Earth's centre from space geodesy[END_REF] indicates a N141ºE trend of convergence at a rate of 4.93 mm/yr in this region, supported by regional GPS data (Fadil et al., 2006). This regional deformation produces a NNW-SSE shortening which, in the central part of the Alboran Sea, may be constrained by the MALA and ALME stations in Spain, and by MELI station in Morocco (Fig. 1). 2015). The most recent seismic sequences were in 1993-94, 2004 and 2016-2017 (Fig. 2). The 1993-94 seismic sequence affected the coastal region nearby Al Hoceima, with a main earthquake of Mw=5.6 (May 26, 1994), after an earlier earthquake in the Campo de Dalías area, of Mw= 5.3 (Dec 23, 1993). In 2004, another seismic sequence with a devastating earthquake event of Mw= 6.4 (Feb 24) occurred onshore in the Al Hoceima region causing nearly 600 deaths and with aftershocks reaching the Alboran Sea (Van der [START_REF] Van Der Woerd | The Al Hoceima Mw 6.4 earthquake of 24 February 2004 and its aftershocks sequence[END_REF]. Earthquake focal mechanisms were very similar in all cases, pointing mainly to sinistral strike-slip along the NNE-SSW oriented deformation zone (El Alami et al., 1998;[START_REF] Stich | Kinematics of the Iberia-Maghreb plate contact from seismic moment tensors and GPS observations[END_REF][START_REF] Stich | Moment tensor inversion for Iberia-Maghreb earthquakes 2005-2008[END_REF], although also possible is the activity of WNW-ESE dextral faults in the 2004 sequence that affect Al Hoceima onshore areas [START_REF] Akoglu | The 1994-2004 Al Hoceima (Morocco) earthquake sequence: Conjugate fault ruptures deduced from InSAR[END_REF][START_REF] Van Der Woerd | The Al Hoceima Mw 6.4 earthquake of 24 February 2004 and its aftershocks sequence[END_REF].

The 2016-17 seismic sequence (Figs. 2 and3) was initiated by a moderate earthquake (Jan 21, 2016, Mw=5.1) followed closely by a stronger event (Jan 25, Mw= 6.3) and by a long seismic series of decreasing activity during 2016-17 [START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF][START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF][START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF]. The relatively long distances from hypocentres to seismic stations, generally greater than 50 km and in some cases reaching more than 80 km, decreases the quality of earthquake locations. inclined compression and orthogonal horizontal extension in both segments linked to prolate stress ellipsoids. Inclination is higher (47º) in the main alignment and its axial ratio is closer to triaxial stress.

Deep structure from gravity data

Bouguer complete gravity anomalies decrease in the Alboran Sea, from 110 mGal to 30 mGal, in a smooth transition from the east to the west up to 4.5ºW (Casas & Carbó, 1990). As the amplitude decreases, it narrows to the west, so that the greatest values are located in the central part. affecting the most recent sediments, some of them reaching up to the seafloor (Fig. 9).

Discussion

This starting with the highest magnitude event (Mw= 6.3, Jan 25) where they join (Fig. 3a). The depth of seismicity is not well constrained because of the variability of crustal velocities and the few and far onshore stations; still, they correspond to crustal levels (<35 km depth), in agreement with Buforn et al.

(2017), [START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF] and [START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF]. New seismic faults started to develop at shallow crustal levels (roughly 5 to 10 km depth), as generally occurs in continental crust [START_REF] Meissner | Limits of stresses in continental crusts and their relation to the depth-frequency distribution of shallow earthquakes[END_REF]. If an elliptical fault surface shape is considered [START_REF] Watterson | Fault dimensions, displacements and growth[END_REF] In the ENE-WSW alignment, the morphostructural pattern and deformation behaviour are different. The presence of MTDs (Figs. 5 and6) and reverse earthquake focal mechanisms (Fig. 3d), and the absence of clear reverse faults affecting the near-surface, would suggest the presence of blind reverse faults at the core of the Alboran Ridge antiform (Fig. 10) (Martinez-Garcia et al., 2013).

Earthquakes may be assumed to be the main mechanism triggering the MTDs, deforming the near-surface sediment of the ENE-WSW alignment (Figs.

5 and 6) [START_REF] Casas | The Baraza Slide: model and dynamics[END_REF]. The initiation of slope failure is due to cyclic loading applied on the sediment and a decreasing shear strength through the development of pore overpressure. Other factors, such as tectonic deformations resulting in seabed, may also contribute to increasing shear stress on the slope, or a decreasing sediment strength due to shearing, dilatancy and possible sediment creep [START_REF] Locat | Submarine landslides: advances and challenges[END_REF].

Geodynamic implications and seismic hazard

The former NE-SW sinistral Trans Alboran Shear Zone [START_REF] Larouzière | The Betic segment of the lithospheric Trans-Alboran shear zone during the Late Miocene[END_REF]) constituted a main tectonic structure during the Miocene, later becoming inactive and overprinted by the recent NNE-SSW deformation zone [START_REF] Stich | Kinematics of the Iberia-Maghreb plate contact from seismic moment tensors and GPS observations[END_REF][START_REF] Stich | Moment tensor inversion for Iberia-Maghreb earthquakes 2005-2008[END_REF] between Campo de Dalías (Balanegra Fault) and the Al Hoceima region (Trougout Fault), including the Al Idrisi Fault (Fig. 10). This evolution reveals a progressive offset to the west and rotation of the central Alboran Sea active deformation zone. The recent shear zone has incipient low deformation.

Al Idrisi Fault, which constitutes the longest fault in this deformation zone, has short strike-slip, however. It is expressed by low interaction at the edge where the Al Idrisi Fault orientation trace changes, and also by the short displacement between the Alboran Ridge and Francesc Pagès seamount antiform axes (Fig. 10).

At present, seismological and GPS studies hold the active seismic zone in the central Alboran Sea to be a main weak zone related to the Eurasian-African plate boundary (Fadil et al., 2006;Grevemeyer et al., 2015;[START_REF] Palano | The Diffuse Plate boundary of Nubia and Iberia in the Western Mediterranean: Crustal deformation evidence for viscous coupling and fragmented lithosphere[END_REF][START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF][START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF] Northwards, towards the Betic Cordillera, the deformation zone is connected with the Campo de Dalías and its seismicity during the 1993-94 series [START_REF] Marín-Lechado | Active faults, seismicity and stresses in an internal boundary of a tectonic arc (Campo de Dalías and Níjar, southeastern Betic Cordilleras, Spain)[END_REF] (Fig. 10).

The present day stresses determined from regional seismicity studies (de [START_REF] De Vicente | Inversion of moment tensor focal Mediterranean: Constraints from an offshore-onshore seismological network and swath bathymetry data[END_REF][START_REF] Stich | Moment tensor inversion for Iberia-Maghreb earthquakes 2005-2008[END_REF] agree with the data obtained in the context of the 2016-17 seismic sequence (Figs. 3 and 10). The NW-SE compression is prolate to triaxial, and inclined towards N319ºE in the NNE-SSW seismicity alignment, while N332ºE in the ENE-WSW alignment, with related orthogonal extension. Thus, compression is rotated in the two alignments, and is compatible with sinistral fault kinematics (Fig. 10). Whereas at the Earth´s surface, main stresses should be horizontal or vertical due to an absence of shear stress [START_REF] Anderson | The dynamics of faulting and dyke formation with applications to Britain[END_REF][START_REF] Bott | The mechanics of oblique slip faulting[END_REF], the inclination of main compression towards the NW at crustal depths suggests activity of northwestward thrusting to some extent. Moreover, this setting could be a consequence of the presence of a low-deformed resistant domain attached to the African plate, here corresponding to the external Rif units on the Variscan basement (Pedrera el al., 2011;Galindo-Zaldívar et al., 2015a;Estrada et al., 2018).

There is furthermore some disagreement between the maximum horizontal compression (between N139ºE and N152ºE) and maximum local shortening (N173ºE) (Fig. 10), estimated by the MALA and MELI GPS stations across the seismic active area. It may be that fault activity is not due to a simple strike-slip fault driven by a far field stress in the sense of [START_REF] Anderson | The dynamics of faulting and dyke formation with applications to Britain[END_REF], but could accommodate crustal block displacement. The NE-SW extension that occurs towards the fault tips, at the northern edge of Campo de Dalías normal faults [START_REF] Marín-Lechado | Active faults, seismicity and stresses in an internal boundary of a tectonic arc (Campo de Dalías and Níjar, southeastern Betic Cordilleras, Spain)[END_REF]Galindo-Zaldívar et al., 2013), is transferred towards the southern edge, where normal faults splay at the Nekor Basin (Fig. 10) (Galindo-Zaldívar et al., 2009;[START_REF] Lafosse | Evidence of Quaternary transtensional tectonics in the Nekor basin (NE Morocco)[END_REF] and crustal thinning is confirmed by gravity results (Fig. 4a).

The epicentres of three seismic series are located west of the main faults recognized at surface, both in the Alboran Sea (Al Idrisi Fault) and in the northern Rif (Trougout Fault) (Figs. 2 and 10) (Galindo-Zaldívar et al., 2009, 2015a;d'Acremont et al., 2014). Hence, the development of the seismic active fault zone west of the well exposed Al Idrisi and Trougout faults might be attributed to the growth of recent faults owing to westward migration [START_REF] Vitale | Polyphase deformation of the Dorsale Calcaire Complex and the Maghrebian Flysch Basin Units in the Jebha area (Central Rif, Morocco): New insights into the Miocene tectonic evolution of the Central Rif belt[END_REF] of the deformation during development of the Gibraltar Arc.

The westward migration of the Gibraltar Arc is well constrained by geological [START_REF] Crespo-Blanc | Clues for a Tortonian reconstruction of the Gibraltar Arc: Structural pattern, deformation diachronism and block rotations[END_REF]GPS data (Fadil et al., 2006;[START_REF] Koulali | New GPS constraints on active deformation along the Africa-Iberia plate boundary[END_REF][START_REF] Palano | The Diffuse Plate boundary of Nubia and Iberia in the Western Mediterranean: Crustal deformation evidence for viscous coupling and fragmented lithosphere[END_REF]. However, the driving mechanism remains a matter of debate, with geodynamic models considering delamination (e.g. Seber et al., 1996;[START_REF] Lis Mancilla | Delamination in the Betic Range: Deep structure, seismicity, and GPS motion[END_REF] and subduction (e.g. [START_REF] Pedrera | Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field[END_REF][START_REF] Ruiz-Constán | Stress distribution at the transition from subduction to continental collision (northwestern and central Betic Cordillera)[END_REF][START_REF] Gutscher | The Gibraltar subduction: A decade of new geophysical data[END_REF]González-Castillo et al., 2015;[START_REF] Spakman | Puzzling features of western Mediterranean tectonics explained by slab dragging[END_REF]), while roll-back is a suitable mechanism for Gibraltar Arc The development of active fault zones has vast implications for seismic hazard. The reactivation of an existing fracture calls for shear stress on the surface to attain the value of the friction and cohesion [START_REF] Bott | The mechanics of oblique slip faulting[END_REF]. Immediately previous to the formation of new fractures, shear stress would have to be above the values of friction in addition to cohesion -considerably higher [START_REF] Anderson | The dynamics of faulting and dyke formation with applications to Britain[END_REF][START_REF] Hajiabdolmajid | Modelling brittle failure of rock[END_REF]. Given the same background setting, the development of a recent fault zone would allow for the accumulation of higher elastic deformation at the fault edges than along a well-developed previous fracture. These factors determine that the propagation of a recent fault, as occurs in the southern Alboran Sea, can produce earthquakes of higher magnitudes than pre-existing faults. The highest magnitude earthquakes of 2004 (Galindo-Zaldívar et al., 2009;d'Acremont et al., 2014) and 2016-17 occurred in areas of recently developed fault segments with scarce evidence of deformation at the surface (Fig. 10). Such faults tend to be particularly active in their initial stage of development, entailing high seismic hazard, and are moreover difficult to detect because of the low amount of accumulated deformation due to their recent age.

Conclusions

The Galindo-Zaldivar, J., Gonzalez-Lodeiro, F., Jabaloy, A., Maldonado, A., & Schreider, A. (1998) Gonzalez-Castillo, L., Galindo-Zaldivar, J., de Lacy, M., Borque, M., Martinez-Moreno, F., García-Armenteros, J., & Gil, A. (2015). Active rollback in the Gibraltar Arc: Evidences from CGPS data in the western Betic Cordillera. Tectonophysics, 663, 310-321. Grevemeyer, I., Gràcia, E., Villaseñor, A., Leuchters, W., & Watts, A.B. (2015).

Seismicity and active tectonics in the Alboran Sea, Western

Figure captions

Fig. 1. Geological setting including regional faults and seismicity. a, Plate boundaries in the Azores-Gibraltar area (modified from Galindo-Zaldívar et al., 2003) and geological sketch of the main structural features and basins of the Alboran Sea (modified from Comas et al., 1999) [START_REF] Michael | Determination of stress from slip data: faults and folds[END_REF]improved by Vavryčuk, 2014). e, cross sections of seismic activity orthogonal to the main alignments.

Ras Tarf Ridge

?

  GPS stations surrounding the central Alboran Sea served as the reference for present-day deformation in the region. MALA and ALME stations (respectively by Malaga and Almeria) along the Betic Cordillera coast, and MELI (by Melilla) on the African coast, time series were obtained from the EUREF permanent Network. Data were considered up to June 2017, and these stations were in operatation: ALME since 2001; MALA since 2005 and MELI since 2012. GNSS data were processed by means of Bernese Software to determine the displacement vectors.

  [START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF] recognize average horizontal errors of 5 km and vertical errors of 10 km, comprised between 2 to 10 km horizontally and 3 to 17 km vertically. The location of 2016-17 seismic activity established by the IGN standard terrain velocity model (Carreño-Herrero & Valero-Zornoza, 2011) (Figs. 2a and 3) or El Moudnib et al. (2015) velocity model (Fig. 2b) shows a deformation band with two alignments, the main one, oriented NNE-SSW, changing sharply to a ENE-WSW trend. Considering the IGN standard terrain velocity model (Carreño-Herrero & Valero-Zornoza, 2011) (Figs. 2a and 3), the NNE-SSW alignment is about 40 km in length and 10-20 km wide, and includes shallow earthquakes (< 35 km depth) that affect the crust and upper mantle. It is parallel and significantly displaced westwards (up to 10 km) with respect to the Al Idrisi Fault (Fig. 2). The ENE-WSW alignment, about 20 km long and 10-15 km wide, is located along the northern side of the westernmost Alboran Ridge and the Francesc Pagès seamount, and is also characterized by a shallow seismicity (< 35 km). During month 1 (Jan 2016), the seismicity clearly defines the two alignments with widths of less than 10-20 km, the main southern one being displaced 5 to 10 km westwards with respect to the trace of the main Al Idrisi Fault (Figs. 2 and 3a).During month 2 (Feb 2016), seismic activity decreases and both alignments become wider (10-20 km) (Fig.3b). Then, from month 3 (March 2016) to present, seismicity gradually decreased and affected a broader area, more than 15-25 km wide (Fig.3c), in between the one clearly defined in month 1, and bounded eastward by the main trace of Al Idrisi Fault.[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF] relocate the earthquakes considering El Moudnib et al. (2015) velocity model and also obtain the same pattern formed by two alignments, the NNE-SSW with a westward offset with respect to the Al Idrisi Fault seabottom location.The earthquake focal mechanisms of the two seismicity alignments show different behaviours (Fig.3d). The main NNE-SSW alignment is characterized by sinistral earthquake focal mechanisms related to NNE-SSW subvertical faults, roughly parallel to the elongation of the alignment. There is also heterogeneity of earthquake focal mechanisms with inclined P and T axes and normal faults, supporting ENE-WSW extension toward the southern deformation zone. In contrast, the ENE-WSW alignment is characterized by highly homogeneous reverse earthquake focal mechanisms associated with ENE-WSW faults. Present-day stress (Fig.3) supports low NNW (N319ºE to N332ºE)

The

  Fig. 4b) nearly parallels the Al Idrisi Fault, though displaced westward. The plots

  multidisciplinary focus on the 2016-17 seismic sequence in the Alboran Sea -in the wake of previous sequences in1993-94 and 2004, and the former Al Idrisi Fault-sheds light on the seafloor deformation and relevant implications in terms of geological hazard. The results help to constrain the processes that occur during migration and propagation of active tectonic brittle deformations.4.1. Constraining active tectonics: seismicity and seafloor deformationsEpicentres related to wrench faults are expected to be located along the fault zone trace. Vertical nodal planes of earthquake focal mechanisms from the main 2016 earthquake and most of the events in the southern Alboran Sea undoubtedly evidence the vertical dip of the seismic active faults (Fig.3), in agreement with the seismological results of[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF]. Anyway, aftershock sequences can affect a wide zone (e.g. >10 km wide with respect to the main fault during the Landers 1992 seismic sequence,[START_REF] Hauksson | The 1992 Landers earthquake sequence: Seismological observations[END_REF] favoured by the structural complexity and pre-existing structures. In spite of[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF] establish horizontal and vertical errors for the 2016-17 seismic sequence location, and[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF] as well as[START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF] relate the seismicity with the Al Idrisi Fault, the careful analyses of epicentre locations for most of the NNE-SSW seismicity alignment (IGN, www.ign.es;[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF][START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF][START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF] points to they are displaced westwards with respect to the seafloor trace of the Al Idrisi Fault determined by marine geophysical data[START_REF] Martínez-García | Strike-slip tectonics and basin inversion in the Western Mediterranean: the Post-Messinian evolution of the Alboran Sea[END_REF], 2017; Estrada et al., 2018) (Figs. 2, 3 and 5). This displacement of the earthquakes is higher when the standard velocity model[START_REF] Carreño-Herrero | Sismicidad de la Península Ibérica en el periodo instrumental: 1985-2011[END_REF]) is considered (IGN, www.ign.es;[START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF][START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF]: roughly 10 km for the first stage earthquakes (Figs.2a and 3a), as opposed to the roughly 5 km[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF] with the model by El Moudnib et al. (2015) (Fig.2b). Although the mislocation of earthquakes by the poorly constrained velocity models and far seismic stations, may produce a westward shift of the seismicity with respect to the Al Idrisi Fault, all the available seismological studies (IGN, www.ign.es;[START_REF] Buforn | The 2016 south Alboran earthquake (Mw=6.4): A reactivation of the Ibero-Maghrebian region?[END_REF][START_REF] Medina | The South-Western Alboran Earthquake Sequence of January-March 2016 and Its Associated Coulomb Stress Changes[END_REF][START_REF] Kariche | The Al Hoceima earthquake sequence of 1994, 2004 and 2016: Stress transfer and poroelasticity in the Rif and Alboran Sea region[END_REF] support the offset to the west of the seismicity and fault activity with respect to the former Al Idrisi Fault that roughly constitutes the eastern boundary of the fault zone. Our findings based on the marine geophysical dataset demonstrate the development of recent faults west of the former Al Idrisi Fault related to the westward propagation of the deformation (Figs. 2, 3, 8, 9 and 10). The early earthquakes of the 2016-17 series are clearly grouped in the two relatively narrow (~10-20 km wide) NNE-SSW and ENE-WSW alignments,

  , the recent fault should extend in depth northward and southward into the seismogenic crustal layer. When deformation propagates upwards, the triggering fault is expected to reach the seafloor in the NNE-SSW epicentres alignment, due to the high dip of fault plane solutions in most of the earthquake focal mechanisms. According to empirical fault scaling relationships (Wells & Coppersmith, 1994), the main seismic event would be related to a surface rupture between 15 and 20 km and might deform the area. In the 2016-17 seismic series, a recent fault zone (10-20 km wide) reaching up to the seafloor was recognized in the main NNE-SSW seismicity alignment, south of Francesc Pagès seamount. This recent zone comprises NW-SE oriented conjugated normal faults coeval with the recent sedimentation, reaching different near-surface stratigraphic levels, probably rotating and connecting in depth with the major recent NNE-SSW vertical sinistral fault (Fig. 10). These shallow faults accommodate the NE-SW extension compatible with the activity of the main wrench fault, and are in agreement with crustal thinning in the Nekor Basin confirmed by Bouguer gravity anomalies (Fig. 4); they would be in line with the recent normal faults described by Lafosse et al. (2017) that demonstrate that the Nekor Basin is floored by a set of splay faults with normal slip component related to the southern prolongation of Al Idrisi Fault. During later earthquakes, the 2016-17 seismic series deformation extended to a broad 10-20 km-wide fault zone in the NNE-SSW alignment, bounded westward by the recent faults and eastward by the Al Idrisi Fault and the Alboran Ridge. Al Idrisi Fault, in contrast, only deforms discontinuously near-surface sediments along the fault trace by recent or active faults and folds, suggesting the recentmost activity of some segments. Although the discontinuous evidences of activity may be the consequence of a seismic character of Al Idrisi Fault, with segments accumulating elastic deformation before the seismic rupture, alternatively they could indicate that the activity of those segments has been replaced by that of other recent faults offset to the west.

  westward displacement and Alboran Sea development. The growth of this seismic NNE-SSW fault zone occurs in the area of weakest and most attenuated continental crust, corresponding to the central Alboran Sea, bounded by the thick continental crust of the Betic and Rif cordilleras. In this regional setting, the Eurasian-African convergence developed indentation tectonics(Estrada et al., 2018) accommodated by the Al Idrisi fault zone, which now extends to westward areas.

  Alboran Sea 2016-17 seismic sequence constitutes a unique opportunity to analyse the development of recent faults in conjunction with seismic hazard. This sequence occurred in the southern part of an active deformation zone crossing the central Alboran Sea and had a main event (Mw=6.3, Jan 25, 2016) located along the corner between the NNE-SSW and ENE-WSW deformation alignments, which are 10-20 km wide. The 40 km-long NNE-SSW alignment is located in the Nekor Basin and Francesc Pagès seamount, west of the seabottom Al Idrisi Fault trace, and the ENE-WSW one is along the northern side of Alboran Ridge and Francesc Pagès seamount. Data recorded during the INCRISIS cruise reveal that the ENE-WSW alignment is mainly characterized by recurrent MTDs that could be linked to earthquake and tectonic activity owing to uplift of the Alboran Ridge and Francesc Pagés antiforms. The NNE-SSW seismicity alignment is related to deep vertical sinistral faults, demonstrated by earthquake focal mechanisms, offset 5 to 10 km westwards from the former Al Idrisi Fault. The major NNE-SSW deep sinistral fault zone responsible for the 2016-17 main events would have activated fault segments up to 15 to 20 km in length for single events. The INCRISIS cruise reveals evidence for recent near-surface ruptures in NW-SE normal faults. These recent faults are related to the western boundary of the deformation zone, bounded eastward by the Al Idrisi Fault; although it is the main fault with seafloor expression, it has segments without recent activity. The present-day stress from earthquake focal mechanisms of 2016-17 constrains a maximum prolate to triaxial compression inclined towards N319ºE in the NNE-SSW alignment, and N332ºE in the ENE-WSW alignment, with related orthogonal extension. Stresses are oblique to the N173ºE shortening determined by GPS data that support sinistral slip behaviour, with some extent of northwestward thrusting. These data reveal a westward migration and offset of active deformation with respect to the already developed Al Idrisi Fault (Fig. 10) that may be linked to the westward development of the Gibraltar Arc. Moreover, the NNE-SSW deformation zone is segmented and was progressively reactivated near the African coast in the 1993-94 seismic sequence, the southern onshore part being affected in 2004 and probably activating NNE-SSW sinistral and WNW-ESE dextral faults, with deformation later propagating offshore toward the northeast in 2016-17. The present-day NW-SE Eurasian-African plate convergence in the westernmost Mediterranean and the inherited heterogeneous rigid basement structures determine the location of deformation areas. At present, a main NNE-SSW sinistral deformation fault zone, including the Al Idrisi Fault and recent developed faults offset to the west, connects the NW-SE extensional faults of the Campo de Dalias´s northern edge (Betic Cordillera) with the southern edge´s normal splay faults located at the Nekor Basin in the Rif (Fig. 10). This setting also contributes to the local crustal thinning of Nekor Basin supported by gravity data.Geological hazard in the central Alboran Sea is closely related to the seismicity that constitutes a main triggering mechanism of MTDs and is moreover responsible for coseismic seafloor displacements. The development of recent faults that condition the westward widening of the fault zone compared with the already developed faults would imply the activity of the largest segments and the greatest accumulation of elastic energy, producing high magnitude earthquakes that increase seismic hazard. In any case, low accumulated deformation and recent activity are predominant features making it possible to recognize such faults, and therefore deserving further analysis through a multidisciplinary approach. mechanisms for active stresses around the microcontinent Iberia: S.O., Tadili, B.A., Cherkaoui, T.E.,Medina, F., Ramdani, M., Brahim, L.A., & Harnafi, M. (1998). The Al Hoceima earthquake ofMay 26, 1994 and its aftershocks: a seismotectonic study, Ann.Geophys., 41, 519- 537. El Moudnib, L., Villaseñor, A., Harnafi, M., Gallart, J., Pazos A., Serrano, I., … Chourak, M. (2015). Crustal structure of the Betic-Rif system, western Mediterranean, from local earthquake tomography, Tectonophysics, 643, 94-105, doi: 10.1016/j.tecto.2014.12.015 Estrada, F., Galindo-Zaldívar, J., Vázquez, J.T., Ercilla, G., D'Acremont, E., Alonso, B., & Gorini, C. (2018). Tectonic indentation in the central Alboran Sea (westernmost Mediterranean), TerraNova, 30, 24-33, doi: DOI: 10.1111/ter.12304 Fadil, A., Vernant, P., McClusky, S., Reilinger, R., Gomez, F., Sari, D.B., ... Barazangi, M. (2006). Active tectonics of the western Mediterranean: Geodetic evidence for rollback of a delaminated subcontinental lithospheric slab beneath the Rif Mountains,Morocco, Geology, 34, 529-532, doi: 10.1130/G22291.1. Frasca, G., Gueydan, F., & Brun, J.P. (2015). Structural record of Lower Miocene westward motion of the Alboran Domain in the Western Betics, Spain, Tectonophysics, 657, 1-20.

Fig. 4 .

 4 Fig. 4. Gravity anomaly maps and main tectonic features. a, Bathymetry map,

Fig. 9 .Fig

 9 Fig. 9. Segments of TOPAS seismic records displaying the recent fault zone

  . Models of magnetic and Bouguer gravity anomalies for the deep structure of the central Alboran Sea basin, Geo-

	Mar. Lett., 18, 10-18.
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	and geological observations of the Al Hoceima (Morocco, Rif) 2004
	earthquake (M=6.3) contradictory?, Tectonophysics, 475, 59-67, doi:
	10.1016/j.tecto.2008.11.018.
	Galindo-Zaldívar, J., Azzouz, O., Chalouan, A., Pedrera, A., Ruano, P., Ruiz-
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	Ras	Afraou	area),	Tectonophysics,	663,	140-149	doi:
	10.1016/j.tecto.2015.08.029.
	Galindo-Zaldivar, J., Gil, A.J., Sanz de Galdeano, C., Lacy, M.C., García-
	Armenteros, J.A., Ruano, P., … Alfaro, P. (2015b). Active shallow
	extension in central and eastern Betic Cordillera from CGPS data,
	Tectonophysics, 663, 290-301, doi: 10.1016/j.tecto.2015.08.035.

  . Displacement of GPS stations around the Alboran Sea with respect to stable Eurasia are indicated (red arrows). The dotted area within the inset indicates the deformation area. b,

	Fig. 3. Seismicity during the 2016-17 seismic sequence. a, Epicentres month 1
	(January 2016). b, Epicentres month 2 (Feburary 2016). c, Late epicentres
	(March 2016 to June 2017). d, Earthquake focal mechanisms from 2016-17
	seismic crisis (data from I.G.N., www.ign.es). NNE-SSW alignment in blue,
	ENE-WSW alignment in red. The main earthquake, located at the edge of the
	two alignments, is shared by the two groups. Present-day stress is determined
	in the two main sectors of 2016-17 Alboran Sea seismic crisis area (by the
	Tracklines of multibeam, very high resolution seismics (TOPAS) and gravity methods of
	surveyed simultaneously during the INCRISIS cruise. c, Tracklines of single-
	and	multi-channel	seismics	(airguns)	from	the	ICM	database
	(http://gma.icm.csic.es/sites/default/files/geowebs/OLsurveys/index.htm).
	Legend: AF, Al Idrisi Fault; AR, Alboran Ridge; AC, Alboran Channel; BF,
	Balanegra Fault; CD, Campo de Dalías; CG, Cabo de Gata; CF, Carboneras
	Fault; DB, Djibouti Bank; EAB, East Alboran Basin; FP, Francesc Pagès
	seamount; NB, Nekor Basin; NF, Nekor Fault; SAB, South Alboran Basin; TF,
	Trougout Fault; WAB, West Alboran Basin; YF, Yusuf Fault.
	Fig. 2. Seismicity distribution during the 2016-17 seismic sequence and other
	recent seismicity (including 1993-94 and 2004 series) (2004 seismic sequence
	from Van der Woerd et al., 2014; other seismicity from www.ign.es database).
	a, Epicentres considering a standard velocity model. b, Relocated epicentres for
	the highest magnitude earthquakes of the 2016-17 seismic sequence according
	to El Moudnib et al. (2015). Legend: FP, Francesc Pagès seamount; NB, Nekor
	Basin.

Acknowledgements

We acknowledge the comments of Prof. N. Niemi, Prof. J. Bruce H. Shyu, Prof. M-A Gutscher and three anonymous reviewers which served to improve the quality of this paper. We also thank A. Mandarieta for his help with the figures. The geological data referenced in this paper are available in the text, the seismicity data are available on the IGN website at http://www.ign.es/web/ign/portal/sis-catalogo-terremotos and the marine geophysical data are available on the ICM-CSIC website at http://gma.icm.csic.es/sites/default/files/geowebs/OLsurveys/index.htm . This study was supported by the Spanish projects INCRISIS (CGL2016-80687-R AEI/FEDER) DAMAGE Project FEDER, SARAS and FAUCES (CTM2015-65461-C2-1-R) and Marlboro cruises. French program Actions Marges, the EUROFLEETS program (FP7/2007-2013; n°228344). The participation of D. Palomino and J.T. Vazquez was funded by the IEO project RIGEL.

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