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Kinematic analysis of secondary faults within a distributed shear-zone reveals fault linkage and increased seismic hazard

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Complex multifault earthquake ruptures involving secondary faults emphasize the necessity to characterize their seismogenic potential better and study their relationship with major faults to improve the seismic hazard assessment of a region. High-resolution geophysical data were interpreted to make a detailed characterization of the Averroes Fault and the North Averroes Faults, which are poorly known secondary right-lateral strike-slip faults located in the central part of the Alboran Sea (western Mediterranean). These faults appear to have evolved since the Pliocene as part of a distributed dextral strike-slip shear zone in response to local strain engendered by the diverging movement of the Carboneras Fault to the north, and the Yusuf and Alboran Ridge faults to the south. In addition, the architecture of these faults suggests that the Averroes Fault may eventually link with the Yusuf fault, thus leading to a higher seismogenic potential. Therefore, these secondary faults represent a hitherto unrecognized seismogenic hazard since they could produce earthquakes up to moment magnitude (Mw) 7.6. Our results highlight the importance of the role played by secondary faults in a specific kinematic framework. Their reciprocal linkage and their mechanical relationship with the main faults could lead to future complex fault ruptures. This information could improve fault source and earthquake models used in seismic and tsunami hazard assessment in this and similar regions. Keywords: Active faults; strike-slip faults; fault linkage; distributed shear zones; earthquakes; Alboran Sea; western Mediterranean
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Confidential manuscript submitted to Marine Geology
https://doi.org/10.1016/j.margeo.2018.02.002
© 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license
http://creativecommons.org/licenses/by-nc-nd/4.0/ 1
Kinematic analysis of secondary faults within a distributed shear-zone
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reveals fault linkage and increased seismic hazard
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Hector Pereaa,b, Eulàlia Gràciaa, Sara Martínez-Lorientec, Rafael Bartolomea, Laura
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Gómez de la Peñaa,d, Ben de Mole, Ximena Morenoa, Claudio Lo Iaconof, Susana Diezg,
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Olvido Telloh, María Gómez-Ballesterosh and Juan José Dañobeitiag
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(a) B-CSI, Institut de Ciències del Mar-CSIC, 08003 Barcelona, Spain
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(b) GRD, Scripps Institution of Oceanography - University of California San Diego, La Jolla
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92093, United States
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(c) iCRAG (Irish Centre for Research in Applied Geosciences), University College Dublin,
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Belfield, Dublin 4, Ireland
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(d) GEOMAR Helmholtz Centre for Ocean Research, 24148 Kiel, Germany
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(e) VNG Norge AS, Oslo 0252, Norway.
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(f) Marine Geoscience, National Oceanography Centre, University of Southampton Waterfront
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Campus, Southampton SO14 3ZH, United Kingdom
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(g) Unitat de Tecnologia Marina-CSIC, 08003 Barcelona, Spain
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(h) Instituto Español de Oceanografía, 28002 Madrid, Spain
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Corresponding author: Hector Perea (hperea@icm.csic.es, hperea@ucsd.edu)
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19
2
Abstract
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Complex multifault earthquake ruptures involving secondary faults emphasize the necessity to
21
characterize their seismogenic potential better and study their relationship with major faults to
22
improve the seismic hazard assessment of a region. High-resolution geophysical data were
23
interpreted to make a detailed characterization of the Averroes Fault and the North Averroes
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Faults, which are poorly known secondary right-lateral strike-slip faults located in the central
25
part of the Alboran Sea (western Mediterranean). These faults appear to have evolved since the
26
Pliocene as part of a distributed dextral strike-slip shear zone in response to local strain
27
engendered by the diverging movement of the Carboneras Fault to the north, and the Yusuf and
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Alboran Ridge faults to the south. In addition, the architecture of these faults suggests that the
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Averroes Fault may eventually link with the Yusuf fault, thus leading to a higher seismogenic
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potential. Therefore, these secondary faults represent a hitherto unrecognized seismogenic hazard
31
since they could produce earthquakes up to moment magnitude (Mw) 7.6. Our results highlight
32
the importance of the role played by secondary faults in a specific kinematic framework. Their
33
reciprocal linkage and their mechanical relationship with the main faults could lead to future
34
complex fault ruptures. This information could improve fault source and earthquake models used
35
in seismic and tsunami hazard assessment in this and similar regions.
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Keywords
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Active faults; strike-slip faults; fault linkage; distributed shear zones; earthquakes; Alboran Sea;
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western Mediterranean
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3
1 Introduction
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Detailed structural mapping, as well as kinematic and seismogenic characterization of
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active submarine fault systems, is essential for determining possible fault linkages for potential
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rupture scenarios and, consequently, for improving the assessment of the seismic and tsunami
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hazard in densely populated coastal areas. Recent advances in seafloor and subsurface imaging
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have made it possible to obtain high-resolution seismic and bathymetric data, which can be used
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to accurately characterize the kinematic patterns and tectonic architecture of submarine faults
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with unprecedented detail (Armijo et al., 2005; Barnes, 2009; Bartolome et al., 2012; Brothers et
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al., 2015; Escartín et al., 2016; Gasperini et al., 2011a,b; Gràcia et al., 2012, 2006; Martínez-
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Loriente et al., 2013; McNeill et al., 2007; McNeill and Henstock, 2014; Moreno et al., 2016;
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Perea et al., 2012; Polonia et al., 2012; Sahakian et al., 2017). In addition, methods have been
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developed to determine the seismogenic potential and earthquake history of single faults based
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on on-fault marine paleoseismology (e.g. Barnes and Pondard, 2010; Brothers et al., 2011, 2009).
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These studies have generally focused on large continental fault systems. Nevertheless, surface
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rupture mapping of recent large magnitude earthquakes shows complex multifault ruptures
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involving unknown or poorly known secondary faults (e.g., Elliott et al., 2012; Hamling et al.,
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2017; Quigley et al., 2012). These observations highlight the need to characterize onshore and
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offshore secondary fault systems as well as their relationship to major faults to improve seismic
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hazard assessment (e.g., Field et al., 2014; Perea and Atakan, 2007; Stirling et al., 2012). Here
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we present an example of a previously unknown secondary active fault system located in the
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central Alboran Sea (western Mediterranean), which is related to the largest faults controlling the
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regional geodynamics.
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4
The Alboran Sea is a Neogene basin formed by crustal extension related to the
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subduction system in the Gibraltar Arc (e.g., Booth-Rea et al., 2007; Comas et al., 1999; van
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Hinsbergen et al., 2014). At present, left-lateral and right-lateral strike-slip faults trending NE-
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SW and WNW-ESE, respectively, accommodate part of the strain related to the NW-SE
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convergence (4-5.5 mm/yr) between the African and Eurasian plates (e.g., DeMets et al., 2010;
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Palano et al., 2015; Vernant et al., 2010). Consequently, the Alboran Sea shows a remarkable
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seismic activity, mainly concentrated along what is known as the Trans-Alboran Shear Zone (De
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Larouzière et al., 1988). Although this seismicity is mainly characterized by low to moderate
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magnitude events (Buforn et al., 1995; Stich et al., 2006), large and destructive earthquakes have
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occurred in the region, such as the 1522 Almería (IEMS98 IX; Spain), the 1790 Oran (IMSK IX-
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X; Algeria), the 1910 Adra (IEMS98 VIII and Mw 6.1; Spain), the 1994 and 2004 Al-Hoceima
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(Mw 6.0 and 6.4, respectively; Morocco), and the 2016 Al-Idrissi (Mw 6.4; Morocco) events (e.g.,
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Biggs et al., 2006; Buforn et al., 2017; Calvert et al., 1997; Gràcia et al., 2012; Martínez Solares
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and Mezcua, 2002; Stich et al., 2003; Tahayt et al., 2009; van der Woerd et al., 2014) (Fig.1).
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The main active faults in the Alboran Sea are the left-lateral Carboneras and Al-Idrissi
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strike-slip faults, the right-lateral Yusuf strike-slip fault and the Alboran Ridge thrust fault
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(Gràcia et al., 2006; Martínez-García et al., 2013; Medaouri et al., 2014; Moreno et al., 2016)
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(Fig. 1). These faults converge at the center of the Alboran Sea between the East and West
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Alboran basins. Their structural relationship, however, is still under debate. In between these
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main faults there is a secondary fault-system, the Averroes Fault (AF) and North Averroes Faults
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(NAFs) with a WNW-ESE trend (Fig. 1). Understanding the tectonic evolution of these
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secondary faults and their relationship and potential linkage with the large fault systems will help
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to characterize the present kinematics of the area and define potential fault rupture scenarios.
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5
Accordingly, the overarching goals of this work are: (a) to characterize the WNW-ESE trending
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AF and NAFs based on newly acquired swath-bathymetry, sub-bottom acoustic profiles, and
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high-resolution multichannel seismic data; (b) to determine the role of these secondary faults to
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further understand the kinematic pattern and style of deformation in the central part of the
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Alboran Sea; and (c) to evaluate the contribution and relevance of secondary faults to the
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seismogenic potential of the western Mediterranean region.
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2 Data and methods
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Our dataset comprises swath-bathymetry and recently acquired high-resolution seismic
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reflection profiles, including sub-bottom (SBP) and multichannel (HR-MCS) data (Fig. 2). The
93
integration of these datasets makes it possible to accurately map fault traces for offshore areas
94
and estimate vertical and horizontal displacements.
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During the EVENT (2010) and SHAKE (2015) cruises, 30 m grid size swath-bathymetry
96
was acquired with the ATLAS Hydrosweep DS multibeam echosounder, hull mounted system of
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the RV Sarmiento de Gamboa. This dataset has been included in the existing bathymetric
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compilation covering most of the Alboran Sea with a grid size of 50 m (e.g., Gràcia et al., 2012,
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2006; Ballesteros et al., 2008) (Figs. 2, 3). The different datasets were processed and integrated
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using the CARIS software.
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The HR-MCS profiles were acquired during the IMPULS (2006) and EVENT (2010)
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cruises on board the RV Hesperides and RV Sarmiento de Gamboa, respectively. Multichannel
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seismic data were collected in the IMPULS cruise using a 300-m long GeoEel Geometrics digital
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streamer with 48 channels, towed at 2 m water depth. The streamer was composed by 6 active
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sections of 50 meters, and each section contained 8 channels with 6.25 m spacing. The seismic
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6
source consisted of an 8-airgun array with a total volume of 4.75 l. In the EVENT cruise, a 600-
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m long Sercel SEAL 96 multichannel seismic streamer with channel spacing of 6.25 m was used
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and towed at 2.5 m water depth. The source array was composed of 10 airguns, with a total
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volume of 13.1 l. Data acquired during the two surveys were quality controlled on board and
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processed, time-migrated and interpreted at the seismic laboratory of the Institut de Ciències del
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Mar-CSIC using the ProMAX, Globe ClaritasTM, and IHS Kingdom softwares, respectively.
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The applied processing seismic flow included: statics correction; trace editing (deleting noisy
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channels in terms of RMS quality control); top mute from 0 time to seabed; bandpass filter; FK
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filtering for a specific dip noise; true amplitude recovering due to spherical divergence loss
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energy; common depth point (CDP) sorting; normal move out after picking a velocity model;
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CDP stack; trace equalization; time migration; and SEGY export format.
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The SBP profiles were acquired at the same time as the HR-MCS profiles during the
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IMPULS survey, using the Simrad TOPAS PS18 hull mounted system of the RV Hesperides.
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The collected profiles show the detailed sedimentary architecture of the uppermost tens of meters
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below the seafloor (up to 100 m), mainly Plio-Quaternary sedimentary units, and evidence recent
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fault activity.
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3 Seismostratigraphy of the central part of the Alboran Sea
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During the last decades, the seismostratigraphic units in the central part of the Alboran
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Sea have been successively re-defined due to the availability of high-quality and high-resolution
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seismic datasets (e.g., Booth-Rea et al., 2007; Comas et al., 1999; Gomez de la Peña, 2017 Juan
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et al., 2016; Martínez-García et al., 2013; Moreno et al., 2016). In addition, the combination of
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the seismic data with radiometric dating and biostratigraphic analyses of sediments and rocks
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collected in sediment cores and commercial and scientific boreholes (e.g. Ocean Drilling
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7
Program sites; Comas et al., 1999) has made it possible to establish accurate ages for the
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seismostratigraphic units (Fig. 4).
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In our HR-MCS profiles, five seismostratigraphic units (Ia1, Ia2, Ia3, Ib1, and Ib2) are
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identified above the regional Messinian reflector (top of unit II) (Figs. 5, 6). These units were
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previously characterized and described by Moreno et al. (2016) (Fig. 4) and their ages are, from
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youngest to oldest: Ia1 - upper Quaternary (present day to 0.78 Ma); Ia2 - lower Quaternary
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(0.78 to 2.45 Ma); Ia3 - Upper Pliocene (2.45 to 3.28 Ma); Ib1 - upper Lower Pliocene (3.28 to
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4.57 Ma); Ib2 - low Lower Pliocene (4.57 to 5.33 Ma); and II - Upper Miocene (> 5.33 Ma).
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4 A secondary fault system in the central Alboran Sea: The Averroes and North Averroes
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Faults
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In the central part of the Alboran Sea, the seafloor morphology of the Djibouti Plateau
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shows the presence of an elongated and narrow trough, striking N115º to N125º, which
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corresponds to the surface expression of the AF (Fig. 2). Over time, its vertical and horizontal
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displacement has generated a 2-km wide and 13.2-km long basin, referred to as the Averroes
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Basin (AB in Figs. 2, 5 ,6), which becomes progressively narrower as it crosses the Adra Ridge.
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To the southeast, across the Alboran Channel and up to the Alboran Ridge, there is an
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escarpment aligned with the AF that bends slightly to the WNW-ESE and uplifts the seafloor
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locally at the center of the channel, forming an elongated ridge (Figs. 2, 3a). Considering that the
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AF extends from the Djibouti Plateau to the base of the Alboran Ridge, its total length is 46.6 km
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(Table 1). The bathymetric map (Figs. 2, 3a, 3b) shows a geomorphologic step-over where the
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AF intersects the northwestern edge of the Alboran Channel, in which the block north of the fault
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has been displaced towards the southeast. To estimate the lateral offset on the AF, we identified
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piercing points on both sides of the fault, geomorphologic similarities on the seafloor and the fit
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8
with the bathymetric contours. Accordingly, this fault scarp has a right-lateral cumulative offset
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of about 4181 m (Fig. 3b; Table 2).
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The HR-MCS profiles reveal that the AF is a right-lateral sub-vertical fault with a vertical
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component, dipping ~70º to 90º, offsetting all seismostratigraphic units from the Miocene to the
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Quaternary, and showing seafloor expression (Figs. 5, 6). The seismic profiles show that the AF
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splits into two branches when it crosses the Adra Ridge South, and that bounding to the
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northeast, the Averroes Basin has numerous secondary faults that may link in depth with the AF
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(Fig. 5a). The average vertical offset, measured on different HR-MCS profiles crossing the
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Averroes Basin, is 0.61 s two-way travel time (TWTT) at the base of unit Ib1, which is depth
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converted to a range between 473 and 488 m (Fig. 5a). However, this should be considered as a
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minimum offset range since the time-depth conversion is based on a nominal interval velocity of
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1550-1600 m/s in the uppermost sediments, as precise seismic velocities are not available in this
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area.
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Across the Djibouti Plateau and Adra Ridge North, there are several escarpments, small
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sub-basins and lineaments parallel to the AF (Figs. 2, 3a). These features correspond to the
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surface expression of the NAFs, which are labeled NAF1 to NAF4. Broadly, these fault systems
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are characterized by a main escarpment with much smaller vertical throws than in the AF. An
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anastomosed and/or sigmoidal geometry fault system can be observed where the different minor
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escarpments link (Fig. 3a). The main faults of each NAF system cut across the Djibouti Plateau,
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the Adra Ridge North and reach the Alboran Channel and East Alboran Basin. Their lengths
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range between 16.9 km (NAF2b) and 36.5 km (NAF2) (Table 1). Moreover, they have different
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morphological features that exhibit lateral offsets across the faults, indicating that they all have a
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right-lateral movement (e.g., offset linear ridges on the Adra Ridge North or scarps in the
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northwestern margin of the Alboran Channel) (Figs. 3a, 3d-f). Considering the offsets between
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the identified piercing points projected into the NAFs, it is possible to estimate a right-lateral
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movement ranging between 1185 m (NAF2) and 656 m (NAF3) (Figs. 3d-f; Table 2).
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The HR-MCS and SBP profiles show that there is pervasive faulting corresponding to the
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NAFs, which offsets all the sedimentary units and the basement at the Djibouti Plateau and Adra
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Ridge North (Fig. 6). These faults are sub-vertical (dipping 70º-90º), have small vertical throws,
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and some of them branch at depth. Usually, two faults bound each NAF system, which laterally
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join and dip in opposite directions towards the center of the faulted zone and clearly offset the
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Quaternary units and deform the seafloor (e.g., NAF 1 and NAF2 in Figs. 3a, 6). Between these
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bounding faults, there are minor faults that merge laterally at depth and most of them offset
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Pliocene units. The activity of the different NAFs has produced small, elongated depressions in
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the seafloor (Figs. 2, 3a) and some exhibit subdued negative flower structure geometry (e.g.,
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NAF1 and NAF3 in Fig. 6), indicating the accommodation of transtensive deformation.
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Regarding the activity of the AF and NAFs, the interpretation of the HR-MCS profiles
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suggests different tectonic phases based on changes in thickness of the sedimentary units within
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the Averroes Basin and Djibouti Plateau (Fig. 7). The first phase is preceded by the deposition of
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unit Ib2 (low Lower Pliocene) infilling local depressions related to the Messinian unconformity
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(Figs. 7b, 7c). Although the activity of the faults may have started during the deposition of unit
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Ib2, the most intensive and significant transtensive deformation is observed in unit Ib1 (upper
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Lower Pliocene). This activity resulted in the creation of the Averroes Basin and some minor
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local basins related to the activity in the NAFs (Fig. 7d). These observations disagree with
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previous studies that suggest that the Djibouti Plateau was not affected by significant
197
deformation prior to the Quaternary (Martínez-García et al., 2013). The previous tectonic phase
198
10
is followed by the deposition of unit Ia3 (Upper Pliocene; Fig 7e), which shows an almost
199
constant sedimentary thickness throughout the entire area. This suggests a period of tectonic
200
quiescence or, alternatively, pure strike-slip deformation without vertical displacement along the
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AF and NAFs. The presence of a small local depocenter located in the northwestern termination
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of the AF, together with the observation that the AF and NAFs are almost parallel to the WNW-
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ESE regional stress field at that time (MartínezGarcía et al., 2013), might support pure strike-slip
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displacement along the faults. Finally, during the Quaternary, the tectonic activity may have
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increased or changed from strike-slip to transtensional, as recorded by the presence of local
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depocenters associated with the northwestern termination of the AF and NAF faults (Fig. 7f).
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However, the deformation rates might have been quite moderate considering the relatively
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constant thickness of the Quaternary units over the area. In agreement with these observations,
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the measured lateral offsets on the AF and NAFs (Fig. 3; Tables 2) may correspond to the
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cumulative fault displacement since the upper Lower Pliocene (4.57 Ma).
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5 Discussion: kinematics, fault linkage and seismic potential
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The analysis of the bathymetric and HR-MCS data reveals that the AF and NAFs are
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right-lateral strike-slip faults, with transtensive and transpressive deformation patterns in the
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Djibouti Plateau and the Alboran Channel, respectively. The following points are the evidence
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for this kinematic interpretation: (a) right-lateral offsets observed on the northwestern margin of
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the Alboran Channel and across the Adra Ridge North; (b) vertical to sub-vertical fault dip; (c)
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changes in the dip-slip component of the AF (extension in the Averroes Basin to compression in
218
the Alboran Channel), coinciding with variations in the strike of the fault and resulting in
219
releasing and restraining bends; (d) anastomosed to sigmoidal geometry of the NAFs, which may
220
indicate fault segment linkage and lateral growth of the systems (Aydin and Berryman, 2010;
221
11
Schreurs, 2003); and (e) subdued negative flower-structure geometries and formation of small
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elongated depressions along some of the NAFs (Figs. 2, 3, 5, 6).
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The right-lateral strike-slip AF and NAFs are related to the Neogene-Quaternary thinning
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of continental crust intruded by arc magmatism (e.g., Booth-Rea et al., 2007; Mancilla et al.,
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2015), and are bounded by the largest active faults in the Alboran Sea, the Carboneras left-lateral
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strike-slip fault to the northwest, and the Alboran Ridge thrust and Yusuf right-lateral strike-slip
227
faults to the south (Figs. 1, 8). These two facts most likely determine the style of deformation in
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this area. Generally, strike-slip deformation in the continental lithosphere is distributed over a
229
wide shear-zone instead of accumulating in a single fault (e.g., McKenzie and Jackson, 1983). In
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addition, analogue models of fault development and interaction in distributed strike-slip shear
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zones (e.g., Dooley and Schreurs, 2012; Schreurs, 2003) exhibit similar features and geometries
232
at the surface and in-depth as those observed for the AF and NAFs (Figs. 2, 3, 5, 6, 8). Although
233
the present NW-SE trending convergence rate controls the regional deformation in the Alboran
234
Sea, the motion towards the NE of the southeastern block of the Carboneras Fault and to the
235
E/ESE of the northern block of the Alboran Ridge and Yusuf faults may result in the
236
development of a local stress field between these main structures. In agreement with the fault
237
strike, the resulting local field may trend approximately E-W (Fig. 8b). Since the regional stress-
238
field affects the continental crust, we propose that the central Alboran region may have evolved
239
as a distributed right-lateral strike-slip shear zone, forming a WNW-ESE pervasive fault system
240
(i.e., AF and NAFs) to accommodate local deformation. Analogue models applied to similar
241
settings predict that sub-vertical parallel strike-slip faults developed in a distributed shear zone
242
may strike between 28º and 35º relative to the direction of the bulk shear (Dooley and Schreurs,
243
2012; Schreurs, 2003). Considering that the strike of the AF and NAFs ranges between N115º
244
12
and N125º, the local right-lateral bulk shear may strike approximately N90º, in agreement with
245
the assumed local stress field (Fig. 8b).
246
Understanding and assessing fault linkage to characterize the maximum earthquake
247
rupture length is critical for estimating the expected maximum magnitudes, and thus, improving
248
seismic hazard studies that include active faults as earthquake sources (e.g., Field et al., 2014;
249
Perea and Atakan, 2007; Stirling et al., 2012). In the study area, the structural connection
250
between the AF and the Yusuf Fault is under debate. Martínez-García et al. (2013) proposed that
251
the Yusuf fault terminates at the northern side of the Alboran Ridge where it links with the
252
Alboran Ridge thrust fault, although without any connection to the AF. The geomorphologic
253
analysis of the bathymetry along the southern margin of the Alboran channel shows that there is
254
no large discontinuities or step-overs between the Yusuf and Alboran Ridge faults, suggesting
255
that both are part of the same fault system. In addition, we observed that: (a) the AF offsets the
256
seafloor, generating an elongated ridge across the Alboran Channel (Figs. 2, 3a-c); (b) there is a
257
right-lateral step-over at the base of the NW margin of the Alboran Ridge aligned with the AF
258
and showing a similar strike (Figs. 2, 3c); and (c) there is a minor ENE-WSW escarpment across
259
the Alboran Ridge slope (Figs. 2, 3c). Our observations suggest that the AF cuts across the
260
Alboran Channel and connects to the Yusuf fault system. Moreover, the increased deformation
261
(larger offset and elongated basin) along the AF than in the other NAFs could result from the AF
262
accommodating part of the deformation transmitted by the Yusuf fault. Analogue models that
263
consider interferences between thrust and strike-slip faults, with similar geometries to those
264
observed across the Alboran Ridge and Yusuf faults, reveal the formation of outer faults when a
265
velocity discontinuity is placed in front of the thrust domain (Rosas et al., 2015). The outer faults
266
have a similar direction as the main strike-slip faults and cut across previously formed thrust
267
13
faults. The resulting structural geometries resemble the AF, Yusuf, and Alboran Ridge faults,
268
and therefore are consistent with a connection between the AF and Yusuf Fault to form a
269
continuous right-lateral strike-slip system. Hence, considering the link between the AF and the
270
Yusuf Fault, the maximum fault rupture length of the main fault increases from 175 km to at
271
least 221 km, with evident implications regarding seismic hazard.
272
Despite the current relatively low seismic activity in this area, our observations
273
demonstrate that the AF and NAFs are active since they offset the Quaternary units and deform
274
the seafloor (Figs. 2, 3, 5, 6). In 2009 and 2010 a network of ocean bottom seismometers was
275
deployed in the Alboran Sea and land seismometers were used in the neighboring coastal areas
276
(Grevemeyer et al., 2015). A total of 229 local earthquakes were recorded, most of them of low
277
magnitude (Mw 1.2 to 2.8), and the largest event recorded in the East Alboran Basin was Mw 3.5.
278
The locations of the earthquakes show a cluster of seismicity at the intersection between the
279
Alboran Channel and the AF while other events nucleate near the NAFs (Figs. 2, 3a, 8a). This
280
microseismicity may highlight the present-day activity of the AF and NAFs. In addition, the
281
crustal models obtained in this area reveal that the maximum thickness of the seismogenic zone
282
is 15 km (Grevemeyer et al., 2015), which limits the earthquake nucleation on the faults to this
283
depth.
284
The seismic potential of a given fault is determined by its slip-rate and by the maximum
285
magnitude of the earthquake it can generate. To estimate the long-term slip-rate of the AF and
286
the NAFs we measured the right-lateral offsets recorded for different morphological features,
287
and the vertical offset on the AF (Fig. 3; Table 2). Assuming that these offsets are the result of
288
the fault activity since their formation in the upper Lower Pliocene (4.57 Ma), the Plio-
289
Quaternary lateral slip-rate for the AF is estimated to be 0.91 mm/yr and range between 0.14 and
290
14
0.26 mm/yr for the NAFs (Table 2). In addition, the AF has produced a minimum vertical offset
291
of about 473-488 m at the base of Unit Ib1 (4.57 Ma), resulting in a minimum vertical slip-rate
292
of about 0.1 mm/yr for the same time interval. Assuming that the lateral slip-rate of the AF has
293
been constant over time and considering that the offset at the base of the Alboran Ridge (1011 m;
294
Fig. 3c) is produced by displacement along the AF, it can be postulated that this fault most likely
295
connected to the Yusuf fault system during the Calabrian (about 1.1 Ma). The higher slip-rate for
296
the AF suggests that this fault is accommodating more deformation than the NAFs, probably due
297
to its connection with the Yusuf fault. However, considering that the two faults may have been
298
linked during the Quaternary and that the AF has been more active than the NAFs since their
299
formation in the upper Lower Pliocene (Fig. 7), we can conclude that the AF has always been the
300
main fault of the distributed shear zone. Although there are large uncertainties in the lateral and
301
vertical slip-rate estimates, they appear consistent with those of the bounding faults determined
302
by GPS block kinematic modeling (e.g., Vernant et al., 2010), and with the AF and NAFs
303
representing secondary faults in the regional tectonic framework.
304
To estimate the maximum magnitude earthquake that the AF and NAFs could generate,
305
we considered the mapped fault length and the empirical relationship proposed by Wesnousky
306
(2008) (see Table S1 in supplementary materials for Mw obtained using other relationships).
307
Accordingly, the AF has the potential of generating earthquakes of Mw as large as 7.0, and the
308
NAFs between 6.6 and 6.9 (Table 1). However, considering a scenario in which the AF and the
309
Yusuf fault may be linked and that both could rupture at the same time, the related earthquake
310
reaches a maximum Mw of 7.6 (Table 1). Recent large earthquakes, such as the Kaikoura event
311
(Hamling et al., 2017), have resulted in complex multifault surface ruptures, involving a number
312
of unknown or poorly known secondary faults in the rupture process. In addition, recent studies
313
15
based on the analysis of several surface ruptures (Biasi and Wesnousky, 2016) have shown that
314
earthquake ruptures are equally likely to propagate over fault steps of 3 km. Considering that the
315
distance from the southeastern tip of NAF 1 and NAF2 to the Yusuf fault or the AF is shorter
316
than 3 km (Fig. 3a) a complex multifault rupture could be possible, and even larger magnitude
317
earthquakes cannot be ruled out. Earthquakes with a magnitude equal to or larger than 7.6 and
318
generated in the first 15 km of the upper crust (Grevemeyer et al., 2015) may produce surface
319
rupture, with vertical offsets in certain zones of the fault system (e.g., Averroes Basin and
320
Alboran Ridge). Eventually, they might trigger submarine mass movements on the slopes of
321
surrounding ridges. These landslides could be tsunamigenic depending on their acceleration
322
(Driscoll et al., 2000) and represent an additional threat to nearby coastal areas.
323
6 Conclusions
324
This study identifies and characterizes the seismic potential of previously unknown
325
secondary active fault systems, the AF and NAFs, which displace the Quaternary sedimentary
326
units and deform the seafloor in the Alboran Sea producing microseismicity. We propose that
327
these faults may have developed in a distributed E-W right-lateral strike-slip shear zone during
328
the last 4.57 Ma to accommodate the local stress field resulting from the different kinematics
329
along the Carboneras and Yusuf fault systems. The seismogenic characterization of the AF and
330
NAFs reveals that these structures are characterized by low slip-rates (<1 mm/yr) but have the
331
potential for generating moderate to large earthquakes (6.6<Mw<7.0) in an area presently
332
characterized by low seismic activity. In addition, the data demonstrate a likely linkage between
333
the AF and the Yusuf fault, which would lead to increased earthquake magnitudes that can reach
334
up to Mw 7.6.
335
16
These results demonstrate that, in the offshore, high-resolution geophysical methods are
336
fundamental for identifying, accurately mapping and characterizing secondary fault systems, and
337
thus, they are critical to the understanding of the main active fault systems. In addition, we also
338
demonstrate that specific studies aimed at understanding the role that the secondary faults play in
339
a specific kinematic framework and their relationships with main fault systems, are essential for
340
defining fault interaction and linkage and determining their seismogenic potential more
341
accurately. This information should be included in seismic hazard studies of complex fault
342
linkages and future fault rupture scenarios to improve the characterization of large earthquakes.
343
In addition, this would contribute to producing more realistic fault sources and fault earthquake
344
models, which would improve the assessment of the offshore seismic and tsunami hazards.
345
Acknowledgments
346
We are grateful to Ingo Grevemeyer (GEOMAR) for providing us with the information
347
related to the local earthquakes recorded in the Alboran Sea and published in Grevemeyer et al.
348
(2015), and to Neal Driscoll whose helpful comments and suggestions have considerably
349
improved the manuscript. This research was supported by IMPULS (REN2003-05996MAR),
350
EVENT (CGL2006-12861-C02-02), SHAKE (CGL2011-30005-C02-02), INSIGHT (CTM2015-
351
70155-R) projects, the EU-COST Action FLOWS (ES 1301) and the European Union's Horizon
352
2020 research and innovation programme under grant agreement No H2020-MSCA-IF-2014
353
657769. Hector Perea was a fellow researcher under the “Juan de la Cierva” program (JCI-2010-
354
07502) and under the Marie Sklodowska-Curie Actions (H2020-MSCA-IF-2014 657769).
355
Grateful thanks are also due to the captain, crew, scientific party and UTM-CSIC technical staff
356
on board the R/V Hespérides during the IMPULS 2006 and R/V Sarmiento de Gamboa during
357
the EVENT-DEEP 2010 cruise. We thank Luca Gasperini and an anonymous reviewer for their
358
17
careful reviews that helped to improve this manuscript. This is a B-CSI publication
359
(2014SGR940).
360
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doi:10.1002/2013TC003349
557
Vernant, P., Fadil, A., Mourabit, T., Ouazar, D., Koulali, A., Davila, J.M., Garate, J., McClusky,
558
S., Reilinger, R., 2010. Geodetic constraints on active tectonics of the Western
559
Mediterranean: Implications for the kinematics and dynamics of the Nubia-Eurasia plate
560
boundary zone. J. Geodyn. 49, 123129. doi:10.1016/j.jog.2009.10.007
561
Wesnousky, S.G., 2008. Displacement and Geometrical Characteristics of Earthquake Surface
562
Ruptures: Issues and Implications for Seismic-Hazard Analysis and the Process of
563
Earthquake Rupture. Bull. Seismol. Soc. Am. 98, 16091632. doi:10.1785/0120070111
564
565
566
27
Figure 1. Topographic and bathymetric map of the central part of the Alboran Sea. Epicenters of
567
historical (I0 MSK) and instrumental (Mw) earthquakes were obtained from the IGN (Spanish
568
seismic catalog) and focal mechanisms from Stich et al. (2010), except for the Al-Idrissi 2016
569
event, provided by the IGN. White arrows indicate the present-day convergence between the
570
African and Eurasian plates (DeMets et al., 2010). AF: Averroes Fault; AIF: Al-Idrissi Fault;
571
ARF: Alboran Ridge Fault; CF: Carboneras Fault; NAFs: North Averroes Faults; YF: Yusuf
572
Fault. Dashed white rectangle localizes Fig. 2. Isobaths every 100 m. Inset: General map of the
573
region showing the Eurasia and Africa plate boundary.
574
Figure 2. High-resolution bathymetric map of the central part of the Alboran Sea. Colored
575
circles are earthquakes coded by depth and sized by magnitude (Mw) (Grevemeyer et al., 2015).
576
Thin light orange and red lines locate the high-resolution multichannel seismic (HR-MCS)
577
profiles acquired during the IMPULS and EVENT cruises, respectively. Black and red arrows
578
indicate the trace of the Averroes Fault (thicker) and the main North Averroes Faults (NAF;
579
thinner). AB stands for Averroes Basin. Thick white lines show the location of the interpreted
580
HR-MCS profiles presented in this work: EVD-125 (Fig. 5a), EVD-123 (Fig. 5b), and IM-23
581
(Fig. 6d). Dashed white rectangle localizes Fig. 3a.
582
Figure 3. (a) Slope-enhanced shade relief map of the central part of the Alboran Sea with the
583
location of the Averroes Fault (AF) and North Averroes Faults (NAFs) (white zones correspond
584
to flat areas and dark grey to black to steep areas). Triangles with the same color indicate
585
homologous piercing points to calculate the faults right-lateral offsets. Black dashed line
586
rectangles localize Figures b to f. Yellow circles correspond to earthquake epicenters
587
(Grevemeyer et al., 2015). (b to f) Detailed shaded relief maps with the location of the piercing
588
points used to calculate the lateral fault offsets for the Averroes Fault and NAF Faults. The
589
28
projection of the piercing points towards the fault plane follows the trend of the identified
590
geomorphic features. Isobaths are every 100 m. The relief maps without interpretation are in the
591
supplementary material Fig. S1.
592
Figure 4. Ages and seismostratigraphic units identified in the central part of the Alboran Sea and
593
their correlation with previous studies focused on the post-Messinian evolution of the Alboran
594
Sea. Limits between units are displayed with the same color as the respective horizon in the
595
seismic profiles. Red wavy lines are indicative of erosive unconformities. TS: This study;
596
M2016: Moreno et al. (2016); MG2013: Martínez-García et al (2013); J2016: Juan et al. (2016).
597
Figure 5. Interpreted HR-MCS seismic profiles EVD-125 (a) and EVD-123 (b). AB: Averroes
598
Basin. In the HR-MCS profiles, thick black lines represent the main faults. Horizon’s (H1 to H5)
599
ages provided in Fig. 4. Vertical exaggeration (V:H) x5. Location of the seismic profiles is in
600
Fig. 2. The seismic profiles without interpretation are in supplementary material Fig. S2.
601
Figure 6. (a, b, c) Interpreted sections (vertical exaggeration x20) of the sub-bottom parametric
602
profile acquired simultaneously with the HR-MCS profile IM-23. (d) Interpreted HR-MCS
603
seismic profile IM-23, where thick black lines represent the main faults. AB: Averroes Basin;
604
NAF: North Averroes Fault. Horizon’s (H1 to H5) ages provided in Fig. 4. Vertical exaggeration
605
(V:H) x5. Location of the profile IM-23 is in Fig. 2. The seismic profiles without interpretation
606
are in supplementary material Fig. S3.
607
Figure 7. (a) Map showing the present-day bathymetry of the central part of the Alboran Sea.
608
Red lines show the position of the newly mapped faults in the area. Dark gray dotted lines
609
localize the high-resolution multichannel seismic profiles used in the seismic mapping, and the
610
white polygon shows the area of interpolation. AB: Averroes Basin; NAF #: North Averroes
611
29
Faults. (b) Map of the interpolated topography of Unit II (Upper Miocene - Messinian erosion
612
surface) in milliseconds (TWTT). (cf) Interpolated isochore maps in milliseconds (TWTT) of
613
units Ib2 (low Lower Pliocene), Ib1 (upper Lower Pliocene), Ia3 (Upper Pliocene) and Ia1+Ia2
614
(Lower and Upper Quaternary). Contour interval indicated on each map. Gray lines indicate the
615
faults. For the interpolation, we used the Flex Gridding algorithm in the IHS Kingdom®
616
software, defining a cell size of 150 m and a distance interpolation of 5 km.
617
Figure 8. (a) High-resolution gray-shaded relief map of the central part of the Alboran Sea
618
showing the main active faults as well as the newly mapped Averroes Fault (AF) and North
619
Averroes Faults (NAF), their kinematic and the interpreted style of deformation. White arrows
620
indicate the present-day convergence between the African and Eurasian plates (DeMets et al.,
621
2010). Yellow circles correspond to earthquake epicenters (Grevemeyer et al., 2015). (b)
622
Schematic sketch showing the location of the distributed E-W right-lateral strike-slip shear zone
623
in relation to the main fault systems and the average strike of each system. ARF: Alboran Ridge
624
Fault; CF: Carboneras Fault; YF: Yusuf Fault.
625
Table 1. Characterization of the lateral offset and slip-rate of the Averroes Fault (AF) and North
626
Averroes Faults (NAFs).
627
Table 2. Calculation of the maximum magnitude earthquake for the Averroes Fault (AF) and
628
North Averroes Faults (NAFs).
629
630
631
30
632
633
31
634
635
32
636
33
637
34
638
639
35
640
641
36
642
643
37
644
645
38
646
647
39
Table 1. Calculation of the maximum magnitude
earthquake for the Averroes Fault (AF) and North
Averroes Faults (NAFs).
Fault
length
(km)
Wesnousky et al
[2008]a
Mw=5.56+0.87logL
46.6
7.0
36
6.9
36.5
6.9
28
6.8
16.9
6.6
33
6.9
21.5
6.7
221.6
7.6
a. L: surface rupture length in km. Rang Mw 5.9-
7.9. L ≥ 15 km. Application: All regions for the
relevant slip types but acknowledging that the
regression dataset will be dominated by plate
boundary earthquakes. The author indicates the
relationship is most relevant to strike-slip
sources.
Table 1_Perea et al.
648
649
40
Table 2. Characterization of the lateral offset and
slip-rate of the Averroes Fault (AF) and North
Averroes Faults (NAFs).
Fault
Length
(km)
Lateral
offset
(m)a
Lateral
slip-rate
(mm/yr)
Start
of
activity
(Ma)
AF
46.6
4181 (a-
a')
0.91
4.57
NAF 1
36
721 (b-b')
0.16
4.57
NAF2
36.5
1185 (c-
c')
0.26
4.57
NAF2a
28
950 (d-d')
0.21
4.57
NAF2b
16.9
-
-
4.57
NAF3
33
656 (e-e')
0.14
4.57
NAF4
21.5
892 (f-f')
0.24
4.57
a. Letters in between brackets indicate the
measured offsets. See figure 3 to localize the
places.
Table 2_Perea et al.
650
651
... In addition, minor N-S to NNW-SSE normal faults have also played an important role in the northern margin. This general fault system explains the main crustal seismic activity in the region (Stich et al., 2006;Grevemeyer et al., 2015;Buforn et al., 2017;Peláez et al., 2018;Stich et al., 2020) and its development strongly affects the seafloor morphology (Estrada et al., 1997;Gràcia et al., 2006;Martínez García et al., 2013Estrada et al., 2018a;Galindo-Zaldívar et al., 2018;Perea et al., 2018;Soumaya et al., 2018; Gràcia et al., (Ballesteros et al., 2008;Vázquez et al., 2008b;Gràcia et al., 2012;d'Acremont et al., 2014;Giaconia et al., 2015;Gómez de la Peña et al., 2016;Vázquez et al., 2016;Lafosse et al., 2018). The seismicity event distribution appoints that the NNE-SSW to NE-SW left lateral strike-slip fault system assumes most of the regional deformation ( Figure 2). ...
... The faults of this system are concentrated in the northeastern continental margin of the basin and in the eastern part of the Alborán Sea Basin, where they show NW-SE to WNW-ESE trends and constitute the outstanding Yusuf Fault Zone, that corresponds to the eastern boundary of the Alborán Ridge Indenter (Moreno et al., 2016;Estrada et al., 2018b;Perea et al., 2018). ...
... At least five WNW-ESE fault zones have been identified in the eastern part of the Motril-Djibouti Marginal Plateau (MDF) (Figure 8).The most penetrative is the Averroes Fault Zone (AVF), but at least other four fault zones have been identified to the northeast, sub-parallel to the Averroes Fault Zone and called NAF1 to NAF4 by Perea et al. (2018). These faults separate elongated ridges that have been interpreted as anticlines between faults (Moreno et al., 2016). ...
Article
Full-text available
The aim of this work is to make a synthesis at regional scale focused on the geophysical characterization of submarine faults around the Iberian margin to identify active structures and analyze their development in the framework of the present plate organization. Most of these submarine faults show seabed morphological expressions mapped with high-resolution swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel seismic profiles. Present active tectonics, deformation, seismicity, and tsunami-affected coastal areas is mainly focused on south Iberia at the Eurasian and Nubia plate boundary. Submarine active faults in these areas are represented by long strike-slip fault systems and arcuate fold-thrust systems. Their development takes place in response to present NW-SE convergence between the Eurasian and Nubia plates. We propose a strain partitioning model of the plate boundary into simple and pure shear zones to explain the distribution and mechanisms of active submarine faults along the Gulf of Cádiz, Gibraltar Arc and Alborán Sea in response to the present-day shear stress orientation. Nevertheless, deformation is also focused in the NW Iberian margin. Thus, along the Galician and Portuguese margin, several submarine faults mapped as thrust fault systems with high-seismic activity along the Iberian ocean-continent transition reflect the re-activation of former structures. We suggest that submarine active faults in the NW and W Iberia are also the response to the eastwards transfer of short-offset transform faults of the Mid Atlantic Ridge into the oceanic Iberian along a weakness as the former plate boundary between the oceanic Iberia and Eurasia domains. The distribution and activity of submarine faults mapped in this work from geophysical and bathymetric data are in good agreement with geodetic data and focal mechanisms.
... Therefore, these landslides could be explained as the result of the interaction between a moderate activity of the strike-slip Yusuf fault and a contourite characterized by reduced friction angle. Seismic reflection studies propose that the Averroes fault links with the Yusuf fault segment since 1.1 Ma (Perea et al., 2018;Martinez-Garcia et al., 2013), suggesting generation of earthquakes up to Mw = 7.6 (Perea et al., 2018). Then, magnitudes high enough to trigger the destabilization at a regional scale and leading to an increase of the volume of the MTD could be possible. ...
... Therefore, these landslides could be explained as the result of the interaction between a moderate activity of the strike-slip Yusuf fault and a contourite characterized by reduced friction angle. Seismic reflection studies propose that the Averroes fault links with the Yusuf fault segment since 1.1 Ma (Perea et al., 2018;Martinez-Garcia et al., 2013), suggesting generation of earthquakes up to Mw = 7.6 (Perea et al., 2018). Then, magnitudes high enough to trigger the destabilization at a regional scale and leading to an increase of the volume of the MTD could be possible. ...
... Moreover, MTD occurrence since ~Q2-Q1 (1.12-0.79 Ma) (Fig. 9), is consistent with the Alboran Sea tectonic reorganization occurring (i) along the Averroes and Yusuf Faults since 1.1 Ma (Martinez-Garcia et al., 2013;Martinez Garcia et al., 2017;Perea et al., 2018); (ii) along the AIFZ and in the Nekor Basin after 1.12-0.8 Ma (Fig. 8) (Galindo-Zaldivar et al., 2018;Giaconia et al., 2015;Lafosse et al., 2016 andLafosse et al., 2020); (iii) along the SAR and Alboran Ridge blind thrusts with their reactivation since 1.8-1.1 Ma (Martinez-Garcia et al., 2013;Lafosse et al., 2016Lafosse et al., , 2020d'Acremont et al., 2020). ...
Article
Earthquakes are the most commonly cited cause of offshore slope failure, followed by high sedimentation rates and ensuing pore pressure build-up. In the South Alboran Sea, the moderate seismicity (Mw = 6.4) of the strike-slip Al Idrissi Fault Zone does not appear to control directly the landslides distribution. To provide a preliminary geohazard assessment, we characterized the spatial distribution, the volume and the ages of the submarine landslides from multibeam and seismic reflection data in the southern part of the Alboran Sea. Since the Quaternary numerous submarine landslide processes have affected the marine sedimentary cover with volumes of the mass transport deposits (MTD) estimated between 0.01 and 15 km³. West of the Al Idrissi Fault Zone, along the South Alboran Ridge's northern flank, the distribution of the MTD follows the SW-NE bank and ridge trend that correlates with blind thrusts and folds covered by a plastered contourite drift. A pockmark field, related to fluid escape, is visible near landslide scars where the contourite drift is relatively thicker. In this area, landslide scars occur on variable slopes (2–24°) and their associated MTDs show variable decompacted volumes (0.01-10 km³). East of the Al Idrissi Fault Zone, between the Alboran Ridge and the Pytheas Bank, the mapped MTDs have uneven volume. The smaller ones (<1 km³) have their slide scars on steep slopes (>10°), whereas those of the largest ones (3–15 km³) occur on gentler slopes (<5°). These observations and a slope stability analysis suggest that the combination of seismic shaking, blind thrusts activity, relatively high sedimentation rate of contourite deposits with potential weak layers, and fluid escape dynamics are likely the main controlling mechanisms. These causal factors would explain the concentration of landslide head scarps at the edge of the thickest parts of the contourite drifts (i.e. crest). Slides may have been controlled locally by fluid overpressures in line with blind thrusts. Additionally, low to moderate seismicity potentially triggered by nearby faults might regionally have played a role in destabilising the landslides since 1.12 Ma (Q2 unit), which coincides with the propagation of the Al Idrissi Fault Zone in the southern Alboran Sea.
... The activity of normal faults on continental shelves can also trigger submarine slope instability and associated tsunamis (e.g., Gamberi et al., 2011;Rovere et al., 2014). Finally, detailed investigations of normal faults on continental shelves are key to identifying the sources of offshore earthquakes, and helping in the completion of regional seismic-risk maps (Perea et al., 2018). ...
... In the fields of marine geology and geophysics, the geometry and evolution of normal faults have been extensively studied in the last three decades (Baudon and Cartwright, 2008a;Alves et al., 2009;Tao and Alves, 2016;Perea et al., 2018). Methods for kinematic analyses of normal faults such as throw-depth (T-z) plots, displacement-length plots (D-x) and expansion indices (Wadsworth, 1953;Thorsen, 1963;Dawers et al., 1993) have expanded our understanding of the growth and kinematics of normal faults along many a continental margin (e.g. ...
Article
Normal faults are ubiquitous on many continental shelves, but have only been considered to play a subordinate role to basin-controlling faults in previous basin studies such as Qiongdongnan Basin. Their detailed geometries and kinematic histories are still poorly known. In this study, high-quality three dimensional (3D) seismic data are used to investigate families of normal faults bordering the continental shelf of the northwest South China Sea. Sixty-six (66) normal faults are interpreted and found to mostly tip out in the Upper Miocene. Three large-scale faults offsetting Horizon T30 (the base of Pliocene) and three other faults terminating beneath this horizon were selected and studied in detail. We distinguished growth and blind faults according to upper tip folding geometries and fault displacement distributions via throw-depth (T-z) plots. The dips of the upper parts of faults are almost twice that of the lower parts. A new three-stage model for fault reactivation in Qiongdongnan Basin is therefore proposed based on our results. In the northwest South China Sea, the propagation of normal faults on the continental shelf can be attributed to: (1) the rotation of the South China block and a reversal in the movement of the Ailao Shan-Red River Fault Zone from left-to right-lateral slip, and (2) large-scale slope instability in the Qiongdongnan Basin at ∼5.5 Ma. The results presented here outline the structural framework of the continental shelf of the northwest South China Sea since 5.5 Ma, and the significance of the movement of regional scale structures such as the Ailao Shan-Red River Fault Zone during the evolution of normal faults on continental shelves in the Qiongdongnan Basin.
... These faults explain the main seismic activity in the Alboran Sea (Fig. 4.2). They have affected the seafloor morphology (Gràcia et al. 2006;Estrada et al. 2018a;Galindo-Zaldívar et al. 2018;Perea et al. 2018) creating morphotectonic features such as linear scarps, elongated pressure ridges, and longitudinal or rhombus-shaped depressions that evince the contemporary variety of Quaternary tectonics (Ballesteros et al. 2008;Vázquez et al. , 2016 (Fig. 4.6). ...
Chapter
The Gulf of Cadiz and the Alboran Sea are characterized by tectonic activity due to oblique convergence at the boundary between the Eurasian and Nubian plates. This activity has favored a variety of tsunamigenic sources: basically, seismogenic faults and submarine landslides. The main tsunamigenic faults in the Gulf of Cadiz would comprise the thrust systems of Gorringe Ridge, Marquês de Pombal, São Vicente Canyon, and Horseshoe faults with a high susceptibility; meanwhile in the Alboran Sea would be the thrust system of the northern Alboran Ridge with high susceptibility, and the thrust systems of north Xauen and Adra margin, the transpressive segment of Al Idrissi fault, and the Yusuf-Habibas and Averroes faults, with moderate to high susceptibility. The areas with the greatest potential to generate tsunamigenic submarine landslides are in the Gulf of Cadiz, the São Vicente Canyon, Hirondelle Seamount, and Gorringe Ridge; and in the Alboran Sea are the southern and northern flanks of Alboran Ridge. Both sources are likely to generate destructive tsunamis in the Gulf of Cadiz, given its history of bigger earthquakes (>7 Mw) and larger landslides. To fully assess tsunamigenic sources, further work needs to be performed. In the case of seismogenic faults, research focus on geometry, offsets, timing, paleoearthquakes, and recurrence, and in landslides on early post-failure evolution, age, events, and recurrence. In situ measurements, paleotsunami records, and long-term monitoring, in addition to major modeling developments, will be also necessary.
... In addition, multifault earthquake ruptures, as observed in the 2016 Kaikoura Earthquake in New Zealand Hamling et al., 2017;Kaiser et al., 2017;Stirling et al., 2017;Litchfield et al., 2018), reveal complex and cascading fault linkages that control rupture lengths and potential earthquake magnitudes. Advances in high-resolution geophysical imaging (i.e., seismic reflection and swath-bathymetry) provide unprecedented resolution of the offshore deformation and fault architecture; such technological developments allow for improved imaging of fault offsets and their recency (Armijo et al., 2005;McNeill et al., 2007;Barnes, 2009;Brothers et al., 2009Brothers et al., , 2011Barnes and Pondard, 2010;Perea et al., 2012;Polonia et al., 2012;McNeill and Henstock, 2014;Escartín et al., 2016;Sahakian et al., 2017;Perea et al., 2018;Gràcia et al., 2019). ...
Article
Full-text available
Identifying the offshore thrust faults of the Western Transverse Ranges that could produce large earthquakes and seafloor uplift is essential to assess potential geohazards for the region. The Western Transverse Ranges in southern California are an E-W trending fold-and-thrust system that extends offshore west of Ventura. Using a high-resolution seismic CHIRP dataset, we have identified the Last Glacial Transgressive Surface (LGTS) and two Holocene seismostratigraphic units. Deformation of the LGTS, together with onlapping packages that exhibit divergence and rotation across the active structures, provide evidence for three to four deformational events with vertical uplifts ranging from 1 to 10 m. Based on the depth of the LGTS and the Holocene sediment thickness, age estimates for the deformational events reveal a good correlation with the onshore paleoseismological results for the Ventura-Pitas Point fault and the Ventura-Avenue anticline. The observed deformation along the offshore segments of the Ventura-Pitas Point fault and Ventura-Avenue anticline trend diminishes toward the west. Farther north, the deformation along the offshore Red Mountain anticline also diminishes to the west with the shortening stepping north onto the Mesa-Rincon Creek fault system. These observations suggest that offshore deformation along the fault-fold structures moving westward is systematically stepping to the north toward the hinterland. The decrease in the amount of deformation along the frontal structures towards the west corresponds to an increase in deformation along the hinterland fold systems, which could result from a connection of the fault strands at depth. A connection at depth of the northward dipping thrusts to a regional master detachment may explain the apparent jump of the deformation moving west, from the Ventura-Pitas Point fault and the Ventura-Avenue anticline to the Red Mountain anticline, and then, from the Red Mountain anticline to the Mesa-Rincon Creek fold system. Finally, considering the maximum vertical uplift estimated for events on these structures (max ∼10 m), along with the potential of a common master detachment that may rupture in concert, this system could generate a large magnitude earthquake (>Mw 7.0) and a consequent tsunami.
... 1c and 2). Tectonic activity initiated along the Averroes Fault during the late early Pliocene 21-24 , giving it an age of 4.57 Ma 21,25 . Considering the age of the fault and the vertical offset along its northern segment, we calculate an average vertical slip rate of 0.1 mm/yr. ...
Article
Full-text available
Tsunamis are triggered by sudden seafloor displacements, and usually originate from seismic activity at faults. Nevertheless, strike-slip faults are usually disregarded as major triggers, as they are thought to be capable of generating only moderate seafloor deformation; accordingly, the tsunamigenic potential of the vertical throw at the tips of strike-slip faults is not thought to be significant. We found the active dextral NW–SE Averroes Fault in the central Alboran Sea (westernmost Mediterranean) has a historical vertical throw of up to 5.4 m at its northwestern tip corresponding to an earthquake of Mw 7.0. We modelled the tsunamigenic potential of this seafloor deformation by Tsunami-HySEA software using the Coulomb 3.3 code. Waves propagating on two main branches reach highly populated sectors of the Iberian coast with maximum arrival heights of 6 m within 21 and 35 min, which is too quick for current early-warning systems to operate successfully. These findings suggest that the tsunamigenic potential of strike-slip faults is more important than previously thought, and should be taken into account for the re-evaluation of tsunami early-warning systems.
... The different tectonics domains are defined by large active strike-slip faults, such as the Al-Idrissi Fault System (FS) (Gràcia et al., 2006; Carboneras FS (Moreno et al., 2016), Averroes-Yusuf FS (Perea et al., 2018), as well as the prominent thrust-fault Alboran Ridge FS (Gómez de la . In consequence, the Alboran Sea is a seismically active area, where moderate to large earthquakes have occurred in the past and recent times Grevemeyer et al., 2015). ...
Thesis
Scleractinian cold-water coral reefs are considered to be key hotspots of benthic biodiversity in the deep ocean. Due to their relevant ecological role and susceptibility to anthropogenic disturbances protection and conservation measures have been applied to these habitats, even though they are far from being completely understood. Throughout the last two decades several studies have quantitatively described the biodiversity of Atlantic cold-water coral reefs, finding considerable differences among biogeographic regions. In contrast, and probably owed to the scarcity of these habitats in the Mediterranean Sea, the knowledge related to coral reef biodiversity in this basin remains modest and almost purely qualitative. On a different note, when coral reefs are under persistent suitable environmental conditions and have a sufficient sediment input, they can develop and form large geomorphic structures known as coral mounds. The latter are sensitive to changes in climate and capable of recording such variations in the chemical composition of the coral skeletons. Numerous surveys in the Atlantic have associated coral mound development patterns to environmental variations caused by glacial-interglacial cycles. Within the Mediterranean, coral mound formation studies have been so far limited to the Alboran Sea and to the last 15 kyr, due to the lack of gravity cores encompassing longer periods of time. In this thesis a wide range of techniques, including ROV video-analysis, multivariate statistics, U-series dating, computed tomography and geochemical analyses were applied to acquire a better understanding of the spatiotemporal distribution of Mediterranean cold-water coral reefs and the processes controlling their evolution into mounds during the last 400 kyr. More precisely, the present study aimed to (1) quantify the density of uncommonly thriving coral reefs and accompanying megabenthic species within the Cabliers Coral Mound Province, and describe their distribution along it; and (2) explore which are the main environmental variables and paleoclimatic events that have controlled coral mound formation in Cabliers and in the newly discovered Tunisian Coral Mound Province. The research presented here revealed the densest and most flourishing cold-water coral reefs witnessed so far in the Mediterranean Sea and brought further insight into their distribution along the crests of ridge-like coral mounds. This thesis also contributed to increase our knowledge on the main species associated to Mediterranean coral reefs and their relative abundances, which showed considerable differences to those found in Atlantic reefs. In regards to coral mound formation, this work has expanded the current knowledge outside the Alboran Sea and back to 400 ka BP. Almost opposite development patterns were observed between the Cabliers and Tunisian coral mound provinces, with the former mainly developing throughout deglaciations and temperate interstadial periods and the latter during glacial periods. Nonetheless, both provinces seem to depend on a high surface productivity and an appropriate depth of the interface between Atlantic and Levantine Intermediate Waters for the coral mounds to develop. Lastly, the oceanographic alterations caused in the Eastern Mediterranean Basin during Sapropel events also seem to have had detrimental effects for coral mound formation in the Western basin.
... Other inversion-related structures have been described further west, most notably the Yusuf fault which probably affect both the basin and the margin (Martínez-García et al., 2011;Medaouri et al., 2012;Perea et al., 2018;d'Acremont et al., 2020;de la Peña et al., 2020). ...
Article
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Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.
... The en échelon NE-SW left lateral strike-slip faults that cross the Alboran Sea and continues in the eastern Betic Cordillera is called the Trans-Alboran Shear Zone (TASZ, Figure 1b; De Larouzière et al., 1988). Recent indentation and block rotation processes (Perea et al., 2018) in the Alboran Sea FIGURE 2 | Geological map of Nekor Basin and swath-bathymetry of the Al Hoceima margin from Marlboro-2 Survey (modified from d' Acremont et al., 2014;Galindo-Zaldívar et al., 2015). Seismicity from IGN catalog 2000-2020; earthquakes with Mw > 1.5, and the position of the main shocks of the seismic series of 2004 and 1994 from Calvert et al. (1997) and Van der Woerd et al. (2014). ...
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