Kinematic analysis of secondary faults within a distributed shear-zone reveals fault linkage and increased seismic hazard
Introduction
Detailed structural mapping, as well as kinematic and seismogenic characterization of active submarine fault systems, is essential for determining possible fault linkages for potential rupture scenarios and, consequently, for improving the assessment of the seismic and tsunami hazard in densely populated coastal areas. Recent advances in seafloor and subsurface imaging have made it possible to obtain high-resolution seismic and bathymetric data, which can be used to accurately characterize the kinematic patterns and tectonic architecture of submarine faults with unprecedented detail (Armijo et al., 2005; Barnes, 2009; Bartolome et al., 2012; Brothers et al., 2015; Escartín et al., 2016; Gasperini et al., 2011a, Gasperini et al., 2011b; Gràcia et al., 2012, Gràcia et al., 2006; Martínez-Loriente et al., 2013; McNeill et al., 2007; McNeill and Henstock, 2014; Moreno et al., 2016; Perea et al., 2012; Polonia et al., 2012; Sahakian et al., 2017). In addition, methods have been developed to determine the seismogenic potential and earthquake history of single faults based on on-fault marine paleoseismology (e.g. Barnes and Pondard, 2010; Brothers et al., 2011, Brothers et al., 2009). These studies have generally focused on large continental fault systems. Nevertheless, surface rupture mapping of recent large magnitude earthquakes shows complex multifault ruptures involving unknown or poorly known secondary faults (e.g., Elliott et al., 2012; Hamling et al., 2017; Quigley et al., 2012). These observations highlight the need to characterize onshore and offshore secondary fault systems as well as their relationship to major faults to improve seismic hazard assessment (e.g., Field et al., 2014; Perea and Atakan, 2007; Stirling et al., 2012). Here we present an example of a previously unknown secondary active fault system located in the central Alboran Sea (western Mediterranean), which is related to the largest faults controlling the regional geodynamics.
The Alboran Sea is a Neogene basin formed by crustal extension related to the subduction system in the Gibraltar Arc (e.g., Booth-Rea et al., 2007; Comas et al., 1999; van Hinsbergen et al., 2014). At present, left-lateral and right-lateral strike-slip faults trending NE-SW and WNW-ESE, respectively, accommodate part of the strain related to the NW-SE convergence (4–5.5 mm/yr) between the African and Eurasian plates (e.g., DeMets et al., 2010; Palano et al., 2015; Vernant et al., 2010). Consequently, the Alboran Sea shows a remarkable seismic activity, mainly concentrated along what is known as the Trans-Alboran Shear Zone (De Larouzière et al., 1988). Although this seismicity is mainly characterized by low to moderate magnitude events (Buforn et al., 1995; Stich et al., 2006), large and destructive earthquakes have occurred in the region, such as the 1522 Almería (IEMS98 IX; Spain), the 1790 Oran (IMSK IX-X; Algeria), the 1910 Adra (IEMS98 VIII and Mw 6.1; Spain), the 1994 and 2004 Al-Hoceima (Mw 6.0 and 6.4, respectively; Morocco), and the 2016 Al-Idrissi (Mw 6.4; Morocco) events (e.g., Biggs et al., 2006; Buforn et al., 2017; Calvert et al., 1997; Gràcia et al., 2012; Martínez Solares and Mezcua, 2002; Stich et al., 2003; Tahayt et al., 2009; van der Woerd et al., 2014) (Fig. 1).
The main active faults in the Alboran Sea are the left-lateral Carboneras and Al-Idrissi strike-slip faults, the right-lateral Yusuf strike-slip fault and the Alboran Ridge thrust fault (Gràcia et al., 2006; Martínez-García et al., 2013; Medaouri et al., 2014; Moreno et al., 2016) (Fig. 1). These faults converge at the center of the Alboran Sea between the East and West Alboran basins. Their structural relationship, however, is still under debate. In between these main faults there is a secondary fault-system, the Averroes Fault (AF) and North Averroes Faults (NAFs) with a WNW-ESE trend (Fig. 1). Understanding the tectonic evolution of these secondary faults and their relationship and potential linkage with the large fault systems will help to characterize the present kinematics of the area and define potential fault rupture scenarios. Accordingly, the overarching goals of this work are: (a) to characterize the WNW-ESE trending AF and NAFs based on newly acquired swath-bathymetry, sub-bottom acoustic profiles, and high-resolution multichannel seismic data; (b) to determine the role of these secondary faults to further understand the kinematic pattern and style of deformation in the central part of the Alboran Sea; and (c) to evaluate the contribution and relevance of secondary faults to the seismogenic potential of the western Mediterranean region.
Section snippets
Data and methods
Our dataset comprises swath-bathymetry and recently acquired high-resolution seismic reflection profiles, including sub-bottom (SBP) and multichannel (HR-MCS) data (Fig. 2). The integration of these datasets makes it possible to accurately map fault traces for offshore areas and estimate vertical and horizontal displacements.
During the EVENT (2010) and SHAKE (2015) cruises, 30 m grid size swath-bathymetry was acquired with the ATLAS Hydrosweep DS multibeam echosounder, hull mounted system of
Seismostratigraphy of the central part of the Alboran Sea
During the last decades, the seismostratigraphic units in the central part of the Alboran Sea have been successively re-defined due to the availability of high-quality and high-resolution seismic datasets (e.g., Booth-Rea et al., 2007; Comas et al., 1999; Gómez dela Peña, 2017; Juan et al., 2016; Martínez-García et al., 2013; Moreno et al., 2016). In addition, the combination of the seismic data with radiometric dating and biostratigraphic analyses of sediments and rocks collected in sediment
A secondary fault system in the central Alboran Sea: the Averroes and North Averroes faults
In the central part of the Alboran Sea, the seafloor morphology of the Djibouti Plateau shows the presence of an elongated and narrow trough, striking N115° to N125°, which corresponds to the surface expression of the AF (Fig. 2). Over time, its vertical and horizontal displacement has generated a 2-km wide and 13.2-km long basin, referred to as the Averroes Basin (AB in Fig. 2, Fig. 5, Fig. 6), which becomes progressively narrower as it crosses the Adra Ridge. To the southeast, across the
Discussion: kinematics, fault linkage and seismic potential
The analysis of the bathymetric and HR-MCS data reveals that the AF and NAFs are right-lateral strike-slip faults, with transtensive and transpressive deformation patterns in the Djibouti Plateau and the Alboran Channel, respectively. The following points are the evidence for this kinematic interpretation: (a) right-lateral offsets observed on the northwestern margin of the Alboran Channel and across the Adra Ridge North; (b) vertical to sub-vertical fault dip; (c) changes in the dip-slip
Conclusions
This study identifies and characterizes the seismic potential of previously unknown secondary active fault systems, the AF and NAFs, which displace the Quaternary sedimentary units and deform the seafloor in the Alboran Sea producing microseismicity. We propose that these faults may have developed in a distributed E-W right-lateral strike-slip shear zone during the last 4.57 Ma to accommodate the local stress field resulting from the different kinematics along the Carboneras and Yusuf fault
Acknowledgments
We are grateful to Ingo Grevemeyer (GEOMAR) for providing us with the information related to the local earthquakes recorded in the Alboran Sea and published in Grevemeyer et al. (2015), and to Neal Driscoll whose helpful comments and suggestions have considerably improved the manuscript. This research was supported by IMPULS (REN2003-05996MAR), EVENT (CGL2006-12861-C02-02), SHAKE (CGL2011-30005-C02-02), INSIGHT (CTM2015-70155-R) projects, the EU-COST Action FLOWS (ES 1301) and the European
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