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

Highlights

• Using high-resolution data to unravel the regional role of offshore fault systems

• Characterization of a pervasive secondary fault system in a distributed shear zone

• Identifying the seismogenic potential of secondary active faults on the offshore

• Fault linkage reveals the potentiality for a Mw 7.6 earthquake in the Alboran Sea.

Abstract

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 (M_w) 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.

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 M_w 6.1; Spain), the 1994 and 2004 Al-Hoceima (M_w 6.0 and 6.4, respectively; Morocco), and the 2016 Al-Idrissi (M_w 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.