Upper Pleistocene to Holocene seismic activity along a large strike-slip fault: The Yusuf Fault System (Alboran Sea)

Abstract

The Alboran Sea, located at the Westernmost Mediterranean, accommodates a large part of the present-day crustal deformation associated to the NW-SE trending convergence (4-5 mm/yr) between Nubia and Eurasia plates. Although seismic activity is mainly characterized by low to moderate magnitude events, large and destructive earthquakes (I > IX) have occurred in this region. The identification and the seismic characterization of the active structures in the Alboran Sea is essential to evaluate better the exposure of the South Iberian Peninsula and North African coasts to different natural hazards. Here we present the Yusuf Fault System (YFS), a dextral strike-slip system that is one of the largest structures in the Alboran Sea and a lithospheric boundary between different crustal domains: the East Alboran Basin to the north and the North African Margin to the south. It trends WNW-ESE, is ~150 km-long and can be divided into two main segments (W and E), forming a pull-apart basin where both segments overlap. In this study, we combine recently acquired high-resolution bathymetric data (from hull mounted to AUV) and a multi-scale seismic imaging to characterize the YFS. The analysis of our dataset reveals that this system is a complex structure composed by an array of strike-slip faults. Most of them reach up and offset the seafloor and the upper Pleistocene to Holocene sedimentary units. According to results from paleoseismic analyses carried out using high-resolution sub-bottom profiles, the YFS might have generated at least 8 earthquakes during the last 200 ka, providing an average recurrence interval of 27.5 ka. The estimated average vertical offset is about 0.64 m while the vertical slip-rate would be around 0.03 mm/yr. However, this value needs to be considered as a minimum since YFS is a strike-slip fault and the lateral slip will be much larger than the vertical one. According to different empirical relationships, the YFS could produce earthquakes over than magnitude Mw 7.0. Finally, our results demonstrate that geomorphological, active tectonic and paleoseismic studies are essential to reveal the present-day activity and to characterize the seismic behavior of the YFS, with crucial implications for seismic (and tsunami) hazard assessment in the surrounding coastal areas