Description: On the 25th January 2016 a magnitude Mw 6.3 earthquake struck 45 km offshore north Morocco, the largest recorded event in the Alboran Sea (western Mediterranean). It was preceded on 21 January by an earthquake of magnitude 5.1 in the same epicentral area, and was followed by numerous aftershocks whose locations mainly migrated south and northeast from the mainshock. The mainshock nucleated at a releasing bend of the poorly known Al-Idrissi Fault. According to slip inversion we assume a 20 to maximum 30 km long rupture zone. We use swath-bathymetry, seismic reflection profiles and seismological data to characterize and document Quaternary activity on the 100 km long Al-Idrissi Fault. We report evidence of left-lateral strike-slip displacement, characterize their fault segments and demonstrate that Al-Idrissi is the fault source of the 2016 earthquake events. Located along a crustal boundary that separates the West and East Alboran Sea, the Al-Idrissi Fault is a young structure. Its central segment, mainly transpressive, was developed during the Early Pliocene while the north and south segments are transtensional and of Quaternary age. All these observations together suggest that the Mw 6.4 earthquake broke across the southern and central segment boundary. Therefore, the complete rupture of the Al-Idrissi Fault should be considered and might generate a greater rupture (Mw 7.2), significantly increasing the potential hazard of the structure.

Description: Recent advances in seafloor and subsurface imaging allow accurately mapping and characterizing the kinematic pattern and the style of deformation of submarine faults with unprecedented detail to better assess seismic and tsunami hazards in coastal areas. The Alboran Sea is a Neogene basin generated by crustal extension associated with the subduction in the Gibraltar Arc. At present, several fault systems absorb part of the strain related to the NW-SE convergence (4-5.5 mm/yr) between the African and Eurasian plates. Consequently, the Alboran Sea shows a significant seismic activity. New high-resolution bathymetric and seismic data reveal the presence of poorly known pervasive fault systems in the central part of the Alboran Sea, the Averroes Fault (AF) and the North Averroes Faults (NAFs). These are secondary fault systems located between two large active faults, the Carboneras and Yusuf/Alboran Ridge faults, and represent a hitherto unrecognized seismogenic potential. The WNW-ESE trending AF and NAFs, which may have evolved since the Lower Pliocene (4.57 Ma), are subvertical right-lateral strike-slip active faults since: a) are offsetting the Quaternary sedimentary units and deforming the seafloor; and b) produce a right-lateral displacement of the northwestern margin of the Alboran Channel and across the Adra Ridge North. Given that the AF and NAFs have formed in a continental crust and that are located in a zone surrounded by some of the main active faults in the Alboran Sea, we postulate that these fault systems have been developed into a distributed dextral strike-slip shear zone with the local bulk shear striking approximately N90º. Considering their surface length they could generate earthquakes with magnitudes (Mw) between 6.3 and 7.2, but reaching 7.6 when AF and Yusuf Fault are linked. The high resolution bathymetry map has allowed us measuring lateral offsets produced by the AF and NAFs. Assuming that these displacements have been accumulated during the last 4.57 Ma, the calculated lateral slip rate for AF is approximately1.5 mm/yr and range between 0.2 and 0.4 mm/yr for the NAFs. Our results evidence the importance of the kinematic and seismogenic characterization of secondary fault systems to better comprehend earthquake and tsunami hazards.