Unveiling the fault source of the January 25th 2016, Mw 6.4 earthquake (Alboran Sea) constrained by seismological and geological data

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