Earthquake crisis unveils the growth of an incipient continental fault system

Large continental faults extend for thousands of kilometres to form boundaries between rigid tectonic blocks. These faults are associated with prominent topographic features and can produce large earthquakes. Here we show the first evidence of a major tectonic structure in its initial-stage, the Al-Idrissi Fault System (AIFS), in the Alboran Sea. Combining bathymetric and seismic reflection data, together with seismological analyses of the 2016 Mw 6.4 earthquake offshore Morocco – the largest event ever recorded in the area – we unveil a 3D geometry for the AIFS. We report evidence of left-lateral strike-slip displacement, characterise the fault segmentation and demonstrate that AIFS is the source of the 2016 events. The occurrence of the Mw 6.4 earthquake together with historical and instrumental events supports that the AIFS is currently growing through propagation and linkage of its segments. Thus, the AIFS provides a unique model of the inception and growth of a young plate boundary fault system.


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Our work focus on large continental faults that extend for thousands of kilometres and form the tectonic boundaries between plates. In these active areas, well-defined faults produce large earthquakes, and thus imply a high seismic hazard. In the Alboran Sea, which hosts an allegedly complex diffuse boundary between the Eurasia and Nubia plates, we discovered one of the few examples worldwide of the initial stages of these tectonic structures. On the 25th January 2016, a magnitude Mw 6.4 submarine earthquake struck the north of the Moroccan coast, the largest event ever recorded in the Alboran Sea. The mainshock nucleated at a releasing bend of the poorly known Al-Idrissi Fault System (AIFS). Here we combine newly acquired multi-scale bathymetric and marine seismic reflection data with a resolution comparable to the studies on land, together with seismological data of the 2016 Mw 6.4 earthquake offshore Morocco -the largest event recorded in the area -to unveil the 3D geometry of the AIFS. We found that, despite its subdued relief, the AIFS is a crustal-scale boundary. We report evidence of left-lateral strike-slip displacement, characterize their fault segments and demonstrate that the AIFS is the source of the 2016 events. The occurrence of the Mw 6.4 earthquake and previous events of 1994 and 2004 supports that the AIFS is currently growing through propagation and linkage of its segments, which eventually might generate a greater rupture increasing the potential hazard of this structure. Authentication Describe the authentication procedures for each cell line used OR declare that none of the cell lines used were authenticated.

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