Abstract
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.
Highlights
Large continental faults extend for thousands of kilometres to form boundaries between rigid tectonic blocks
West Alboran Basin related to the east-dipping Rif-GibraltarBetics slab[16,17], an ~80-km-wide NE-SW trending seismic zone extends for ~500-km-long[18] and runs along the so-called TransAlboran Shear Zone (TASZ)[19]
The TASZ is traditionally interpreted as a complex belt of deformation that crosscuts the Alboran Sea and its two margins, connecting the Rif (North Africa) to the Eastern Betic Shear Zone (SE Iberian Peninsula)[18,19]
Summary
Large continental faults extend for thousands of kilometres to form boundaries between rigid tectonic blocks. We report evidence of left-lateral strike-slip displacement, characterise the fault segmentation and demonstrate that AIFS is the source of the 2016 events. Large historical and instrumental earthquakes have occurred in the region, such as the 1804 and 19108 Adra earthquakes (MSK Intensity VIII–X), and the Mw 6.0, 26 May 19949,10 and the Mw 6.3, 24 February 200411 Al-Hoceima earthquakes (Fig. 2a) This last event caused 629 fatalities and left 15,600 homeless[12], making it the most catastrophic earthquake in the region during the last century. On 25 January 2016, a Mw 6.4 earthquake (white star in Fig. 1) hit the area offshore the city of Al-Hoceima on the Moroccan coast[13,14] This is the largest event recorded in the Alboran Sea. The earthquake caused one casualty in Al-Hoceima and 30 injured in Melilla. A recent work that combines geological, geodetic and 3D numerical modelling[17] demonstrates that crustal deformation in the Alboran
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