The Al Hoceima seismic zone (SW prolongation of the EBSZ in the Rif): GPS evidences of deep tectonic displacements on a main active blind sinistral fault

Abstract

4th Fault2SHA Workshop, Fault Complex Interaction: Characterization and Integration into Seismic Hazard Assessment (SHA), 3-5 June 2019, BarcelonaThe Al Hoceima seismic zone is located at the southwestward prolongation of the Eastern Betic Shear Zone that crosses the Alboran Sea and reaches up to the Rif. This region was affected by the 1994, 2004 and 2016 onshore and offshore seismic crisis and constitutes one of the most active areas of the westernmost Mediterranean. The January 24, 2004 (M= 6.4, depth 7 to 10 km) earthquake was related to a NNE-SSW sinistral or a WNW-ESE dextral vertical fault, but in spite of its shallow origin, no tectonic field ruptures were recognized. Anyway, the epicentral region evidences NE-SW to E-W brittle extensional structures: Quaternary fractured pebbles, conjugate normal faults and open joints. The main normal and transtensional faults in the region are located eastwards of the zone affected by seismicity, onshore and offshore of the Nekor bay. A non-permanent GPS network composed by 6 sites and measured since June 2007 evidence very moderate or absent activity of the main outcropping faults surrounding the Nekor Basin. However, very high rates (up to 2- 3mm /yr) of ENE-WSW extension have been detected in the epicentral zone. These field data are in agreement with those obtained on the analysis of the 2016 seismic series in the Alboran Sea, that evidence a westward migration of the deformation in the region. While the main sinistral EBSZ propagates southwestward up to the Rif affecting the basement, shallow tectonics units probably are detached and only being affected by the extensional deformation above the deeper main crustal fault. This complex setting may constitute a key area to advance in paleoseismological and seismic hazard assessment studies because the main recent outcropping faults have become inactive and the new main active structures are blind strike-slip faultsPeer Reviewe