Very recent seabottom deformation in the area affected by the 2016-2017 seismic crisis in the Alboran sea (Western Mediterranean)

Abstract

The Alboran Sea (Westernmost Mediterranean) is a Neogene basin located between the Betic and Rif cordilleras that undergoes shortening and orthogonal extension due to the Eurasian-African NW-SE oblique plate convergence. A NNE-SSW broad band of deformation and seismicity affects its central part extending from the northern Rif (Trougout Fault), the Alboran Sea (Al Idrisi Fault), to southern Betics (Balanegra Fault). After the 1993-1994 and 2004 seismic series, a new crisis shocked mainly the southern Alboran Sea in 2016-2017 (main event Mw=6.3, Jan 25). The nearsurface recent seafloor deformation of this area is investigated using seismic profiles, multibeam and gravimeter data, recovered during the INCRISIS cruise. Epicenters grouped into two main branches. The northern WSW-ENE branch parallels the northern face of the Alboran Ridge- Francesc Pagès seamount antiforms and intersects the Al Idrisi Fault. Mass-transport deposits occurred at least since recent times. The southern branch, that crosses the Francesc Pagès seamount and Nekor Basin, consists of a NNE-SSW vertical sinistral deformation zone, with early epicenters of higher magnitude earthquakes located along a narrow band paralleling the 10 km far east Al Idrisi Fault. Nearsurface deformation includes active NW-SE vertical and normal faults, unmapped until now. Later, the epicenter band spreads eastward, reaching the main Al Idrisi Fault characterized by discontinuous active NE-SW vertical fractures. Seismicity and tectonic structures suggest a westwards propagation of deformation and the growth of very recent incipient faults, comprising NNE-SSW sinistral fault zone in depth that is connected upwards with NW-SE vertical and normal faults. This new fault zone is segmented, including 1994 seismicity in coastal area, that propagates 2004 onshore and 2016 offshore. The new insights for the seismic hazard assessment, point to the growth of new faults can produce potential high magnitude earthquakes than already formed faults