The 2021-2022 seismic sequence in southern Alboran Sea

The Alboran Sea is a complex tectonic region in the westernmost Mediterranean Sea, dominated by the present-day NW-SE convergence between Eurasia and Nubia plates. This compression regime accomodates long strike-slip active fault systems, together with several inverse structures, crossing the Alboran crust in a NE-SW trending shear deformation belt which mainly controlls the shallow seismicity in the area. In fact, the southern sector of the Alboran domain has experienced two large earthquakes in the last two decades, the Mw 6.3 2004 Alhoceima and the Mw 6.4 2016 Alboran events. Since mid-april 2021, and over the following 20 months, tens of moderate-magnitude shallow earthquakes (4≤Mw≤5.3, h<20 km) have been registered in this area, to the northwest of Melilla, between the 2016 main shock and the African coast. The two largest events, a Mw 5.1 on August 28, 2021 and a Mw 5.3 on May 20, 2022, were widely felt in Melilla city (maximum EMS-98 intensities of IV and IV-V, respectively) and along the southern Spanish and the Moroccan coasts. These moderate seismicity occurs together with thousands of low-magnitude events (M<3) in a swarm-type distribution, in contrast to previous seismic sequences in 2004 and 2016 which showed a more typical foreshock-mainshock-aftershock pattern. An accurate hypocentral location of this seismicity is a key point to better image the seismicity distribution and rupture area and, hence, to improve our knowledge of the active tectonics of this region, contributing to improve seismic and tsunami hazard assessments. In this study we perfom a high-precision relocation of a selected good-quality subset of moderate-magnitude earthquakes of the 2021-2022 seismic sequence and we compare them to a similar set of relocated earthquakes of the 2004 and 2016 series, using all the available seismic data. We apply a non-linear probabilistic location algorithm jointly with a 3-D velocity model for the Alboran-Betic-Rif system, to account for differences in wave propagation in the laterally heterogeneous crust. This approach is a powerful tool to improve the hypocentral parametres.