Relocation of Recent Shallow Seismic Activity in the Alboran Sea (Western Mediterranean Sea): The 2021–2024, 2016, and 2004 Seismic Series

ABSTRACT

The Alboran Sea is a complex tectonic region and one of the most seismically active areas in the westernmost Mediterranean Sea. Its southern sector has been the scene of three significant earthquakes in the past 30 yr, the 1994 M 5.9 and 2004 M 6.3 Al Hoceima and the 2016 Mw 6.4 Alboran events. In this study, we perform a high-precision relocation of a selected subset of moderate-magnitude earthquakes of the three main seismic series that have occurred in this region in this century, the 2021–2024, 2016, and 2004 series, using all the available seismic data. We apply a two-step shared relocation procedure, first, a nonlinear probabilistic algorithm with a 3D velocity model for the Alboran–Betic–Rif system, and second, a double-difference relative method. Our results for the 2021–2024 and 2016 series show clustered epicenters along the nearby active fault systems in the area. The 2016 series displays an excellent spatial correlation with the Al-Idrisi fault and may be linked to it, whereas the 2021–2024 series could be associated to an unmapped structure to the east of Al-Idrisi, more fractured and of smaller dimensions, as inferred from its spatial–temporal magnitude distribution. Our solutions for the 2004 series confirm two clear and perpendicular epicentral alignments, which may be related to the Trougout fault. The overall depth distribution reveals shallow hypocenters mainly constrained in a seismogenic layer up to 15–20 km. Our results display a remarkable hypocentral clustering compared with the Instituto Geográfico Nacional (IGN) earthquake catalog solutions and an improvement in the accuracy and precision of earthquake locations, with mean horizontal and vertical uncertainties lower than ∼5 km, giving better constrained hypocenters over the IGN catalog. Our findings highlight the effectiveness of this methodology, especially in the case of offshore seismicity with poor azimuthal coverage, and may improve seismic hazard studies in the region.