Intermediate-depth earthquakes in southern Spain and Alboran Sea

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A striking feature of the seismicity in the Ibero-Maghrebian region is a narrow band of intermediate-depth earthquakes (50 < h < 100 km) beneath the western part of the Alboran Sea, with epicenters following a NNE-SSW alignment. The origin and characteristics of this seismicity are debated, and an accurate analysis of this seismic scenario is provided despite the low to moderate magnitude of these earthquakes. In this study, we collect 20 years of seismic data from permanent and temporary installations and reprocess these data with the aid of advanced seismological techniques, including non-linear probabilistic relocation with a 3D-Earth velocity model and a probabilistic moment tensor inversion scheme, to shed new light on intermediate-depth seismicity in Southern Spain and the Alboran Sea. We relocated 238 intermediate-depth earthquakes (M >= 3) using a nonlinear probabilistic approach and a recent regional 3D tomography lithospheric velocity model for the Alboran-Betic Rif Zone. Maximum likelihood hypocenters confirm the NNE-SSW distribution in a depth range between 50 and 100 km, depicting three clusters of epicenters with a seismic gap that may be correlate to the boundary between the sunken slabs of the Iberian and African plates around Gibraltar. We simultaneously determined the focal mechanisms of 25 mb > 3.9 earthquakes using P-waves and moment tensors by fitting body-wave amplitude spectra and waveform cross-correlations. We performed an accurate resolution study by repeating the inversion using different 1-D velocity models. The results show predominant horizontal T axes with a rotation on the direction from NE-SW in southern Spain to E-W near the African coast. The distribution of intermediate-depth earthquakes and their source geometries provide new evidence of the seismotectonic complexity of the region, which is possibly controlled by the stopping or slowing down of subduction.
CRUE-CSIC (Acuerdos Transformativos 2022) © 2022 The Authors. This work has been partially supported by the Spanish Ministry of Economy and Competitiveness (CGL2017-86070-R). We want to thank the Instituto Geografico Nacional (IGN, doi: 914/sn/es) for providing part of the data. C. L-S also wants to thank the Erasmus+ Program of the Universidad Complutense de Madrid for supporting the internship at the German Research Centre for Geosciences (GFZ) and to the GFZ itself. The authors wish to thank the anonymous reviewer, the reviewer Dr. Maria Mesimeri and the editor Dr. Ramon Carbonell for their valuable comments and suggestions, which allowed us to improve our manuscript.
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