<p>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&#8804;Mw&#8804;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.</p>
<p>The occurrence of moderate magnitude earthquakes in intermediate depth (40<h<150 km) is a characteristic of the seismicity of the Ibero-Magrebian region. The most important concentration of this activity is in the western part of the Alboran Sea, with the epicenters following an N-S direction. In order to improve the knowledge of the geometry of these seismogenic structures, we have carried out a study of the hypocenters distribution and focal mechanisms for earthquakes that occurred in the period 2000-2020 (M>4.0). For the hypocentral location, we have used a non-linear probabilistic approach (NonLinLoc algorithm) jointly with 3-D lithospheric velocity tomography models recently developed for the Alboran-Betic-Rif zone. Focal mechanisms have been obtained from moment tensor inversion of stations at regional distances (Kiwi tools). Maximum likelihood hypocentres confirm a near vertical N-S distribution in a depth range between 50 and 100 km. Focal mechanisms show a different stress pattern, changing from a vertical tension axis for earthquakes located off-shore and western of 4.5&#186;W to vertical pressure axis for earthquakes inland and at eastern of 4.5&#186;W.</p>
<p>The aim of this study is to make a review, actualization and homogenization of the seismic parameters of the Seismic Catalogue of the National Seismic Network of Spain, which belongs to the National Geographic Institute. Our analysis focusses on the region that spans from 36.0 to 39.5&#176; N and from 3.25&#176; W to 1&#176; E, which is a seismically very active region. The studied time period refers to earthquakes occurred between 1900 and 1923, where most information comes from macroseismic data and macroseismic effects.</p><p>The study begins by searching and collecting information from seismic bulletins and seismic catalogues, seismograms, seismic surveys, photographs, specific studies, historical newspapers and different digital archives. Then, the achieved information from all the different sources were reviewed and, whenever possible, the seismic parameters such as localization, seismic intensity and magnitude were recalculated.</p><p>The objective of this work is, from one hand, to establish the study methodology that allow to develop an overall review of all the earthquakes occurred in Spain from 1900 to date, and on the other hand, to provide good quality seismic data (improving the completeness and homogeneity of this seismic catalogue). Seismic data is important because it is used to make seismic hazard maps, studies of seismic risk, to update the seismic building standards and it is also used to make seismic characterization of the territory.</p>
<p>One of the characteristics of the seismicity in the Ibero-Maghrebian region is the occurrence of intermediate depth earthquakes (50<h<100 km), their largest concentration located at the western part of the Alboran Sea, with epicenters following an NNE-SSW alignment. In this study, we have relocated over 200 intermediate depth earthquakes (M&#8805;3) occurred in this region in the period 2000-2020, using a non-linear probabilistic approach (NonLinLoc algorithm) together with 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. We have determined the focal mechanisms of 26 of these earthquakes with magnitudes (mb) greater than 3.9. We first derived focal mechanisms using the P-wave first motion polarity method and then performed a moment tensor inversion, using a probabilistic inversion approach based on the simultaneous fit of waveforms and amplitude spectra of P and S phases. We performed an accurate resolution study, by repeating the inversion using different 1-D velocity models and testing different moment tensor (MT) constraints: a full moment tensor, a deviatoric moment tensor and a pure double couple (DC). Misfit values are similar for different MT constraints. Most solutions have a non-DC component larger than 30%. This may be due to the tectonic complexity of the region and the use on the inversion of 1-D Earth model. The DC components obtained from the inversion show different orientations of the nodal planes. A first group of events to the northern part with epicenters inland on south Spain have horizontal tension axes in NE-SW direction. A second group of earthquakes with epicenters off-shore, but close to the Spanish coast, presents near-vertical pressure axes. The third group, formed by deeper earthquakes, with epicenters on the center of the Alboran sea have dip slip focal mechanisms of either normal or reverse motion with planes either vertical or dipping 45&#186; plane oriented in NNE-SSW direction, approximately the same orientation as the alignment of their epicenters. The distribution of these intermediate depth earthquakes and their focal mechanisms evidence the seismotectonic complexity of the region related with a possible subduction.</p>
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