Estudi neotectònic de l’àrea font dels sismes d’Os de Civís

Abstract

Since 1st February 2021 there have been more than 630 earthquakes in the surroundings of Os de Civís (Alt Urgell, Spain), two of them reaching magnitudes up to Ml=3.8 and Ml=3.6. The absence of destructive earthquakes in the historic and instrumental record is reflected by the lack of neotectonics studies in this area, so the earthquakes source is unknown. However, the evidence of active faults close (<50km) to the study area and the trace of an E-W alpine thrust (Estaron thrust) crossing it, suggest that there could be a connection between the structure and the earthquakes. This study sets out two objectives to identify the seismic source of the earthquakes. On one hand, it is proposed to relocate the earthquakes available in the ICGC’s catalogue by using NonLinLoc, a non-linear location algorithm that provides reliable and complete solutions. The obtained locations would contribute to the knowledge of the seismic source in depth and also to enclose the study area. On the other hand, it is intended to elaborate a neotectonics map, considering elements and materials both affected or generated by tectonic deformation. This would allow to suggest a possible relation between the registered seismicity and its source. The obtained earthquake locations define an ESE dipping region that has been related to two NE-SW faults, interpreted as reactivated branches of the Estaron thrust. No unequivocal superficial rupture has been identified in the neotectonics study, but some displacement indicators have been seen in La Ginebrera region, as well as a scarp up to 35 m in the south slope of Bony de Trescull. The latter has been interpreted as a possible old rupture of another segment of the branch. The present study has been a first approach to the area. More investigation is need to be done, so different methods and studies are proposed for future research.