Local Seismic Tomography Reveals the Crustal Structure Beneath Arraiolos (Central Portugal): Earthquake of ML 4.9 (15 January 2018)

Abstract An ML 4.9 earthquake occurred in Arraiolos, central Alentejo, on the 15 January 2018. As the mainshock was recorded as the highest magnitude in the region, a thorough investigation of the cause of this seismicity was carried out. In this study, local seismic tomography was conducted for the first time to demonstrate the importance of a uniform spatial distribution of earthquakes for obtaining accurate crustal images via seismic inversion. Two different inversions were performed using local tomographic software. The main difference between both inversions is that the first integrated the total amount of aftershocks, whereas the second one only incorporates one part. In addition, in both cases, additional events spread throughout a larger part of the region were selected. This investigation confirmed that the spatial spread of earthquakes has a greater impact on the accuracy and reliability of tomographic models than the number of earthquakes present in a region. The boundaries between the distributions of the P- and S-velocity anomalies are significantly correlated with the existing outcropping faults, revealing the probable segmentation of the São Gregorio (S.G.) fault and extension of the Santana do Campo (S.C.) fault. Moreover, a source alignment oriented west-southwest/east-northeast in the seismogenic zone was highlighted at the end of the second segment of the S.G. fault, below the main rupture zone. The velocity anomalies observed in both the P and S waves were strongly correlated with the geological formations at the surface, indicating the reliability of the seismic inversion; this contributes to a deeper comprehension of the Arraiolos seismic zone and offers valuable insights into its tectonic structure.