3D crustal seismic structure of southern mainland Portugal from combined local earthquake and active source tomography

Summary We present P- and S-wave seismic tomographic models for southern mainland Portugal, specifically for the region south of the 39° N parallel. These models were obtained using local seismicity and wide-angle refracted and reflected seismic profiles, considering a spatial resolution of approximately 10 km in latitude and longitude, and 3 km in depth, the highest spatial resolution so far compared to previous models of similar or broader regional scope. The aim is to improve the understanding of continental seismicity and to correlate seismic heterogeneities with crustal tectonic structures. For this purpose, the Fast Marching method was employed to solve the forward problem, while subspace inversion was used for the inverse problem, both implemented within the FMTOMO tomography package. The resulting tomographic model shows a segmented crust between upper, middle and lower. Some of the seismic structures show orientations consistent with Variscan tectonic features, generally following a northwest–southeast direction within the Ossa-Morena and South Portuguese Zone. Contacts between major geological units are discernible and well delineated in the model. Within the Ossa-Morena Zone, a negative velocity anomaly was identified which may be related to a possible fractured zone between the Boa Fé Shear Zone and Ciborro-Aldeia da Serra Fault. In the South Portuguese Zone, the most pronounced velocity contrast is associated with the syenite intrusion of the Monchique Massif in the southwest, characterized by a positive velocity anomaly, extending down to 20 km depth. To the southeast, seismic activity may be related to the Alcoutim and Ribeira do Vascão Faults. These structures impose a velocity anomaly suggesting that they are near-vertical faults extending to the top of the lower crust. A good correlation is observed between seismicity, seismic anomalies, and regional geology.