Crustal structure beneath the Lower Tagus Valley, southwestern Iberia using joint analysis of teleseismic receiver functions and surface-wave dispersion

Abstract

SUMMARY We conducted a joint analysis using a genetic algorithm on body wave teleseismic receiver functions and Rayleigh wave phase velocities computed from seismic waveform data recorded at six broadband seismic stations deployed in the Lower Tagus Valley, southwestern Iberia. The study region is located on the central western Portuguese mainland; a few hundred kilometres north of the Africa–Iberia Plate boundary and has a record of historical damage and casualties which are caused by destructive earthquakes occurring either to the southwestern offshore area or onland as a result of intraplate seismic activity. The latter may represent a response of old basement fractures to the current convergence between Africa and Iberia. However, little is known about the deep structure of the crust beneath this region. For this reason, we employed the genetic algorithm to analyse the seismic data recorded in the Lower Tagus Valley. The crustal thickness beneath the study region varies from 26 to 28 km with an average of 27.5 km, which is clearly lower than the global average. The crustal thickness shows a tendency of decrease towards the west near the coast of the Atlantic Ocean, being consistent with the extensional tectonic episodes that led to the opening of the Atlantic in the Middle Jurassic.