Fault activity studies in the Lower Tagus valley and Lisbon region using geophysical data

Abstract

Abstract The Metropolitan Area of Lisbon and the Lower Tagus Valley (LTV) region are located in central Portugal and inhabited by nearly 4 million people. The region has suffered throughout its history the effect of destructive earthquakes caused by hidden faults, possibly related to the plate boundary, which is sited approximately 400 km south of the region (Figure 1). In spite of low slip-rates and big recurrence times that have been estimated for these local, regional faults, they can produce moderate-to-large earthquakes that cause large damage and loss of life, as in 1344, 1531, or 1909 (e.g. Justo and Salwa, 1998; Cabral et al., 2003; 2013). The shorter occurrence time of the earthquakes might be owing to the existence of multiple active faults and/or time clustering owing to stress drop caused by proximal faults (e.g. Carvalho et al., 2006). Therefore, the seismic hazard and risk evaluation of the region has long been a reason of concern. Figure 1 Seismotectonic of the Azores-Iberia region (after Custodio et al., 2016 and references therein), where the study area is included (dashed square). Earthquakes recorded instrumentally since 1996 are shown by small brown dots. Historical earthquakes reported by the SHARE European Earthquake Catalogue – SHEEC – with magnitudes larger than 5.5 are shown by circles with a radius corresponding to the earthquake magnitude (Stucchi et al., 2013). The earthquakes with magnitudes equal to or larger than 7.0 are labelled with year of occurrence and magnitude. The plate boundaries of the global plate tectonics NNR-MORVEL56 model are shown by dark-blue lines (DeMets et al., 2010; Argus et al., 2011). The surface projection of potentially active faults compiled on the SHARE database are shown by red rectangles, the surface trace of the faults is marked by a thick red line (Basili et al., 2013; Vilanova et al., 2014). The SHARE faults in western Iberia are underlain by the original fault traces proposed by Cabral (2012) (green) and Duarte et al. (2013) (blue). The topography in the background is taken from the global SRTM30+ model, obtained from satellite altimetry and ship depth soundings (Smith and Sandwell, 1997; Becker et al., 2009).