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

Abstract

he 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. Geological outcrop and geomorphologic mapping identified several regional faults in the LTV region that could be the source the historical earthquakes, but some of them do not affect. Quaternary sediments and lacked the proofs that they were active faults. On the other side, in the vast quaternary alluvial plains that cover the region, it was difficult to identify active faults, as the sedimentation/erosion rates erase any possible surface rupture caused by the low slip-rate faults (<0,35 mm/y). By the late-20th century, seismic reflection data that had been acquired for the oil-industry till the beginning of the 1980s began to be used to identify the major hidden fault zones (e.g. Cabral et al., 2003; Vilanova and Fonseca, 2004; Carvalho et al., 2006). Potential field data was also used to locate active faults in the areas where no seismic data is available (Carvalho et al., 2008; 2011). Though a few more active faults have been proposed, the vast majority of authors agree that the following active faults threaten the region: Nazaré-Caldas da Rainha, Lower Tagus Valley, Ota, Azambuja, Vila Franca de Xira (VFX), Pinhal Novo and Porto Alto faults (Garcia-Mayordomo et al., 2012; Vilanova et al., 2014). In this work, we discuss the acquisition, processing and interpretation of near surface geophysical works carried out over three of these faults — the VFX, Porto Alto and Azambuja faults — in order to confirm they have had activity during the Holoceneera. Their location is shown in Figure 2. We further estimate some of its fault parameters (vertical displacement, slip-rate, length, etc.) and respective implications in terms of seismic hazard.