A Sismicidade de Portugal

Abstract

Portugal can be considered to have a moderate seismicity characterized by small events (M<5.0) and occasional moderate/large/major (5.0<=M<=7.8) earthquakes. The northern and central part of Portugal are characterized by a low seismicity (M<5.0) except Lower Tejo Valley region, which exhibits the relatively high seismic activity. At least three important historical earthquakes have occurred in this zone in 1344 (Ms=6.0), 1531 (Ms=7.1) and 23 April 1909 (Mw=6.0). The southern part of Portugal is the region with the most significant seismicity and can be divided into two areas: the Atlantic adjacent zone; and the continental zone. The first one is characterized by more intense seismic activity associated with the convergence between Eurasian and African plates. The southwest of cape of São Vicente area, where the earthquakes of 28 February 1969 (Ms=8.0) and 1 November 1755 (so-called Lisbon earthquake, Io=X-XI) occurred, is probably the most seismically active zone. According to the Portugal southern mainland neotectonic map it is possible to identify the following main tectonic structures: a) the Messejana fault, with an extension of 500 km crossing all of the southern region with NE-SW orientation and which is probably associated with the Azores-Gibraltar fault; b) close to the city of Évora, the Moura-Vidigueira fault has a WNW-ESE strike; and c) the Loulé fault, located in Algarve with an approximately W-E orientation, probably responsible for large historical earthquakes. The seismic studies of the western zone of the Iberian Peninsula has revealed four zones of high seismic strain elease: the Lower Tejo Valley, the Gorringe Bank zone, the southern Algarve and the area around the city of Évora. The southern region of Portugal (mainly the Algarve) and the Lisbon region (including the Lower Tejo Valley region) are the zones that have suffered the greatest number of disastrous earthquakes.