The earthquakes of 29 July 2003, 12 February 2007, and 17 December 2009 in the region of Cape Saint Vincent (SW Iberia) and their relation with the 1755 Lisbon earthquake

Abstract

The Cape Saint Vincent region is of major seismological interest for its tectonic complexity and for the occurrence of the great 1755 Lisbon earthquake. No structure capable of generating such a large earthquake has yet been convincingly identified, but all authors agree that there is a possibility of a similar earthquake occurring offshore of the Cape sometime in the future. To shed some light on the region's dynamics, we here examine the mechanism of the three largest earthquakes to have occurred in the last 40 years west of the Cape – 29 July 2003 (Mw=5.3), 12 February 2007 (Mw=6.1), and 17 December 2009 (Mw=5.5). By inversion of the body waves and the kinematic slip distribution, we estimated the three earthquakes to have had similar characteristics (dimensions, maximum slip, stress drop, source time function, focal depth, and rupture velocity), although there were differences in the geometry of the rupture that reflect the great seismotectonic complexity of the zone. The focal mechanisms of the 2003 and 2007 earthquakes were similar, corresponding to thrusting motion, but the 2009 earthquake had a dip-slip motion in a vertical plane. As deduced from the slip distributions, the three shocks show NE–SW rupture planes, with the energy released propagating to the NE, compatible with the regional NW–SE horizontal compression produced by the convergence of the Eurasian and African plates. Applied to the generation of the 1755 Lisbon earthquake, this direction of faulting would correspond to a complex rupture along NE–SW trending thrust faults at the Gorringe Bank, the Horseshoe Scarp, and the Marquis de Pombal Fault, with the rupture propagating to the NE towards the Portuguese coast. Such a model could explain that unusually large and tsunami-generating earthquake.