Simulations of strong ground motion in SW Iberia for the 1969 February 28 (Ms= 8.0) and the 1755 November 1 (M~ 8.5) earthquakes - I. Velocity model

Abstract

This is the first paper of a series of two concerning strong ground motion in SW Iberia due to earthquakes originating from the adjacent Atlantic area. The aim of this paper is to build and calibrate a velocity model that will be used in the companion paper for seismic intensity modelling of the 1969 (Ms = 8.0) and 1755 (M = 8.5–8.7) earthquakes. Taking into account the geological evolution of the region since the Palaeozoic, we build a 3-D velocity model down to the Moho discontinuity, substantially different from a simple 1-D layered model. The velocity model presented in this paper is built a priori , using information originating from a variety of geological and geophysical studies. Its resolution is sufficient to simulate realistically seismic wave propagation in the low-frequency (f < 0.5 Hz) range, which is the most significant for the study of the destructive effects of large earthquakes at a regional scale. To validate the model, we compute synthetic seismograms for three recent earthquakes of moderate magnitudes (4.6 < Mw < 5.3) located offshore, in the most seismically active area in the region. Synthetics are generated using a wave propagation code, based on the finite-difference method, which was chosen for its simplicity and accuracy in the frequency range considered in this study (0.1–0.5 Hz). We compare simulated waveforms with threecomponent seismograms for 9 different stations. Traveltimes of the direct P waves, and the amplitude of ground motion, are accurately reproduced at all stations. The frequency content of the seismograms fits the observations, especially for the lowest frequencies investigated (0.1–0.3 Hz). For each earthquake, the estimated seismic moment is in good agreement with values obtained by other authors, using different methods.We conclude that the velocity model provides encouraging results for the computation of low frequency seismograms in the region, and can be used for the study of larger earthquakes, for which the radiated wavefield has a predominant low-frequency spectrum.