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

  1. Raphaël Grandin1,*,
  2. José Fernando Borges1,2,
  3. Mourad Bezzeghoud1,2,
  4. Bento Caldeira1,2,
  5. Fernando Carrilho3

Article first published online: 14 SEP 2007

DOI: 10.1111/j.1365-246X.2007.03570.x

Issue

Geophysical Journal International

Geophysical Journal International

Volume 171, Issue 3, pages 1144–1161, December 2007

Additional Information

How to Cite

Grandin, R., Borges, J. F., Bezzeghoud, M., Caldeira, B. and Carrilho, F. (2007), 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. Geophysical Journal International, 171: 1144–1161. doi: 10.1111/j.1365-246X.2007.03570.x

Author Information

  1. 1

    Centro de Geofísica de Évora, Universidade de Évora, Évora, Portugal

  2. 2

    Departmento de Física, Universidade de Évora, Évora, Portugal

  3. 3

    Instituto de Meteorologia, Lisbon, Portugal

* Now at: Institut de Physique du Globe de Paris (IPGP), Laboratoire de Tectonique et Mécanique de la Lithosphère, 4 place Jussieu, 75252 Paris, France. E-mail: grandin@ipgp.jussieu.fr

Publication History

  1. Issue published online: 8 OCT 2007
  2. Article first published online: 14 SEP 2007
  3. Accepted 2007 July 30. Received 2007 July 30; in original form 2006 November 24

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Keywords:

  • Iberian region;
  • ocean–continent transition;
  • strong ground motion;
  • waveform modelling

SUMMARY

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 three-component 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.

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