GROUND MOTION IN MEXICO-CITY DURING THE APRIL 25, 1989, GUERRERO EARTHQUAKE

Instrumental observations of ground motion in Mexico City during the April 25, 1989, Guerrero earthquake were analyzed. Our aim was to understand various aspects of the seismic response of the valley that had not been completely resolved. Such understanding of the basic mechanisms that control the seismic behavior of the valley sediments is crucial in any modeling attempt. The study of vertical motion for this event, which was shown to be practically unaffected by site conditions, lead to the identification of a prominent long-period Rayleigh wave. This, together with the availability of absolute time for some stations, allowed the establishment of a common time reference for all recordings. Horizontal motion, in contrast, was significantly amplified, with large increases in duration, at lake bed sites. In order to interpret the observed complexity of ground motion we studied two simplified models of soft alluvial valleys. One of these is two-dimensional and it is excited by plane S waves with variable polarization and incidence angles. This model allows three-dimensional response. The other is a three-dimensional axi-symmetric flat valley with a rigid base. Computations were performed in the frequency domain by means of a boundary integral method for the two-dimensional model and using a collocation least-squares technique for the three-dimensional one. Seismograms were obtained through Fourier synthesis. It was found that the irregular soft layer response produces polarization patterns which are similar to the observations, suggesting that the latter are a consequence of three-dimensional effects.