RESONANT RESPONSE MODELS FOR THE VALLEY OF MEXICO .2. THE TRAPPING OF HORIZONTAL P-WAVES

The strong ground motion recorded during the 1989 April 25 earthquake (M(s) = 6.9) shows many of the previously observed features in the soft lacustrine clays over which downtown Mexico City is built: a dominant response frequency around 0.4 Hz, enormous time prolongation of the seismic signal with characteristic beating phenomena and conspicuous differential amplification. Following the two-step process suggested in Part I (Seligman et al. 1989), we add two new theoretical aspects: the generation of horizontally propagating SP waves in the layered structure beneath Mexico City for incoming SV waves beyond critical incidence, and the subsequent lateral trapping of these SP waves in the lacustrine clays. The time evolution of the lateral resonant pattern produced by these waves is calculated for the realistic geometry of the lake beds. Synthetic accelerograms and Fourier spectra at different sites within the clay region are thus obtained. Our results can explain the monochromaticity and the long duration of the strong motion observed systematically in the soft clays of the Valley of Mexico.