SITE RESPONSE ANALYSIS IN THE VALLEY OF MEXICO - SELECTION OF INPUT MOTION AND EXTENT OF NONLINEAR SOIL BEHAVIOR

It is well known that seismic waves in the lake-bed zone of Mexico city are dramatically amplified, and that the soft clays in this region exhibit an unusually large elastic range. New field and laboratory data show that clays in certain areas within the Valley of Mexico have lower elastic thresholds and, hence, non-linear soil behaviour would be more likely to take place. In this paper, we analyse geotechnical and strong-motion data to model, using a one-dimensional (1-D) approach, the non-linear soil response at two sites in Mexico City during the September 19,1985 (M(s) = 8.1) Michoacan earthquake. Results suggest that, even for motions as large as those recorded during the 1985 Michoacan event, a linear viscoelastic behaviour must be assumed for the examined area, although, as far as it is presently known, the clays from this area show the greatest modulus reduction among the Mexico City clays. Thus, a linear 1-D propagation analysis continues to be a suitable engineering option in computing expected ground motions at soft sites for the Valley of Mexico. Caution, however, is warranted when selecting input motions, since, as shown in this paper, large differences can result depending on the choice of outcrop motions.