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 cit y are dramatically amplified, and that the soft clays in this region e xhibit an unusually large elastic range. New field and laboratory data show that clays in certain areas within the Valley of Mexico have low er elastic thresholds and, hence, non-linear soil behaviour would be m ore likely to take place. In this paper, we analyse geotechnical and s trong-motion data to model, using a one-dimensional (1-D) approach, th e non-linear soil response at two sites in Mexico City during the Sept ember 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 examin ed area, although, as far as it is presently known, the clays from thi s area show the greatest modulus reduction among the Mexico City clays . Thus, a linear 1-D propagation analysis continues to be a suitable e ngineering option in computing expected ground motions at soft sites f or the Valley of Mexico. Caution, however, is warranted when selecting input motions, since, as shown in this paper, large differences can r esult depending on the choice of outcrop motions.