ON THE DECOUPLING OF SURFICIAL SEDIMENTS FROM SURROUNDING GEOLOGY AT MEXICO-CITY

The geotechnical conditions at Mexico City suggest strongly the idea o f decoupling the vertical resonance of the thin, very soft, clay layer at the surface, from the two-dimensional (2-D) effects of a much stif fer underlying sedimentary valley. The purpose of this paper is to exp lore whether numerical modeling supports this idea. This problem has a lso a bearing on the contradiction between the extent of local models usually assumed by seismologists and those considered by geotechnical engineers. We present results of 1-D models and of two different appro aches to a 2-D modeling in an effort to explore the extent of local ge ology that must be considered in evaluating site effects at Mexico Cit y. Our results indicate that there is a significant degree of coupling between the surficial clay layer and the underlying sediments, despit e the extremely large impedance contrast. As a consequence, a full 1-D analysis gives better results than a 2-D ''decoupled'' one. It is als o shown that deeper layers affect significantly ground motion at the s urface. Thus, a simplified 1-D model that would consider only the uppe rmost very soft layers would be wrong in its prediction of surface mot ion by a factor about 3 both in frequency and time domains.