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On the decoupling of surficial sediments from surrounding geology at Mexico City

OBSERVATOIRE DE GRENOBLE LABORATOIRE DE GÉOPHYSIQUE INTERNE ET TECTONOPHYSIQUE, IRIGM, B.P. 53X, 38041 GRENOBLE CEDEX, France
LABORATOIRE CENTRAL DES PONTS-ET-CHAUSSÉES, 58, BD. LEFEBVRE, 75732 PARIS CEDEX 15, France
INSTITUTO DE INGENIERÍA, UNAM CIUDAD UNIVERSITARIA, APDO. POSTAL 70-472, COYOACÁN, 04510 MÉXICO, D.F., Mexico
CENTRO DE INVESTIGACIÓN SÍSMICA, A.C., CAMINO AL AJUSCO 203, 14200 TLALPAN, D. F., Mexico

Abstract

The geotechnical conditions at Mexico City suggest strongly the idea of decoupling the vertical resonance of the thin, very soft, clay layer at the surface, from the two-dimensional (2-D) effects of a much stiffer underlying sedimentary valley. The purpose of this paper is to explore whether numerical modeling supports this idea. This problem has also 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 approaches to a 2-D modeling in an effort to explore the extent of local geology that must be considered in evaluating site effects at Mexico City. Our results indicate that there is a significant degree of coupling between the surficial clay layer and the underlying sediments, despite the extremely large impedance contrast. As a consequence, a full 1-D analysis gives better results than a 2-D "decoupled" one. It is also shown that deeper layers affect significantly ground motion at the surface. Thus, a simplified 1-D model that would consider only the uppermost very soft layers would be wrong in its prediction of surface motion by a factor about 3 both in frequency and time domains.

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