DYNAMIC SOIL PARAMETERS DETERMINATION BY GEOPHYSICAL-PROSPECTING IN MEXICO-CITY - IMPLICATION FOR SITE EFFECT MODELING

Seismic experiments were conducted in Mexico City in order to determin e the dynamic characteristics of the soft clay layer present at the su rface in the lake bed zone. In-situ measurements of V-p, V-s, Q(p) and Q(s) were performed in the Texcoco lake area. Q(s) values ranging fro m 8 to 60 have been found from the surface down to 30 m depth, while V -s values are between 30 and 115 m/s for the same depths. These relati vely low Q values support the idea that surface waves generated at the edges of the Mexico basin cannot propagate very far in the structure. During this study, the elastic characteristics of the surficial basal t layers outcropping in the hill zone have also been investigated. Ver y low P-wave velocity values (500-800 m/s) have been found in the basa lt. In the hill zone where basalts are interbedded with soil, numerica l modeling has shown that such low values can only be partially explai ned by the relation between wavelength and basalt thickness and that o ther mechanisms, like fracturation, must be involved. Copyright (C) 19 96 Elsevier Science Limited.