Dynamic soil parameters determination by geophysical prospecting in Mexico City: implication for site effect modeling

Abstract

Seismic experiments were conducted in Mexico City in order to determine the dynamic characteristics of the soft clay layer present at the surface 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 from 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 relatively 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 basalt layers outcropping in the hill zone have also been investigated. Very low P-wave velocity values (500–800 m/s) have been found in the basalt. In the hill zone where basalts are interbedded with soil, numerical modeling has shown that such low values can only be partially explained by the relation between wavelength and basalt thickness and that other mechanisms, like fracturation, must be involved.