3D Analysis of S-Wave Propagation in Soft Deposits

Abstract

During the 1985 Michoacan earthquake (surface-wave magnitude = 8.1), large-amplitude seismograms of extremely long duration were recorded in the lake-bed zone of Mexico City. Large amplification in soft surficial deposits has not been well understood. Ground subsidence of the soft clay deposits might heavily influence S-wave propagation. In the present study, we attempt to explain both S-wave amplification and propagation time between surface and borehole recordings under the action of gravity on the basis of a 3D nonlinear finite-element (FE) technique where a bilinear model with a Mohr-Coulomb criterion is used. We identify body-wave (S-wave) time sections in the recordings with a cross-correlation analysis. The FE simulation successfully predicts the S-wave amplification and propagation time, which cannot be explained by a purely 1D linear model. Although subsidence changes S-wave linear amplification, S-wave propagation time decreases little.