AN INDIRECT BOUNDARY ELEMENT METHOD APPLIED TO SIMULATE THE SEISMIC RESPONSE OF ALLUVIAL VALLEYS FOR INCIDENT-P, INCIDENT-S AND RAYLEIGH-WAVES

A boundary integral formulation is presented and applied to model the ground motion on alluvial valleys under incident P, S and Rayleigh wav es. It is based on integral representations for the diffracted and the refracted elastic waves using single-layer boundary sources. This app roach is called indirect BEM in the literature as the sources' strengt hs should be obtained as an intermediate step. Boundary conditions lea d to a system of integral equations for boundary sources. A discretiza tion scheme based on the numerical and analytical integration of exact Green's functions for displacements and tractions is used. Various ex amples are given for two-dimensional problems of diffraction of elasti c waves by soft elastic inclusion models of alluvial deposits in an el astic half-space. Results are displayed in both frequency and time dom ains. These results show the significant influence of locally generate d surface waves in seismic response and suggest approximations of prac tical interest. For shallow alluvial valleys the response and its reso nant frequencies are controlled by a coupling mechanism that involves both the simple one-dimensional shear beam model and the propagation o f surface waves.