DIFFRACTION OF P-WAVES, S-WAVES AND RAYLEIGH-WAVES BY 3-DIMENSIONAL TOPOGRAPHIES

The diffraction of P, S and Rayleigh waves by 3-D topographies in an e lastic halfspace is studied using a simplified indirect boundary eleme nt method (IBEM). This technique is based on the integral representati on of the diffracted elastic fields in terms of single-layer boundary sources. It can be seen as a numerical realization of Huygens' princip le because diffracted waves are constructed at the boundaries from whe re they are radiated by means of boundary sources. A Fredholm integral equation of the second kind for such sources is obtained from the str ess-free boundary conditions. A simplified discretization scheme for t he numerical and analytical integration of the exact Green's functions , which employs circles of various sizes to cover most of the boundary surface, is used. The incidence of elastic waves on 3-D topographical profiles is studied, We analyse the displacement amplitudes in the fr equency, space and time domains. The results show that the vertical wa lls of a cylindrical cavity are strong diffractors producing emission of energy in all directions. In the case of a mountain and incident P, SV and SH waves the results show a great variability of the surface g round motion, These spatial variations are due to the interference bet ween locally generated diffracted waves. A polarization analysis of th e surface displacement at different locations shows that the diffracte d waves are mostly surface and creeping waves.