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Seismic response of three-dimensional alluvial deposit with irregularities for incident wave motion from a point source

Department of Architectural Engineering Faculty of Engineering Kyoto University, Yoshida Honmachi, Sakyo-Ku, Kyoto 606, Japan
Urban and Architecture Design Department NTT Power and Building Facilities NTT Shinagawa TWINS, 1-9-1, Konan Minato-ku, Tokyo 108, Japan
Construction Department The Kansai Electric Power Company, Inc., 3-3-22, Nakanoshima, Kita-Ku, Osaka 530, Japan

Abstract

The purpose of this article is to further investigate the influence of local site conditions of surface soil layers on seismic ground motion by studying the response of a semi-spherical alluvial deposit model under the more realistic type incident wave field. The incident wave field is a spherical wave radiated from a point source that is situated at the near focal distance and the far focal distance. A plane SH wavelet is also considered for comparison. A least-squares technique is applied to solve the boundary problem. The surface displacements on the alluvial deposit and in the elastic half-space are evaluated for various focal distances and source incidence angles. The numerical results indicate that there are significant differences concerning scattering and diffraction of wave motions between the two horizontal components when the semi-spherical alluvial deposit is subjected to incident wave motions from a point source. In the near field and for the component involving predominantly P and SV waves, the coupling of these waves induces a great variation in the spatial distributions, the amplitudes, and the time histories of the surface displacements compared to the tangential component, which involves mostly SH waves. The wave propagation characteristics of this component are very similar to those for a plane SH wavelet. In the far field and for the tangential component, there is not such strong coupling among waves. It is also shown that the first reflected waves at the curved boundary are deeply related to scattering and to diffraction of wave motion in a surface soil layer with irregularities and play an important role in the development of later arrivals.