境界要素法と断層震源モデルによる堆積盆地におけるSH波とLove波の地震波動解析

抄録

In recent large earthquakes (Nihonkai-chubu 1983 and Michoacan 1985 earthquakes etc.), unexpected types of damage in long-period structures on basin-shaped soft sedimentary layers were noticed. Measurements of ground motion in Kanto plain (Kanto sedimentary basin, in and around Tokyo, Japan) show that seismic waves depend greatly on the characteristics of a seismic fault origin. In order to study both the characteristics of source faults and topographical irregularity of the surface layers, we developed BEM (boundary element method) analysis of a basin model with a fault adjacent to it in 1987. In this study, we constructed a two-layered model of Kanto basin and simulated long-period SH and Love waves in a basin-shaped layer. Following conclusions are obtained; 1) When a fault is located under the basin, the seismic waves are mainly the body waves propagated upward from the bedrock. In this case we can use a classical theory of multiple wave reflection. On the other hand, when a fault is shallow and is laterally away from the basin, the body waves are superposed by local surface waves which are generated at the basin edge. Thus the wave duration becomes longer. If we use the multiple reflection theory, we would underestimate the wave energy at the surface. Because the energy of the local surface waves can be large. 2) The surface waves generated at the basin edge are mainly composed of fundamental mode, and predominate in longer period. Moreover, shorter period components of the surface waves tend to be damped largely while traveling through the low-Q sediments, as compared with the longer period components. Thus influence of these surface waves on long-period structures can not be neglected, if a natural period of the structure is almost equal to a presumed dominant period of surface waves.