Seismic Ground Motion in Sedimentary Valleys: Focusing or Chaos?

1985年的墨西哥地震把人们的注意力集中在从基岩传播到上覆沉积层时强烈地面运动的放大上。美国的主要城市也可能存在类似的情况，例如洛杉矶盆地。本研究项目的主要目的是通过计算机模拟研究由基岩-沉积层边界界定的盆地底部形状对地震期间地面运动时空分布的影响。迄今为止，几乎所有的研究都认为盆地的底部是凹的。最近的一篇科学文章指出，一个同时包含凹面和凸面的底面会引起地面的混沌运动。这种混沌运动对工程结构所经历的地面运动水平的影响尚不清楚，但它可能是有益的。这将在以入射平面SH波的形式受到地震激励的二维沉积山谷的背景下进行探讨。参数化研究将确定凸底表面上的地面运动与通常凹底表面上的地面运动的差异。本研究还将考虑山谷几何形状和地震激励（如入射波的入射角）的微小变化是否会对山谷响应的时空分布产生显著变化。如此大的变化将对地震危险性评估和结构抗震设计产生重要影响。＊＊＊

9319339 Bielak The 1985 Mexico earthquake focused attention on the amplification of strong ground motions when propagating from bedrock through the overlying sedimentary deposits. Similar situations may exist with regard to major cities in the United States, such as with the Los Angeles basin. The main objective of this research program is to investigate, by computer simulation, the effect of the shape of the bottom of the basin, as defined by the bedrock-sedimentary layer boundary, on the spatial and temporal distribution of surface ground motion during earthquakes. Practically all studies to date have considered the bottom of a basin to be concave. A recent scientific article suggests that a bottom surface which includes both concave and convex sections can cause ground surface motions which are chaotic. The effect of such a chaotic motion on the level of ground motion experienced by engineered structures is unknown, but it is likely to be beneficial. This will be explored in the context of two-dimensional sedimentary valleys subjected to seismic excitation in the form of incident plane SH waves. Parametric studies will determine the difference in the surface ground motion above a convex bottom surface as compared to that above the usual concave shape. This study will also consider whether small changes in valley geometry and seismic excitation, such as the angle of incidence of the incoming wave, can produce significant changes in the spatial and temporal distribution of the valley response. Such large variations would have important implications for seismic hazard assessment and the seismic design of structures. ***