Topography Effects Are Not Dominated by Ground Surface Geometry: A Site Effects Paradox

Abstract

The material properties and the geometry of near surface soil layers, known as local site conditions, can significantly change the input seismic motion compared to the simple case of homogeneous linear elastic half-space. Our recent studies have shown that the effects of topography coupled to site response can lead to ground motion aggravation larger than the superposition of site and geometry amplification. These soil-topography coupling effects arise from seismic waves trapped in the near surface soil layers, are amplified or deamplified as a consequence of stiffness contrast, and are further modified due to scattering caused by irregular interface and ground surface. In this study, we investigate the coupling effects for 2D idealized convex features through a systematic analysis. The resulting trends, which are presented in the form of dimensionless amplification factors, clearly demonstrate the nonlinear nature of coupling effects, which cannot be predicted by modifying simulations of topography effects on rock by 1D site amplification factors, a posteriori. We then quantify these coupling effects through 3D site-specific analyses at selected strong ground motion stations in California, which yield more realistic amplification patterns (using 1 arc-second DEM extracted surface topographies and measured Vs profile). The results of coupling effects provide a basis as to how it can be incorporated in the proposed design motion of seismic code provisions.

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