Geometry and stratigraphy parameterization of topography effects: From the infinite wedge to 3D convex features
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Although the problem of seismic wave scattering by topographic irregularities has been studied for several decades, only recently it has attracted the attention of geotechnical earthquake engineering researchers. Macroseismic observations and recorded evidence from large earthquakes have highlighted that structural damage intensity is frequently higher on the surface of irregular topographies than on adjacent flat ground sites. Numerical and semi-analytical published studies have qualitatively corroborated these observations, but when compared to field recordings, have been shown to systematically underestimate the absolute level of topographic amplification up to an order of magnitude or more in some cases. This discrepancy between theory and observations has been attributed, at least in part, to idealizations of the above studies such as the assumptions of 2D geometry, homogeneous medium, linear elastic response, and monochromatic or narrowband ground shaking. In this research, we bridge the quantitative gap between previous theoretical studies and observations by systematically studying the role of geometry, stratigraphy, and ground motion characteristics through a series of elaborate numerical analyses. We specifically start from the topographic amplification caused by a 2D infinite wedge on the surface of a homogeneous elastic halfspace, and extend the state-of-the-art understanding of wave focusing and scattering by this fundamental block of irregular ground surface geometries. From there, we gradually increase the geometric and stratigraphic complexity up to a 3D convex layered topographic feature, identifying in each level the controlling factors of topographic amplification. Our results provide new insights into the effects of surface topography and its nonlinear coupling with subsurface soil layering, and suggest that in real conditions, topographic amplification can only be quantitatively captured when geometry and stratigraphy of the site are simultaneously accounted for in theoretical predictive models.