Abstract

The phenomenon of localized resonance in sedimentary basins with irregular basement topography (Rial et al., 1992) was recently independently confirmed by Moczo et al. (1996) in a numerical study that shows how a single, concave upwards trough in the basement of an otherwise horizontal layer of sediment overlying rock can give rise to localized resonance. In an effort to complement these studies this note turns to the opposite effect: localized de-amplification, as created by a convex section of the sediment/rock interface. It is shown here that de-amplification is indeed localized, being strongest directly above convexities in the basement whereby seismic risk should be substantially reduced. Further, it is found that even a small increase in topographic height of a convex segment of the sediment/rock interface can produce orders of magnitude decrease (nonlinear de-amplification) in ground motion amplitude at the site directly above it, especially if the contrast in elastic impedances between bedrock and sediment is high. Thus, it becomes possible, at least theoretically, to identify areas that will stand safe, as it is to identify areas of high seismic risk, provided the topography of the basement is known.

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