A brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis. Amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed, but the numerical simulations often underestimate the actual observed effects. We propose that this disagreement is because current models are not complex enough.
We therefore computed the response to incident SH waves of a set of different complex configurations that include two-dimensional surface topography, with or without periodic ridges and subsurface layering. The results show that: (1) the topographic effect in itself is difficult to isolate from other effects, like surface layering, and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography; and (2) the high crest/base amplification ratio observed in the field cannot, usually, be matched even with complex two-dimensional structures with incident plane SH waves, which suggests that more complex models are needed to incorporate more complex wave fields (e.g., SV, surface) and three-dimensional geologic configurations.