The influence of site conditions on the spatial incoherence of ground motions

We have found evidence for large differences in the spatial incoherence of ground motions under different site conditions. At sites located on flat-lying alluvial sequences, such as the Imperial Valley, California and Lotung, Taiwan, the spatial incoherence increases smoothly as a function of station separation and wave frequency. This uniform behavior is consistent with wave scattering in an otherwise laterally homogeneous seismic velocity structure. In contrast, on sites located on folded sedimentary rocks, such as the Coaling anticline in California, the spatial incoherence does not show a strong dependence on station separation and wave frequency, and is higher at close station spacing and low frequency than in flat-lying alluvial sites. This chaotic behavior is consistent with wave propagation in a medium having strong lateral heterogeneities in seismic velocity. We have modeled this behavior using synthetic seismograms calculated for a basin structure in which overlying sedimentary material is separated from basement rock by an undulating interface. Even simple undulations in a smooth interface produce caustics, giving rise to ground motions whose spatial incoherence characteristics resemble those of the Coalinga data. We conclude that the spatial incoherence measured on flat-lying sedimentary sites such as the Imperial Valley and Lotung may not provide a good description of the spatial incoherence at sites where there is significant lateral heterogeneity. These include folded sedimentary rocks exposed at the surface, basins generated by the folding of sedimentary rocks, and alluviated river valleys. At present, we do not have measurements of spatial incoherence at enough sites to know whether the spatial incoherence characteristics of ground motions can be readily correlated with simplified categories of site conditions. In order to obtain reliable estimates of the spatial incoherence of ground motion at a particular site, it may be necessary to use accelerograms of a small earthquake or explosion recorded on a small array at the site.