Wave propagation in a three-dimensional circular basin model is studied by using a ray technique which employs the method of principal curvature for geometric spreading calculation. Three components of ground motion are calculated at surface stations assuming incident plane P and SH waves with various incidence angles. Computed results show that seismic energy is substantially enhanced in the central region of the basin while it diverges rapidly toward the edge of the basin. Compared to a two-dimensional basin with the same cross section, amplitudes are larger for stations in the central region and smaller for stations near the edge. Waveforms and amplitudes can change drastically between two nearby stations due to caustics and geometric focusing for different paths. It is found, for the particular basin geometry and velocity contrast used, that the first multiple P wave and other particular phases experience two caustics near the basin center which are associated with the principal curvatures of the wave front. This results in the reversal of the polarity of the waveform. As incidence angle increases, later arrivals at near-edge stations and the stations in the diagonal direction are more strongly developed from the curved boundary of the basin, but amplitudes of ground motions generally decrease as incidence angle increases. In general, wave responses strongly depend on the incidence angle and station location. These results have important implications for site amplification of strong ground motions since wave behavior is so complex even for this simple idealized structure.

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