Considering the possibility that random-velocity fluctuation of sediments may affect spatial variation of high-frequency seismic motions in small areas (a few hundred meters in diameter), we examine this possibility experimentally and numerically. By comparing seismic-array data obtained at three geologically different sites (rock, stiff-sediment, and soft-sediment sites), we make two observations related to the effect of random-velocity fluctuation of sediments. The first observation is that above 3 Hz, waveform-spatial variation evaluated in terms of the cross-correlation is larger at the stiff-sediment site than at the rock site. The second is that above 2 Hz, the amplitude-spatial variation evaluated by the spectral ratio shows more observation-station-specific behavior at the soft-sediment site than at the stiff-sediment site. We try to explain these two observations by modeling the sediments as random media. Two-dimensional finite-difference simulations for plane SH- and SV-wave incidences demonstrate that the sediments may affect the seismic waveform variation above 3 Hz if the autocorrelation functions of the velocity fluctuations are in the range derived from well-log velocity data. The velocity structure at the soft-sediment site is modeled as a two-layered random media having a low-velocity surface layer. The finite-difference simulations for this sediment model demonstrate that it is also possible that amplitude variation above 2 Hz is caused by random-velocity fluctuations in the low-velocity surface layer.

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