Abstract
To evaluate the effects of velocity inhomogeneity in propagation path on seismic ground motion, numerical simulations were performed using inhomogeneous medium models with fluctuation in seismic velocity. In this study inhomogeneity effects in short distance region from seismic source are focused on. Epicentral distance is about less than 50km and is approximately equal to 50 times ratio to the analyzed central wave length. 3D inhomogeneous models were generated and a double-couple type point source was located in the medium models. Then the finite difference method was applied to the models to obtain velocity records at observation points on the surface. The simulated records were revealed to have coda waves similar to observed records and to have modulated wave forms and amplitudes differed from calculated velocity records in a homogeneous medium model. Such effects were observed even in relatively short distance. Fluctuation in amplitudes was observed in simulated waves in inhomogeneous medium even if the source radiation effect was removed. The fluctuation was revealed to depend on the inhomogeneous parameters and frequency. The ratios of the amplitudes in inhomogeneous medium to those in homogeneous medium were decreasing with increasing distance to show the scattering attenuation. The effect of source radiation pattern on the scattering attenuation has not been seen. Also the fluctuation in amplitudes is seen to increase gradually with distance. As a result even in relatively short distance, such as several tens of kilometer, the effects of inhomogeneity in velocity can cause effects on wave forms and amplitude fluctuation.
