Characterization of scattered seismic wavefields simulated in heterogeneous media with topography

We performed numerical simulations of seismic waveforms with frequencies up to 10 Hz in heterogeneous media with topography to investigate the effects of topography and structural heterogeneity on seismic scattering. We used the simulated waveforms to test the source location method assuming isotropic radiation of S waves for long-period events and tremor at volcanoes. The assumption of isotropic radiation has been shown previously to be valid in a high frequency band because of the path effect caused by the scattering of seismic waves. Our simulation results showed that distortion of the wavefields increased as the correlation distance of structural heterogeneity (a) decreased, as the frequency increased, and as the travel distance increased. Wavefield distortion was greatest at ka =~ 1, where k is the wavenumber of the S waves. Topography alone considerably distorted the wavefields. However, we found that strong scattering due to topography was suppressed if the correlation distance of structural heterogeneity was longer than the S-wave wavelength. Isotropic radiation of S waves by scattering due to topography was not achieved in our simulations. Our results indicated that scattering due to structural heterogeneity becomes stronger than that due to topography only at ka =~ 1. This suggests that strong structural heterogeneity satisfying ka =~ 1 (i.e., a specific correlation distance for a particular frequency band) is required to achieve isotropic radiation of S waves. Although we could not reproduce isotropic radiation, our results support the validity of the assumption of isotropic radiation in the source location method.