Scattering of high-frequency seismic waves caused by irregular surface topography and small-scale velocity inhomogeneity

Based on 3-D finite difference method simulations of seismic wave propagation, we examined the processes by which the complex, scattered high-frequency (f > 1 Hz) seismic wavefield during crustal earthquakes is developed due to heterogeneous structure, which includes small-scale velocity inhomogeneity in subsurface structure and irregular surface topography on the surface, and compared with observations from dense seismic networks in southwestern Japan. The simulations showed the process by which seismic wave scattering in the heterogeneous structure develops long-duration coda waves and distorts the P-wave polarization and apparent S-wave radiation pattern. The simulations revealed that scattering due to irregular topography is significant only near the station and thus the topographic scattering effects do not accumulate as seismic waves propagate over long distances. On the other hand, scattering due to velocity inhomogeneity in the subsurface structure distorts the seismic wavefield gradually as seismic waves propagate. The composite model, including both irregular topography and velocity inhomogeneity, showed the combined effects. Furthermore, by introducing irregular topography, the effects of seismic wave scattering on both body and coda waves were stronger than in the model with velocity inhomogeneity alone. Therefore, to model the high-frequency seismic wavefield, both topography and velocity inhomogeneity in the subsurface structure should be taken into account in the simulation model. By comparing observations with the simulations including topography, we determined that the most preferable small-scale velocity heterogeneity model for southwestern Japan is characterized by the von Karman power spectral density function with correlation distance a = 5 km, rms value of fluctuation epsilon = 0.07 and decay order kappa = 0.5. We also demonstrated that the relative contribution of scattering due to the topography of southwestern Japan is approximately 12 per cent.