Strong seismic wave scattering in the low-velocity anomaly associated with subduction of oceanic plate

Abstract

Analyses of dense seismic records in Kanto, Japan, revealed distinct pulse broadening and peak delay of high-frequency S waves at central Chiba. These phenomena are observed at frequency range of 1–8 Hz and exist only for ray paths passing through the low-velocity (LV) zone at depth of 20–40 km beneath northwestern Chiba. To obtain a more detailed understanding of these phenomena, we conducted 2-D and 3-D finite difference method simulations of seismic wave propagation using a realistic heterogeneous structure model. Through numerous simulations we demonstrated that strong seismic scattering, due to localized strong small-scale heterogeneities in the LV zone and in the oceanic crust, is a major cause of strong pulse broadening and peak delay of high-frequency S waves. After comparing simulation results with observations, the most preferable small-scale velocity heterogeneity in the LV zone is characterized by a Gaussian power spectral density function (PSDF) with correlation distance a of 1–2 km and rms value ε = 0.07–0.09, superposed on a background exponential PSDF (a = 3 km, ε = 0.07). Assuming strong velocity heterogeneities, observed amplitude decay at Chiba is also well explained by strong scattering attenuation in the LV zone. Because the LV zone, which has been reported by seismic tomography studies, is interpreted as being constructed by the dehydration of the subducting oceanic crust of the Philippine Sea Plate, strong small-scale velocity heterogeneity in the LV zone may be related to the random distribution of fluid in this volume.