Seismo Lab Brown Bag Seminar

Subduction zone earthquakes occur in some localized areas, probably controlled by material and stress heterogeneity on the plate interface, which have been little understood. Here we provide high-resolution observations of material and stress heterogeneity in the Japan subduction zone through an integration of material and source imaging. We present evidence for localized, anisotropic structures with anomalous Vp/Vs ratios in the medium surrounding 1-2 km-sized clusters of deep interplate earthquakes in the Kanto and Off-Miyagi regions in Japan. Thanks to the dense coverage of Hi-net borehole stations and off-shore S-net stations, we image the near-source medium using the differential P- and S-wave travel times between pairs of similar earthquakes and measure earthquake stress drops through source spectral ratios. We find a median Vp/Vs ratio of 1.44 in the Kanto earthquake source regions, equivalent to a Possion's ratio of nearly 0, suggesting the existence of a heterogeneous near-fault medium with distinct material properties from those of surrounding rocks. We also observe for the first time a strong azimuthal dependence of Vp/Vs ratios in both Kanto and off-Miyagi regions, which can be caused by a highly damaged and foliated medium. The typical stress drop values of M>3.4 earthquakes support a relatively low effective normal stress, potentially due to elevated fluid pressure in fault zones. Thus, our results from material and source imaging suggest the fault medium is damaged, foliated, and enriched with fluid. Such localized structures can cause stress perturbations on faults that in turn favor the frequent occurrence of deep interplate earthquakes in the Japan subduction zone.