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Anisotropic Kilometer-scale Structures With a Near-zero Poisson’s Ratio on the Japan Subduction Zone Plate Interface

Description: 

Geological studies show that subduction zone plate interfaces feature complex fault structures with mixtures of different minerals and foliated rocks, but seismic observations of such near-source structures are still rare in subduction zones. Here we present evidence for localized, anisotropic structures with anomalously low Vp/Vs ratios in the medium surrounding 1-2 km-sized clusters of deep interplate earthquakes in the Kanto region, Japan. Thanks to the dense coverage of Hi-net borehole 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 these 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 the earthquake source region, 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.

Session: Earth’s Structure from the Crust to the Core [Poster]

Type: Poster

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Peng

Student Presenter: No

Invited Presentation: 

Poster Number: 112

Authors