Seismic imaging of the Amur–Okhotsk plate boundary zone in the Japan Sea

We determined high-resolution images of P and S wave velocity and Poisson’s ratio under the Japan Sea off Honshu Island using a large number of arrival-time data from local earthquakes to understand the interaction of the Amur and Okhotsk plates. Our new data collected from the crustal earthquakes under the Japan Sea relocated precisely with sP depth phase are crucial to make this work possible. Our results show that strong lateral heterogeneities exist in the crust and upper mantle under the Amur–Okhotsk plate boundary zone, which may have affected the seismotectonics in the region. The crustal velocity variations under the Japan Sea may reflect the complicated geologic structures that were produced during the opening of the Japan Sea and the present compressional stage of the Honshu arc associated with the collision of the Amur plate with the Okhotsk plate. Low-velocity zones in the mantle wedge are found to extend westward under the Japan Sea, rather than just confined under Honshu Island, suggesting that the back-arc magmatism and tectonics are part of the complex geodynamic system in East Asia.