Mapping the mantle wedge and interplate thrust zone of the northeast Japan arc

We detected sP depth phases from three-component, short-period seismograms from the forearc earthquakes under the Pacific Ocean recorded by the dense seismic network on the Japan Islands. Then we use the sP depth-phase data together with P and S wave arrival times to locate the suboceanic earthquakes accurately. A large number of P and S wave arrival times from the relocated suboceanic events and earthquakes under the land area are used to determine the 3-D P and S wave velocity and Poisson's ratio structures under the entire NE Japan arc from the Japan Trench to the Japan Sea. Our results exhibit strong lateral heterogeneity under the forearc region and a clear correlation between the structural heterogeneity and distribution of large earthquakes (M 7.0-8.2) occurred in the interplate thrust zone. Prominent low-velocity and high Poisson's ratio zones are visible in the mantle wedge under the volcanic front and back-arc region, which show nicely the images of arc magma caused by the slab dehydration and corner flow in the mantle wedge.