Deep structure of the Japan subduction zone

We determined 3-D P-wave velocity structure down to 700 km depth under the Japan Islands using a large number of arrival time data from local and teleseismic events simultaneously. We collected 207,000 arrival times from 7743 shallow and deep earthquakes occurred in and around Japan and 34,148 data from 333 teleseismic events, which were recorded by over 1000 seismic stations on the Japan Islands. Our tomographic model revealed some new features. The Philippine Sea slab is found to subduct down to 500 km depth under southwest Japan though the seismicity within the slab ends at 150–200 km depth. Significant low-velocity anomalies are found to exist in the deep portion of the mantle wedge above the Pacific slab, which may be caused by the deep dehydration process of the slab. Slow anomalies are detected in the mantle beneath the Pacific slab, which may be caused by a mantle plume or upwelling of hot mantle materials associated with the deep subduction of the Pacific slab and its collapsing down to the lower mantle.