DEEP-STRUCTURE OF JAPAN SUBDUCTION ZONE AS DERIVED FROM LOCAL, REGIONAL, AND TELESEISMIC EVENTS

We have determined a detailed three-dimensional P wave velocity structure of the Japan subduction zone to 500-km depth by inverting local, regional, and teleseismic data simultaneously. We used 45,318 P wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the Japan Islands. The arrival times are recorded by the Japan University Seismic Network which covers the entire Japan Islands densely and uniformly. We also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern Japan. In comparison with the previous results obtained from only local and regional events, the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes. In the crust and uppermost mantle, low-velocity zones are clearly visible beneath active volcanoes. In the mantle wedge the low-velocity zones generally parallel with the slab and exist continuously to a depth of about 200-km, which is consistent with the petrological, geochemical and geodynamic studies. We consider that the existence of volcanism-related low-velocity anomalies in the mantle wedge is a general seismological. characteristic of subduction zones, in light of all the available tomographic results for many subduction zones in the world. The Pacific slab beneath Japan is imaged more clearly than in previous studies as a high-velocity zone with a thickness of 80-90 km and a P wave velocity 4-6% higher than the normal mantle. Lower velocity anomalies are found in the mantle below the slab.