DETECTION OF THE SUBDUCTING CRUST OF OCEANIC PLATES BENEATH THE KANTODISTRICT, JAPAN

Low-velocity crustal layers on the top of the subducting slabs beneath the Kanto district, central Japan, were detected by a travel time inv ersion using 40,763 P-wave arrival times from 3038 local earthquakes r ecorded by a local network of 41 seismic stations. The inversion cover ed 188 x 228 x 110 km volume of the target region and the model was pa rameterized by 47 x 57 x 18 rectilinear blocks, measuring 4.0 km per s ide horizontally and 4-10 km varying thickness in depth. A prominent E -W-directed low-velocity band of about 25 km width appeared in the cen tral Kanto region at a depth of 30 km, moving north or northwestward g radually with depth. This band can be traced to the depth of about 60 km. At a depth of 60 km, a slightly lower amplitude, nearly N-S-direct ed low-velocity band was observed at the east coast of the Kanto regio n, moving westward with depth. These low-velocity bands are thin layer s attached to the top of the inclined thick layers with relatively hig h velocity. In the Kanto district, the Philippine Sea (PHS) plate subd ucts under the Eurasian (EUR) plate and the Pacific (PAC) plate also u nderthrusts beneath both the EUR and PHS plates. Some authors already suggested low-velocity anomalies near the surface of those descending plates in this region, mainly from the analyses of later phases and fo cal mechanisms of local earthquakes. The shapes of the low-velocity ba nds are similar to the depth distribution of the upper boundary of the PHS and PAC plates proposed by other authors. The thrust earthquakes that occur at the boundaries between the EUR and PI-IS plates, and bet ween the PHS and PAC plates are almost located in the low-velocity lay ers described above. We conclude that these low-velocity layers are th e crusts that exist at the top of the subducting PHS and PAC plates, w hile high-velocity layers are subducting slabs themselves. For the PHS plate, low-density crust of the Izu-Bonin are is probably stacking at the range from 139 degrees 25'E to 139 degrees 55'E at 35 degrees 55' N, which is observed as a thick low-velocity region to the south of th e range and in a steep inclination to the north of it; this is also re cognized from the gravity anomaly.