Detection of the subducting crust of oceanic plates beneath the Kanto district, Japan

Abstract

Low-velocity crustal layers on the top of the subducting slabs beneath the Kanto district, central Japan, were detected by a travel time inversion using 40,763 P-wave arrival times from 3038 local earthquakes recorded by a local network of 41 seismic stations. The inversion covered 188 × 228 × 110 km volume of the target region and the model was parameterized by 47 × 57 × 18 rectilinear blocks, measuring 4.0 km per side 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 central Kanto region at a depth of 30 km, moving north or northwestward gradually 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-directed low-velocity band was observed at the east coast of the Kanto region, moving westward with depth. These low-velocity bands are thin layers attached to the top of the inclined thick layers with relatively high velocity. In the Kanto district, the Philippine Sea (PHS) plate subducts under the Eurasian (EUR) plate and the Pacific (PAC) plate also underthrusts 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 focal mechanisms of local earthquakes. The shapes of the low-velocity bands 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 PHS plates, and between the PHS and PAC plates are almost located in the low-velocity layers described above. We conclude that these low-velocity layers are the crusts that exist at the top of the subducting PHS and PAC plates, while high-velocity layers are subducting slabs themselves. For the PHS plate, low-density crust of the Izu-Bonin arc is probably stacking at the range from 139°25′E to 139°55′E at 35°55′N, which is observed as a thick low-velocity region to the south of the range and in a steep inclination to the north of it; this is also recognized from the gravity anomaly.