We perform travel-time tomography beneath Kanto, Japan, to obtain detailed structures of the wedge-shaped serpentinized mantle previously observed in the Philippine Sea slab. The results suggest that the western boundary of the serpentinized mantle (serpentine boundary) is subvertical and P- and S-wave velocities vary by 15%-20% across it over the short distance of similar to 10 km. Two intraslab earthquakes, in 1921 (M7.0) and 1987 (east off Chiba earthquake; M6.7), are inferred to have occurred along the serpentine boundary, accompanied by right-lateral movement, based on analyses of focal mechanisms and aftershock distribution. A subvertical earthquake cluster penetrating the entire Philippine Sea slab is observed along the serpentine boundary, and four earthquakes in the cluster have strike-slip focal mechanisms similar to that of the 1987 earthquake. Focal mechanisms obtained for past large earthquakes and present-day microearthquakes suggest a concentration of right-lateral deformation along the mechanically weak serpentine boundary. The Philippine Sea slab may have been torn in two along this boundary, with the eastern portion being left behind relative to subduction of the western portion. Assuming that one of the large aftershocks (M7.1) of the 1923 Kanto earthquake (M7.9), which occurred off the Boso Peninsula, ruptured the same asperity as did the 1987 earthquake, the slip deficit accumulated along the serpentine boundary during the 64 year interval is consistent with the fault slip of the 1987 earthquake. Interaction between the seismic slip along the plate interface and the serpentine boundary can explain the series of M7-class earthquakes before and after the 1923 Kanto earthquake.
