Cause of M ~ 7 intraslab earthquakes beneath the Tokyometropolitan area, Japan: Possible evidence for a vertical tearat the easternmost portion of the Philippine Sea slab

摘要

We perform travel-time tomography beneath Kanto, Japan, to obtain detailedstructures of the wedge-shaped serpentinized mantle previously observed in the PhilippineSea 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 –10 km. Two intraslab earthquakes, in 1921 (M7.0)and 1987 (east off Chiba earthquake; M6.7), are inferred to have occurred along theserpentine boundary, accompanied by right-lateral movement, based on analyses of focalmechanisms and aftershock distribution. A subvertical earthquake cluster penetrating theentire Philippine Sea slab is observed along the serpentine boundary, and fourearthquakes in the cluster have strike-slip focal mechanisms similar to that of the 1987earthquake. Focal mechanisms obtained for past large earthquakes and present-daymicroearthquakes suggest a concentration of right-lateral deformation along themechanically weak serpentine boundary. The Philippine Sea slab may have been torn intwo along this boundary, with the eastern portion being left behind relative tosubduction of the western portion. Assuming that one of the large aftershocks (M7.1) ofthe 1923 Kanto earthquake (M7.9), which occurred off the Boso Peninsula, ruptured thesame asperity as did the 1987 earthquake, the slip deficit accumulated along theserpentine boundary during the 64 year interval is consistent witlithe fault slip of the1987 earthquake. Interaction between the seismic slip along the plate interface and theserpentine boundary can explain the series of M7-class earthquakes before and afterthe 1923 Kanto earthquake.