Two Oceanic Plates Subducting Beneath the Kanto Area and Implications for the Prominent Seismicity Before the 1923 Kanto Earthquake

Though the 1923 Kanto earthquake (M7.9) is the shallow thrust faulting between the overriding Okhotsk plate (OKH) and the descending Philippine Sea plate (PHS) along the northern Sagami Trough, the remarkable seismicity associated with the 1923 earthquake has occurred extensively around the source area. One important constraint of the subduction of PHS and the accompanying seismicity is the existence of another thrusting boundary in depth, which is located in the seismic zone of the overlapped portion of PHS and the Pacific plate (PAC). The slip vectors of earthquakes in the PHS-PAC seismic zone are consistent with the relative plate movement at PHS-PAC. Thus due to the difference of the relative movement at the OKH-PHS, OKH-PAC and PHS-PAC boundaries, both the Philippine Sea plate and Pacific plate have to deform elastically during the subduction. As a remarkable deformation, we demonstrate the striking feature of concave of the Pacific slab, which is located eastward adjacent the PHS-PAC boundary beneath eastern Kanto. According to the catalogue by Utsu (1982) and JMA catalogue revised recently, the seismicity before the 1923 Kanto earthquake was remarkably high in the wide area, in particular in the northward and deeper portion of the imminent 1923 earthquake source area, and also in the sea area far east off the Pacific coast (off Ibaraki Prefecture). Space-time distribution and magnitude time series of these earthquakes show a gradual increase of event size and seismicity starting from about 1906, and an accelerating tendency of the cumulative seismic energy release toward the 1923 earthquake. In particular the intense swarm including two M7 events occurred off Ibaraki Prefecture just three months before the 1923 Kanto earthquake. After the remarkable aftershock activities the seismicity decreased systematically in wide area. All these long-term seismicity pattern imply that the Kanto subduction zone is not always in a statistically stationary critical state, i.e., self-organized criticality, but evolves widely in a critical state before the impending large earthquake. After the large event the state retreat from the criticality and the convergence of the three plates stated above may maintain as a mechanically equilibrating system.