Crustal block kinematics and seismic potential of the northernmost Philippine Sea plate and Izu microplate, central Japan, inferred from GPS and leveling data

摘要

We clarify the contemporary deformation observed by GPS and leveling for the greater Tokyo (Kanto, eastern Tokai, and the Izu islands) region, where the Izu-Ogasawara (Bonin) arc is subducting and colliding with the central part of the Japan arc. From these data, we develop a kinematic model of fault sources with variable components of seismic and aseismic slip. Under the assumption that the contemporary deformation during 1995–2000 is representative of the long-term interseismic strain field, the geodetic data are inverted to estimate the rotation poles of the crustal blocks, the degree of elastic strain accumulation on faults, and the volumetric inflation sources beneath volcanoes. The present crustal movements are explained by four crustal blocks: the Izu microplate, the central Japan block, the Pacific plate, and the Philippine Sea plate. Along the Suruga and Sagami margins of the Philippine Sea plate lie strongly coupled faults, which include sites of the 1854 M = 8.4 Tokai, 1923 M = 7.9 Kanto, and 1703 M ～ 8.2 Genroku Kanto earthquakes. In contrast, the Philippine Sea plate boundary immediately north of the Izu peninsula, site of M ≤ 7.5 collisional earthquakes, is only weakly coupled. The boundary between the Izu microplate and the Philippine Sea plate experiences left-lateral motion with the rate of ～30 mm/yr. Most of this boundary is locked and thus has a large potential of future earthquakes. The Izu microplate is found to rotate rapidly clockwise at 10°/Myr, with a rotation pole relative to the central Japan block located just north of its northern boundary. The pole and angular velocity of the Izu microplate depends, however, on the assumed location of the boundary. With the exception of the site of the 1938 M ≥ 7 earthquake swarm, which we infer has the current potential to produce a M ～ 8.1 earthquake, the Pacific megathrust has a very low seismic potential, although the geodetic data have little resolving power for much of the Pacific plate interface east of Tokyo.