Shear partitioning near the central Japan triple junction: the 1923 great Kanto earthquake revisited-II

SUMMARY We propose the existence of a major right-lateral transform fault which we call the Boso transform fault. It is related to the Sagami trough, a portion of the Philippine Sea plate boundary south of the Kanto area (central Japan). This Boso transform fault is the result of shear partitioning due to oblique subduction and has delimited a Boso sliver for 2 Myr. The rate of motion is estimated at 16 mm yr-' and the total offset at 30 km. The fault cuts through the Miura and Boso peninsulas onland, where it has a multiple surface expression roughly along the limit of a steeply dipping Miocene ophiolitic body. These subaerial faults have been identified as active, and their cumulated rate of slip across the Miura peninsula can be estimated to be greater than 12 mm yr-', in reasonable agreement with the above estimate. We propose that the slip on the Boso transform fault was responsible for two large (M = 7.0 and 7.5) aftershocks which occurred on the second day after the 1923 great Kanto earthquake. This explains the unusual duration of the aftershock sequence, and the large magnitudes of some of the aftershocks.