Crustal structure related to the Philippine Sea plate subduction in the northeastern part of the Sagami Trough, Japan

A seismic refraction experiment was conducted with airguns and 17 pop-up type ocean bottom seismographs to delineate crustal structure related to the Philippine Sea plate subduction beneath Sagami Bay, at the northeastern end of the Sagami Trough, Japan. The P-wavespeed structure beneath Sagami Bay shows large variation in the range less than 10 km. As a whole, however, the structural model of the bay consists of four layers. The upper two layers are sediments, which are thick around the trough axis. The P wavespeed of the top of the fourth layer is consistent among profiles (5.6-6.1 km s(-1)), and the layer dips towards the northeast. This layer is also found as upper crust beneath the Izu-Bonin volcanic are, to the south of Sagami Bay. This suggests that the island are structure on the Philippine Sea plate subducts beneath the landward plate at the bay. The most significant lateral discontinuity in P-wavespeed structure across the trough axis is detected at the Sagami Tectonic Line. Faults along this structure are thought to be the locus of the 1923 Kanto Earthquake (M = 7.9).