ELSEVIER Physics of the Earth and Planetary Interiors 93 (1996) 21-36 PHYSICS OFTHE EARTH ANDPLANETARY INTERIORS Crustal structure related to the Philippine Sea plate subduction in the northeastern part of the Sagami Trough, Japan Azusa Nishizawa a, *, Toshihiko Kanazawa b, Takaya Iwasaki b, Hideki Shimamura c a Ocean Research Laboratory, Hydrographic Department, Maritime Safety Agency, 3-1, Tsukifi 5-chome, Chuo-ku, Tokyo 104, Japan b Earthquake Research Institute, The University of Tokyo, Tokyo 113, Japan c Laboratory for Ocean Bottom Seismology, Faculty of Science, Hokkaido University, Sapporo 060, Japan Received 7 July 1994;revision accepted 19 July 1995 Abstract 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 kin. 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-l), and the layer dips towards the northeast. This layer is also found as upper crust beneath the Izu-Bonin volcanic arc, to the south of Sagami Bay. This suggests that the island arc 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). I. Introduction Around the Japan arc, there are several trenches and troughs where plate subduction oc- curs (Fig. 1). Typical oceanic plates are subduct- ing beneath the Japan island arc at most of these subduction zones. An exception occurs at the Sagami Trough, which is situated at the northern end of the Philippine Sea (PHS) plate boundary. The PHS plate in this region is characterized not * Corresponding author. by normal oceanic but by island arc structure produced by the subduction of the Pacific plate along the Izu-Bonin Trench. The PHS plate is, as a whole, moving northwestward (310 ° + 5°) with a velocity of 3-5 cm year-1 (Seno, 1977). However, the manner of the plate subduction is not simple: the forearc structure of the PHS plate is subduct- ing at Sagami Bay, the Izu Peninsula on the PHS plate is colliding with the Japan arc, and the oceanic structure of the PHS plate is descending beneath the island arc plate along the Nankai Trough. Sagami Bay is located at the northernmost part of the Sagami Trough. Bottom topography in 0031-9201/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0031-9201(95)03086-7