Abstract
The Sagami Bay region is situated in the eastern part of the South Fossa Magna on the Pacific coast of central Japan, where the north-south running Izu-Bonin arc on the eastern margin of the Philippine Sea (PHS) plate joins with the Honshu arc. In this region, the non-volcanic Izu outer arc (IOA) lithosphere is being subducted beneath the Kanto district north-northwestward, and the volcanic Izu inner arc (IIA) on the west of IOA is colliding with Honshu to the north of the Izu Peninsula because of its buoyancy. I infer that the subduction entrance of the IOA lithosphere is at the southern margin of the Sagami Basin in Sagami Bay, and estimate the depth of the upper surface of the IOA lithosphere beneath the Ashigara Plain on the north of Sagami Bay at more than 10km. On the other hand, the surface collision boundary between the IIA lithosphere and the Honshu block is located about 50km north from the subduction entrance of the IOA lithosphere. Therefore, there should exist a north-south striking fracture within the PHS plate which separates the subducting IOA lithosphere from the IIA lithosphere at rest on the earth's surface.
