Abstract
The Philippine Sea plate is subducting beneath the NE Japan plate in the Sagami trough and the Nankai trough. The Izu peninsula, which has a thick crust, is located between the both troughs. the plate boundary of the Philippine Sea plate and the NE Japan plate is not distinct be cause of the existence of the Izu peninsula. That is why there are several hypotheses about the location of plate boundary between the Philippine Sea plate and the NE Japan plate.
Hydrographic Department, Maritime Safety Agency carried out multi-channel seismic reflection survey of the Sagami Bay. Several facts were revealed by the survey.
1) The extension of northern slope of Izu-Oshima island subducting beneath the Manazuru knoll which is located on the slope of the Izu peninsula.
2) The erosion is dominant in the axis of the Sagami trough.
3) A reverse fault is recognized on the shelf slope foot of the Izu peninsula to the east of Hatsushima island.
The reverse fault on the south of Manazuru knoll suggests that the bottom of the Sagami trough may be subducting beneath the Izu peninsula and that the plate boundary between the Philippine Sea plate and the NE Japan plate is located on the east of the Izu peninsula.
