The northern boundary of the Philippine Sea plate is defined topographically by the Nankai, Suruga and Sagami Troughs and it is considered to form a subduction boundary along the troughs. Tectonic landforms in and around the Suruga and Sagami Troughs are described and their relations to the subduction are considered in this paper. There is a significant difference in geomorphology between the Suruga Trough and the Sagami Trough. The Suruga Trough is narrow and runs straight in the direction of N10°E, while the Sagami Trough is sinuous off the southern tip of the Boso Peninsula and has a broad basin in Sagami Bay.
The tectonic landforms around the Suruga Trough indicate that a subduction is going on from the southeast with a rather steep inclination. Riedel shear faults running in the direction of N40°W occur along the southeastern part of the Sagami Trough. A steep fault scarp with the direction of N70°E develops off the southern tip of the Boso Peninsula and is considered to originate from reverse faulting. These tectonic landforms indicate that the northern end of the outer belt of Izu-Ogasawara arc is moving northwestwards along the Sagami Trough. Regional difference in the nature of crustal deformations deduced from the tectonic landforms can be explained in terms of the relationship between the direction of plate motion and the change in strikes of the troughs.
The mode of crustal deformations of the“seaward”side of the troughs is distinctively different from that of the“landward”one. The seaward side of the troughs including the izu Peninsula bends and form a broad flexure before subducting beneath the troughs. The landward side of the troughs is characterized by severe active deformations such as faults and folds with short wavelengths.
