In 1971, for about a period of ten days, OBS observations were carried out in Sagami Bay, south of Tokyo, where the 1923 Kanto Earthquake (M=7.9) occurred, and where seismic activity has been very low since 1926. Although the OBS on the Sagami Trough detected one near earthquake of magnitude 0.1, it is concluded that the shallow seismic activity in Sagami Bay around the terminus of the Sagami Trough was very weak in late 1971. It is concluded that the 1923 earthquake fault posulated in Sagami Bay is still quiet at present in spite of the recent unusual crustal deformation in the adjacent region, the Boso Peninsula.
Use of an OBS along with several land-based stations expanded the detection capability to the area south of the Boso Peninsula. The hypocenter distribution revealed a linear alignment extending to the southeast from the tip of the Boso Peninsula at a mean depth of about 60km. The vertical distribution of hypocenters along the linear alignment resemble the bathymetric feature of the sea floor. There was a depth discontinuity of hypocenters at the foot of the inner wall of the Sagami Trough at 34.8°N.