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Microseismicity in the sea and land area on the west coast of Sagami Bay, the Pacific coast of central Japan, which has been almost unknown so far, is investigated for the period from 1980 August to 1981 May based on the observation at Yugawara microearthquake observation station of IISEE. The area is noteworthy in that the west part of Sagami Bay may be playing some role in the Philippine Sea plate convergence and that the recent seismic activities along the Izu Peninsula appear to migrate toward this area. Yugawara station, which began working in June 1980, is situated nearly at the center of the study area and has the highest detection capability among all stations in and around the area. Through analysis of S-P times at Yugawara station for the net observation time of 181 days, it has been proved that rather high and stationary microearthquake activity exists in the area in question. 98 hypocenters among 165 earthquakes whose S-P times at Yugawara station are not longer than 3.0 sec during the 181 day period have been located by means of Yugawara and 13 additional stations belonging to 4 different institutes (IISEE, HSRI, ERI, and NRCDP). Their magnitudes, which have been determined from F-P times at Yugawara station by a newly derived formula, range from 0.3 to 3.1. The located earthquakes can be divided into two groups; shallow (2.6 N 6.5 km) earthquakes inside Hakone caldera, and other earthquakes. The former group shows a swarm tendency and seems somewhat active during the study period. The latter presents a north-south trending zonal distribution of hypocenters along the west coast of Sagami Bay, which appears to dip toward the west. Their focal depths range from several to nearly 20 km. The west side of Hakone caldera and the Tan'na fault has turned out to be almost aseismic during the study period. Two small-scale swarm activities, near Ito and off Manazuru, have been also located precisely.

Microseismicity in the Vicinity of Yugawara Station on the west Coast of Sagami Bay, Central Japan