Eight pop-up-type ocean bottom seismometers (OBS's) were deployed in western Sagami Bay, central Japan, for about one month in order to investigate the seismicity in the northernmost part of the Philippine Sea plate, where seismic activity has been intense for the past decade. About 100 earthquakes were located in the western Sagami Bay region, clustering into mainly three groups. The most active group is in the area off the east coast of the central Izu Peninsula, where swarm activity has

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... red since late 1978. The epicentral distribution of events of this group is aligned WNW-ESE and the focal depth range is shallower than 15 km. A small cluster was located 5 km to the south of the main alignment, with focal depths shallower than 10 km located beneath the submarine volcano Nishi-Chigasaki-Kaikyu. The second group is located west of Izu-Oshima Island, where foreshocks of the 1978 Izu-Oshima-kinkai earthquake occurred. The epicenters in this group are aligned NW-SE, showing en echelon pattern with the first group. Their focal depths are determined to be shallower than 5 km. The third group is roughly located between the Izu-Oshima and Niijima Islands, but could not be located precisely because these events are out of the OBS array. Swarmlike occurrences were found for the second and third groups. Particularly in the second group, two burst-type activities were found in the observational period. The directions of the epicenter alignments of the first and the second group are in agreement with the maximum tectonic stress direction, crossing the fault traces of the major earthquakes occurring there. This seismicity pattern is interpreted as a result of the presence of a "weak strength zone" caused by magmatic intrusion along the maximum tectonic stress direction.