ALBERT

Description: A seismicity observation with 10 pop-up-type ocean bottom seismometers were conducted along the Suruga trough, south off central Japan, for about one month in 1984. This observation detected more than 300 earthquakes, among which about 60 earthquakes were located around the Suruga trough. In this observational period, seismically the most active area is the middle part of the Suruga trough. Applying station corrections, we precisely located the earthquakes around the Suruga trough. The hypocenters show a thin seismic zone dipping westward from the trough axis. The focal depths at the trough axis, 0-10 km, are shallower by 5 to 10 km than the principal focal depths of hypocenters obtained by the land-based seismic network. The hypocentral distribution by the present survey is interpreted as the seismic activity in the uppermost part of the Philippine Sea plate subducting from the Suruga trough axis. In contrast to the high seismicity at the middle part of the trough, only a small number of earthquakes were located in the northern and southern parts. The microearthquakes along the Suruga trough observed in the present survey tended to occur simultaneously in the segment over about 50 km length, synchronizing with the activation of the small swarm around the northeastern edge of the Nankai trough. © 1988, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: 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 occurred 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. © 1989, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.