Seismicity at Northwestern Sagami Bay

摘要

It has been pointed that an M 7 class earth- quake might occur in the near future around northwestern Sagami Bay, southwest Kanto district, central Japan, and this hypothetical earthquake has been named the “Western Kanagawa Prefecture Earthquake (WKPE)”. In order to investigate the mechanism of WKPE, the Hot Springs Research Institute of Kanagawa Prefecture (HSRI) has carried out continuous monitoring of seismicity and crus- tal deformation since 1989, covering the anti- cipated rupture zone estimated from historical data. In this paper, the relationship between seismicity and geological structure in and around northwestern Sagami Bay is investi- gated in detail based on a hypocenter precisely determined by the HSRI during the period 1990-1998. Hypocenters of earthquakes are calculated from 16 stations for the present seismicity anal- ysis. Seismic stations are deployed 2-10km apart. The accuracy of the hypocenter deter- mination in this study area was within 2-3 km with application of the method of prediction analysis. The minimum magnitude of observ- able earthquakes was estimated to be M0 in the Hakone area and M l in northwestern Sagami Bay. Comparison between seismicity and geolog- ical structure can be summarized as follows : (1) The higher seismicity areas are located in the prefectural border between Kanagawa and Yamanashi, and in the Hakone area. This higher seismicity can be attributed to the uplift of the Tanzawa Mountains and to volcanic activity, respectively, in the two areas. (2) The cutoff depth of seismicity becomes deeper from the center toward the east