Seismicity at Northwestern Sagami Bay.

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  • 相模湾北西部およびその周辺地域の地震活動
  • サガミワン ホクセイブ オヨビ ソノ シュウヘン チイキ ノ ジシン カツドウ

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Abstract

It has been pointed that an M7 class earthquake 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 crustal deformation since 1989, covering the anticipated rupture zone estimated from historical data. In this paper, the relationship between seismicity and geological structure in and around northwestern Sagami Bay is investigated in detail based on a hypocenter precisely determined by the HSRI during the period 1990-1998.<br>Hypocenters of earthquakes are calculated from 16 stations for the present seismicity analysis. Seismic stations are deployed 2-10km apart. The accuracy of the hypocenter determination in this study area was within 2-3km with application of the method of prediction analysis. The minimum magnitude of observable earthquakes was estimated to be M0 in the Hakone area and M1 in northwestern Sagami Bay.<br>Comparison between seismicity and geological structure can be summarized as follows:<br>(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.<br>(2) The cutoff depth of seismicity becomes deeper from the center toward the eastern flank of Hakone volcano. The depth change of the seismogenic layer should be attributed to geothermal effects of the Hakone volcano.<br>(3) The lower seismicity areas are located in the area (Ashigara Mountains and Oiso Hills) where the Philippines Sea plate is colliding with central Honshu, and in Quaternary volcanic areas (Yugawara and Taga).<br>(4) Seismicity shallower than 10km has not been observed around active faults, such as Kannawa and Kozu-Matsuda faults, during observation periods.<br>(5) Two remarkable seismic events occurred in 1990 (M5.1) and 1994 (M4.8), the first earthquakes in 57 and 74 years, respectively. Some researchers had postulated a period of seismic quiescence in the area, but this seems unlikely.

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