Abstract
Characteristics of seismic activity in the western part of Kanagawa prefecture, central Japan, where a large earthquake is expected to occur in the near future, are investigated. There is a conspicuous seismic zone along the western border of Kanagawa prefecture in the direction connecting the Yamanashiken-tobu active region to Odawara city. We tentatively call the seismic zone Doshi-Odawara seismic zone in this paper. Temporal correlation is observed between seismic activity in the northern part and that in the southern part of the seismic zone. The Yamanashiken-tobu seismic region is divided into two parts at the northern extension of the zone; the western part is more active than the eastern part. “The Nishi-Sagamiwan Danretsu”, which was proposed by ISHIBASHI (1988a, b) to represent a split of the Philippine Sea plate, is parallel to the Doshi-Odawara seismic zone. However, the southern half of “the Nishi-Sagamiwan Danretsu”, from the headland of Manazuru to the Ito-oki swarm region, is not seismically active. The western boundary of the Doshi-Odawara seismic zone is sharply cut, and the plane which separates the seismic zone from western aseismic region seems almost vertical. It is proposed that the subducting Kanto slab moves left laterally relative to the colliding Izu block at the western boundary of the Doshi-Odawara seismic zone, and the southern extension of the block boundary is continued to the Tan-na fault partially through the Hakone volcanic area. Large activity of the Tan-na fault can be understood by considering that it represents partly the relative motion between the Kanto slab and the Izu block. A notable doughnut pattern is observed to the east of the Doshi-Odawara seismic zone. Within that gap the Oiso hills, the Ashigara plain and the Hadano basin are located. It is to be noted that the seismicity around Odawara city which corresponds to the southern end of the Doshi-Odawara seismic zone has remarkably decreased since the beginning of 1988.
