1990 年 43 巻 1 号 p. 27-42
Seismic quiescence which has recently appeared in the west coast region of Suruga Bay is investigated. The quiescence is clearly recognized for earthquakes with M≥2.5 since the beginning of 1988. Southern boundary of the quiescence region corresponds to the Irozaki-Shizuoka tectonic zone and its extension to the north-west. However, the whole focal region of the expected Tokai earthquake and its surrounding areas show quiescence since the middle of 1988, when earthquakes with M≥3 are considered. The seismic quiescence seems to have started first in the northern part and extended to the south. The feature that the Irozaki-Shizuoka tectonic zone forms boundary of the quiescence region is very meaningful when it is combined with the fact that a gap exists in the hypocentral depth of earthquakes at the tectonic zone. It suggests that the Philippine Sea slab is split at the Irozaki-Shizuoka tectonic zone and the stress states in the two segments of both sides are more or less independent. It is noted that seismic activity in the regions where the quiescence appeared had been high in the long period of recent several decades.