北陸地方の地震活動の時間変化

Abstract

Hypocenter data obtained by the Hokuriku Observatory of Disaster Prevention Research Institute, Kyoto University, during the recent 17 years since June, 1976 have been extensively analyzed, especially from the viewpoint of long-term variation of seismic activity in the Hokuriku area. During the study period the 1984 Western Nagano Prefecture earthquake (M6.8) and its aftershocks were the most prominent activity in the present area, Hokuriku. The seismic activity in the study period has been nearly stationary except the above two periods. Temporal variation in the number of earthquakes shows a periodicity 5-6 years and its variational pattern is well correlated among the following three sub-areas: the Fukui earthquake fault area, the Yanagase fault area, and the Neodani fault area. On the otherhand, in the Hanaore fault and its vicinity, about 50 km west of the Yanagase fault area, the number of earthquakes shows a different pattern of temporal variation of earthquakes from the two sub-areas.As to the secular variation of seismicity in the present area, the number of earthquakes has been decreasing with time in the sub-areas including the Fukui earthquake fault and the Yanagase fault, almost with a constant rate, but in and around the Neodani fault, seismicity has been increasing, which became obvious in particular since 1984.