The mechanism of the occurrence of the Matsushiro earthquake swarm in central Japan and its relation to the 1964 Niigata earthquake

Abstract

The frequency curve of the Matsushiro (central Japan) earthquake swarm which began in August 1964 is analyzed from the point of view that earthquakes are fracturing events in the earth's crust. This analysis is based on fracture experiments in the laboratory and estimations of the temporal variation in stress in the focal region of the earthquake swarm. The change in the estimated stress is attributed to pore-water pressure in the earth's crust, which was caused by the upward intrusion of ground water from depth. Just after the 3rd peak of seismic activity, the estimated stress (the pore pressure) decreased markedly. This occurred simultaneously with the outflow of an enormous volume of ground water from a wide area around the focal region of the earthquake swarm. This result supports the water eruption model which was first proposed by Nakamura (1971). A hypothesis is proposed that the Matsushiro earthquake swarm was triggered by the 1964 Niigata earthquake of M = 7.5. This earthquake occurred in an adjacent region along the same active tectonic belt one year before the Matsushiro earthquake swarm. This is strongly supported by the space-time distribution of earthquakes which have occurred in the Niigata-Matsushiro region.