Seismometric Observations of Matsushiro Swarm Earthquakes

Abstract

This paper is the report of seismometric observations of Matsushiro swarm earthquakes made by Abuyama Seismological Observatory and Disaster Prevention Research Institute, independently or co-operatively. Continuous observation at Shinko has been carried out to investigate the time variation of the seismic activity since the joint observation of ultramicro-earthquakes at the end of 1965. As the seismicity became active, continuous observation was started at Sanada, too, on June, 1966, with the main purpose of investigating the "Ishimoto-Iida's relation" and the time variation of seismic activity, but it was abandoned on September, 1966 because of overlapping the station with that of the Earthquake Research Institute. For studying the attenuation of seismic wave in short hypocentral distance and formulating the equation for calculating the magnitude of an earthquake, the Matsushiro swarm earthquakes are a very good source observations. On the basis of this idea, we picked out four observation points on the array line from Matsushiro to Komoro, and installed completely equipped low magnification seismographs of displacement type. The observations were begun on June, 1966. Further, on November the same year, the array line turned about 90 degrees for the first line which was extended from Matsushiro to Toyoshina through Akashina. The observations are still being continued at present. As had been planned in the earthquake prediction program in Japan, mobile observations were carried out three times from June to November, 1966, for the purpose of investigating the field problems of this observation system and the seismic activity outside the so-called seismic region of Matsushiro swarm earthquakes. Although this is a simple method by which only the frequency distribution of S-P duration times were observed, notwithstanding its simplicity the mobile observation system for the purpose of surveying the seismic activity was proved useful in the region where the dense observation network had not then been set up. Though the idea that the earthquakes are almost always generated in the vicinity of the fault is exceedingly Americanized, we carried out the observation of ultramicro-earthquakes by the seismometer-array, with an expectation that some relation between the cracks around Mt. Minakami and ultramicro-earthquakes may exist. However this expectation has been proved completely to the contrary.