Abstract
The regional seismicity before the Matsushiro Earthquake Swarm was studied for large, small and micro-earthquakes. Findings are as follows.
1) Past major earthquakes and swarms near Matsushiro occurred outside the swarm region, and the swarm region had been left unaffected for many years.
2) For a period of 8 years prior to the swarm, the region where the swarm initially started was comparatively quiet, and a higher activity was found in the surrounding region where the swarm was to extend in its later stages.
3) A small swarm activity o ccurred from April 1963 to June 1964 with several rises and falls. These earthquakes, however, did not occur in the initial swarm region. The Matsushiro swarm started after this activity had once diminished.
4) The “m” value was smaller (m=1.65) before the swarm and increased up to m=2.02 after the start of the swarm.
5) The regional seismicity before the swarm showed a pattern that is usually found for large earthquakes, i.e. the regional seismicity is abnormally low for a certain period of time before the occurrence of a large earthquake.
