On August 3, 1965, seismographs at the Matsuchiro Seismological Observatory recorded several earthquakes whose S-P times are shorter than 1 second. The number of earthquakes having approximately the same S-P times has increased day after day, and more than 6,000 felt and unfelt events a day were observed at the observatory at the most active period of the earthquake swarm. The number of felt and unfelt earthquakes amounted to 690,000 up to the present.
In the earlier stage of the swarm, the events occurred in a very narrow and nearly circular area whose center is located at Mt. Minakami. After September 1966, the earthquake area expanded toward both the northeast and southwest directions.