(7)The Matsushiro earthquake swarms

The Matsushiro earthquake swarms began on August 3, 1965. There were two periods of activity in 1966 that later subsided (Fig.6-40) and had almost completely stopped by the end of 1970. By 1970, a total of 62,821 felt earthquakes had occurred, and 711,341 overall. The largest of these earthquakes was M 5.4, but the total energy for these earthquake swarms was equivalent to a single M 6.4 earthquake. These earthquake swarms wounded 15 people, completely collapsed 10 houses, and partially collapsed many more houses. In most cases, the damage included tilted roofs, broken groundsills, and broken walls and tile. The earthquakes also caused unease among the local populace.

The earthquake swarms occurred during the following four periods (Fig.6-41).

First period: Up to February 1966, seismic activity occurred within a 5-km radius centering on Mt. Minakami.

Second period: Up to July 1966, the area of activity extended from northeast to southwest. This was the period of the most intense seismic activity and crustal deformation. Such surface phenomena as spring water and ground fissures occurred during this period.

Third period: Up to December 1966, as the area of activity grew larger, the activity near Mt. Minakami diminished. The spring water accompanying the expansion of the ground fissures caused landslides in the Makiuchi region which resulted in damage to houses (Fig.6-42).

Fourth period: Up to May 1967, activity shifted to the surrounding active area and expanded from northeast to southwest. Activity in the central part of the area diminished during this period. Activity quickly dissipated after June 1967.