Abstract
Many swarm earthquakes started near Matsushiro from August,1965, and their coda activity still continues as of May,1968. Fig.1 illustrates the daily frequency of perceptible earthquakes, which outlines the present swarm activity.
From examinations of the distribution of initial motions of these earthquakes, activities of magma pockets were inferred as follows. First, the Matsushiro magma pocket started its activity, and later in April,1966, the Wakaho magma pocket became active in addition to the former, and in May,1966, activity of the Oomine magma pocket perticipated in these. Those pockets are believed to have intruded nearly vertically with an incident angle of 5 degrees (see Fig.8).
At the peak of the swarm activity, Mt. Minakami rose or tilted to the north-west. This change was slightly restored later, which probably indicated the end of the main activity. The auther assumes that the deformation of Mt. Minakami is an indication of magma intrusion and that the present swarm is of volcanic nature, which was originated by heat energy from radioactive elements (Th, U, Ra etc. ).
