抄録
In 1955 Ultra Sensitive seismometers were temporarily set up at the observatory at Mt. Tsukuba by one of the present writers. The results of this experiment led to the conclusion that the equation, logN(M)=a-0.8M, holds for shocks of M from less than zero up to about 5. The present writers made temporary observations of near-by microearthquakes at Matsushiro with Ultra Sensitive seismometers in order to see whether or not this is the case in this region.
The writers found that the above mentioned equation holds good for shocks occurring in the vicinity of Matsushiro also. Microearthquakes of M≈0 are occurring about 20 times every day, and minor shocks of M, 4 or 5, are occurring 1 or 2 times a year in this region. The geographical distribution of foci of these microearthquakes agrees with that of minor shocks. Their foci are seated in the shallow part of the crust.
The E of the smallest shock recorded during the term of this experiment is about 109 ergs.
