抄録
Premonitory change in the ratio of compressional and shear wave velocities was observed for the shallow earthquake of magnitude 6.2 which occurred in the southeastern part of Akita Prefecture in northeastern Japan on October 16, 1970. The time duration of the anomalous velocity preceding this earthquake was about two years, the value being in good accordance with the period expected from the relation between earthquake magnitude and precursor time interval based on the dilatancy model. The size of the anomalous region, in this case, was estimated to be about two times as large as the aftershock area.
The temporal variations of the b value in the earthquake frequency-magnitude relation and the seismic activity near the focal region were also investigated. A positive correlation was obtained between the b value and the velocity ratio of compressional and shear waves. A remarkable foreshock activity was observed just after the recovery of seismic wave velocity to the normal value.
