On October 31, 1976 an earthquake of magnitude 5.0 occurred under the west slope of the Hidaka range, southern Hokkaido.
It is the first remarkable event since a seismic observation through a telemetering system at the Research Center for Earthquake Prediction (RCEP), Hokkaido University started in July, 1976. Using data obtained from three stations of RCEP near the epicentral region, a seismic investigation was made on the above event.
This earthquake occurred at a depth of about 55km in the uppermost mantle, however, its aftershock activity is very similar to that of earthquake in the crust elsewhere.
Background seismicity around the focal region of this shock was estimated as 0.04 shocks (M≥0.5) per day.
Frequency of aftershocks per day has decreased to this level about 140 days after the main shock. Aftershocks are confined in a region of about 6×6×4km³ including the main shock. No distinct seismic activity change is found prior to the main shock.
Focal mechanisms of the main shock and of the largest aftershock are of normal fault type. Maximum tension axes lie in nearly horizontal plane and their directions are perpendicular to the strike of the Hidaka range. A large earthquake (M=6.7) which occurred under the east slope of the Hidaka range on January 21, 1970, showed focal mechanism of reverse fault type. But all of B-axes of above three events are parallel to the strike of the Hidaka range.
These imply some complexity of focal process beneath both sides of the Hidaka range.