The aftershocks of the Hidaka Mountains Earthquake of Jan. 21, 1970 were observed at three temporary and one routine stations of Hokkaido University during the period from Jan. 22 to Mar. 25.
At the temporary stations, the direct analogue recording (DAR) systems were employed. This system can observe for sixteen days by a tape of 740m in a 7 inch reel without any skilled operator.
The aftershock activity was considered from the spacial S-P time distributions with increase in time, and apparent velocity as derived from the Omori's coefficient in the area concerned.
There are two peaks in the S-P time distribution at each station. At Taiki station (TAY), aftershocks of which S-P times are longer than the minimum of the S-P time distribution, show more complicated seismograms compared with ones of which S-P times are shorter than it. At Kamikineusu routine station (KMU), S-P time distributions versus time from the mainshock show uniform decrease and movement of active region from shallower part to deeper part of the aftershock region.
Aftershock region was determined about 20km shallower than that decided by J. M. A.. High apparent velocity appeared to be 20km to 30km deep which obtained from four S-P times from hypocenter to epicenter. This indicates the existence of such a body with higher density as the Ivrea Body of which existence has been estimated under the south western margin of the Alps. The difference between locations of the aftershock regions determined by the temporary and J. M. A. observations might be caused by this singular structure.
Some geological characteristics seem to coincide with the mechanism of mainshock and the crustal structure.
The fault caused by mainshock was infered to be reverse fault type from the shape of aftershock region and the polarities P wave initial motions of J. M. A. data.