On January 13, 1979, a shallow earthquake of magnitude 4.2 after the Research Center for Earthquake Prediction of Hokkaido University (hereafter called RCEP) occurred off Mukawa, southern Hokkaido. Some distinguished foreshocks and aftershocks with relation to this event were observed.
It is a remarkable event because in this region such shallow events have seldom occurred by this time.
After two days of the main shock a seismological observation had set up at Mukawa by means of a cassette tape recorder with a tape speed of 0.10mm/sec. From the view point of disasters, seismic intensity of the main shock was also investigated in the whole of Mukawa town.
From the above detailed seismological survey data, several important facts were found as follows.
1. Some stages of seismic activities were suggested. In the first stage, four small events took place in succession in the furthest places off Mukawa. This activity was completed in a relatively short span of time of half an hour. After a following aseismic period of seven hours, the dramatic main stage appeared near the southeast rim of the source region of the impending main event. At this second stage, two extremely large main event occurred successively in mere two minutes. After that, such a phenomenon of occurrence of earthquakes in a limited region and in a short time interval was sometimes recognized in a vaguely sense.
2. Focal mechanism solution of the main event suggested that type of fault movement was a left-lateral strike slip pattern as well as other shallow events occurred previously in Hidaka district. The fact was attributed to the field of east west trending compressional shearing stress in the whole of Hidaka district. It may be said in this connection that a fault plane with dip direction of 311.4° measured counter-clockwise from the north is in accord with a general direction of source area implied by epicentral distribution of a series of small events with relation to this main shock.
It is furthermore consistent with an edge of “old downed delta” [MOGI (1964)].
3. Based on the results of error analysis in hypocentral parameters with the prediction analysis after WOLBERG (1967), the standard errors of hypocentral coordinates due to random errors in the data and model parameters of velosity structure may be several kilometers at most.
4. Micro-seismic zoning map of earthquake intensity for the main shock suggested that local difference of seismic intensity depends upon variety of geological structures in places.
5. A T-phase, traveling through the ocearn with the velocity of sound in sea water was found on seismograms of coastal station ESH, which is one of RCEP stations. The path of propagation from the epicenter to ESH is mostly oceanic.
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