抄録
Since August 1971, a continuous observation of crustal movement based on the short base-line length in the vault has been conducted at the Erimo Geophysical Observatory, Hokkaido University (this observation will be called COCM hereafter). The purpose of the observation is to obtain such a continuous and precise crustal movement near the observatory as we cannot observe by geodetic surveys. The COCM has been adopted as one of the most important subjects to be studied in the earthquake prediction research, because this observation could probably reveal premonitory phenomena of earthquakes [1]. In fact, the COCM is excellent as a practical method possible to watch continuous crustal movement. However, the COCM has still problems not only whether the results based on the COCM always agree with those to be obtained by the geodetic surveys or not, but also whether the data by the COCM are contaminated with any other disturbances due to the known or unknown factors or not. For the problems, the geodetic surveys around the observatory should be also carried out along with the COCM so as to observe the same quantities of crustal movement, and so as to reveal factors which would affect the data as noise. After careful comparison of the data obtained by both methods was made, discussion of the premonitory or abnormal crustal movement should be given. Under these conditions, the COCM had started at the Erimo Geophysical Observatory in August 1971, where the vault with a total length of about 160 m was constructed for three sets of the quartz-tube extensometer with a length of 30 m and two sets of the water-tube tiltmeter with a length of 30 m. At the time, the occurrence of an earthquake with a magnitude of about 8 had been forecasted off the Nemuro Peninsula in the near future on the basis of the seismicity gap conception by Utsu [2]. On June 17, 1973, the earthquake (Ms=7.7, NOAA) occurred just in the predicted area about 2 years after the beginning of the COCM at Emimo. Just after the event, there was the discussion about the possibility whether a large earthquake would occur again around there or not because the magnitude of the event was smaller than that estimated [3], [4], [5]. Until now, another earthquake in question has not occurred in the area. The 1973 earthquake was the largest one at Erimo during the observation period from 1971 to 1981. According to the data observed by the extensometers during this period an abnormal variation of Crustal strain was recognized only before and after this event. The summary of this phenomenon will be presented.
