1978年函館群発地震初期の地震観測

抄録

An earthquake swarm began south off Hakodate city on October 23, 1978. At the beginning, a seismological survey was temporarily set up in order to investigate a microearthquake activity in the neighbourhood of Minami-Kayabe which is about 30km northeast of the source area of the swarm.
Seismicity around there were monitored by a seismological network of tripartite composed of a routine station ESH (Esan) of the Hokkaido University and two temporary unattended stations in which the portable long-period cassette tape recorders were equipped. These temporary observation systems were the OBS (Ocean bottom seismographs) which were modified for the land use.
It is not a common experience that a temporary network caught the detail of the initiation of a swarm.
In the first stage, the source area of the swarm is concentrated in a region of about 4×8km² of area with a north-south trend. Several weeks later another concentrated area is added at several kilometers southeast of the former region. Therefore, we can refer to the former in early about 40 days at the beginning stage of the whole activity off Hakodate which has been active for more than a year.
Item of the present paper is concentrated to the result of our seismological observation at the early stage of a relatively short period from October 23 through the end of November 1978.
In the first stage of about 40 days, the activity can be further divided into three periods. In the first period, which is five days since the appearance, very small shocks gradually increased in number.
When the number of earthquakes increased abruptly the second period began. It is midday of October 27, 1978. This was the most active period in the first 40 days.
The third period began on October 29, when the activity began decreasing.
The m-values of Ishimoto-Iida's equation were, m=2.8 for the first period, m=1.6 for the smaller events and m=2.0 for the larger events in the second period, respectively, by the monitored records of ESH. The value for the third period could not been obtained easily for the difficulty in applying the Ishimoto-Iida's relationship.
For the large earthquakes (250μ kines-2500μ kines in the maximum velocity amplitude), the m-value was obtained by the records at the nearest station OBS-1 for the period from Oct. 25 through Nov. 8, 1978. Its value was m=2.8.
No earthquake with magnitude more than 3.5 occurred during the first 40 days though the total number of the recorded earthquakes is about 3700 or more.