1977 年 30 巻 1 号 p. 73-90
An earthquake swarm started its activity in the northern somma of the Aso caldera on January 20, 1975. In the course of the earthquake swarm activity, on January 23, an earthquake of magnitude 6.0 representing the largest one among the whole earthquake swarm took place. After this earthquake, the activity of the earthquake swarm declined gradually until the end of January. By using available data obtained from all the seismological stations in the Kyushu district, 120 foci of these earthquake swarm were determined with fairly a high accuracy.
Analysing the time variation of the frequency of earthquakes, the migration of their foci, the variation of b-values, and so on, it was found that these features seen in the earthquake swarm were having quite a similar character in the fracturing of the stressed rocks.
1) The b-values showed fairly a large differrence in the two periods, i. e. before and after the largest earthquake. Before the earthquake, b=0.73 while after the earthquake b=1.01.
2) The series of the earthquake swarm activity can be divided into 6 stages, from the consideration of the migration of the earthquake foci or their characters. The temporal change in the spatial distribution of these foci was noteworthy. In the early stage of the activity a void space (seismicity gap) was observed in the central part of the area in which many foci were distributed. In the next stage, small earthquakes began to take place inside of the above mentioned void space, then at last the largest earthquake occurred at the edge of the void space.
3) A volume of the void space may be taken to be 1.8×1017cm3 from the locations of foci during the stage II and the ultimate stress energy that can be stored up in the void space until a break-down takes place can be estimated to be 5.65×1020 erg. According to the relation between the energy of an earthquake and its magnitude i. e. log E=11.8+1.5M, the magnitude of the largest possible earthquake M=5.97 is obtained. This value agrees with the magnitude which has been determined from the maximum amplitude on the seismograph of the above mentioned largest earthquake (M=6.0) among the whole swarm.
4) Considering the migration of the earthquake foci, it may be concluded that this earthquake swarm activity have not been related to a volcanic activity of the Volcano Aso.