阿蘇外輪域に発生した群発地震活動

抄録

During the period from August of 1970 to January of 1972, four earthquake swarms occurred at the western part of the Aso Caldera. The 1st swarm occurred on 9 and 10 of August, 1970, outside of the southwestern caldera-rim, the 2nd swarm on 24 and 25 of June, 1971, at the Tateno Valley, the 3rd swarm on 5, 6 and 7 of July, 1971, at the region about 10 km northwards from the Tateno Valley and the 4th swarm on 12 and 13 of January, 1972, outside of the northwestern caldera-rim. Analyzing these four swarms, the results obtained are as follows : (1) Epicenters of these swarms appear to be migrated successively from the south to the north within a narrow straight zone along the western rim of the Aso Caldera. The foci of the swarms occurred under the outside of caldera distribute more or less horizontally, and on the other hand, the foci of the swarms occurred under the collapsed part of the caldera-rim, the Tateno Valley, distribute rather vertically. (2) The constituent materials within the spaces occupied by the foci are highly fractured in comparison with the normal one of the earth’s crust. Particularly the state under the Tateno Valley, a part of the Oita-Kumamoto tectonic zone which is running from the east to the west in the central Kyushu, seems to be more fractured than any other. (3) In most cases, the initial phases of earthquakes show the opposite sense at the seismic stations situated symmetrically with respect to the zone along the western calderarim. (4) A new tectonic zone, or a fracture zone, will be inferred from the distributions of foci and initial phases and the strained state within the spaces occupied by foci as above mentioned.