A CHARACTERISTIC OCCURRENCE OF EARTHQUAKE SWAMS AT THE ASO CALDERA-RIM

Abstract

Since the big explosion of October, 1965, at the Volcano Aso, the red-hot bottom in the 1st crater has been observed almost continuously during these several years. At the end of July, 1971, a small pit of 10 m in diameter opened at the bottom of crater and volcanic ash has been ejected intermittently from October, 1971, until now. Under such volcanic conditions, four groups of small earthquakes occurred at the western area of the Aso Caldera during the period from August, 1970, to .January, 1972. The results obtained are as follows: (1) The life of each swarm is shorter than about 40 hours and the foci are restricted to a small region. (2) The epicentral regions of the four swarms lie on a line along the northwestern rim of the caldera and the seismic region appears to be migrated successively from the south to the north. (3) The focal depths are 4-16 km under the Tateno Valley, only a collasped part of the caldera, and 4-8 km under the caldera-rim or the outer side of caldera. (4) The spa._es occupied by the swarms occurred under the caldera-rim or the collapsed part, spread vertically while those under the outer side of caldera spread horizontally. (5) The released energy of the swarm under the Tateno Valley amounted to 10^17 ergs, is smaller than the energies of the others, amounted to 10^18- 19 ergs. Therefore, it seems to reflect the tectonic state under the collapsed part of the caldera.