Some New Phenomena Observed in the 1970-1971 Eruption at Volcano Komagatake -With Special Reference to Its Explosion Mechanism-

Abstract

The eruptive activities of Komagatake, Akita Prefecture, which were recorded during the period from September 1970 to February 1971, are especially interesting on account of some unusual phenomena involved in lava flows and explosions.
The writer expressed the characteristics of the volcano's mechanism of explosion in terms of S/M, that is, the ratio of two phase amplitudes of sound and earthquake in a seismogram of explosion (Fig.6). Referring to the theoretical values of tide generating forces of Komagatake, the writer studied the nature of this activity and explained the depth of the source of explosion and its changes. The mechanism of the underground activities of Komagatake applies also to the Matsushiro earthquake swarm and activities of other volcanoes.
During the eruptive activities, the “smoke-ring” was observed for the first time in Japan. Fortunately, the writer was able to observe the whole process of its formation from the crater, by means of an 8 mm cinecamera and the tapecorder. Then he carried out spectrum analysis and examined the relationship between the fluctuations of explosive energy and the seismic waves, so as to elucidate the cause of this unique phenomenon which is seldom recorded in the history of volcanoes of the world.
At 14: 16 in Oct.24,1970, a pillar-like white smoke emerged out of the crater with a detonation (Fig.2-a-1). It grew with the top part gradually shaping into a horizontal ring as it became taller (Fig.2-a-2). After 12 seconds, at a height of 50meters, the ring went up alone leaving the white pillar of smoke below. The ring was about 7 meters in diameter. The air current was seen to move relatively upwards inside the ring and downwards outside it. The ring travelled up for about 20 minutes until it disappeared in the cloud (AC) about 5,000 meters high.
The detonating sound of the explosion recorded o n magnetic tape was analysed by band pass filters, the results being shown in Fig.3-a,3-b. There is little difference in strength between the detonation accompanied by “smoke-ring” and that without it. Marked difference, however, is seen in the type of spectrum of the detonation between that with “smoke-ring”(Fig.4, top) and those without it. (Fig.4, middle and bottom). Sound energy is more concentrated in the lower band of frequency in the former case than in the latter.