4. The Mechanism of the Explosion Earthquakes(V. Features of the Activity of the 1970-71 Eruption of Akita-Komagatake)

Abstract

Akita-Komagatake Volcano manifested a large number of explosions during the period September 1970 to January 1971, and about 32, 000 explosion earthquakes were recorded by the seismographs installed temporarily at 3 points at the foot of the volcano. In this paper, the mechanisms of the volcanic explosions are investigated by the use of the initial motions of the explosion earthquakes. Though the initial motions of almost all explosion earthquakes were compressional, about 0.6% of them were dilational. The distribution of push and pull of the initial motions at 3 observation points are classified into 6 types. On the basis of these 6 types, it is suggested that the mechanisms of the explosion origins belong to the “asymmetrical push cone type.” The explosion origins are distributed at the depths of 0 to 300m or more below the crater. The origins of the explosions of which initial motions are all push at the 3 points are shallow, while those of all pull ones are deep. Deep focus explosions occurred frequently at the early stage of the activities. The process of a volcanic explosion is surmised as follows : If an explosion earthquake occurs at a depth, a shock starts from there. The velocity of this shock is calculated to be equal to that of the sound wave in the liquid. This shock attacks the material which fill the upper part of the vent, and then a destructive outburst occurs at the crater.