On the Investigation of Explosive Activities of Sakura-zima and Various Earthquakes originating from the Volcano

Abstract

Volcano Sakura-zima resumed explosive eruption in its summit crater after a long repose on Oct. 13. 1955, and is still active up to the present (Oct. 1957). In order to throw a light on the relation between explosive eruptions and volcanic earthquakes, we applied to this volcano the same method of seismometrical observation as being carried out in Volcano Asama. As the result, we observed four types of volcanic earthquakes originating from Sakura-zima: Namely, 1) volcanic earthquakes of the A type, 2) volcanic earthquakes of the B type, 3) earthquakes accompanied by explosive eruptions and 4) volcanic pulsation of continuous train (or harmonic tremor). On the basis of the seismometrical observation at four or six stations, we determined localities of these earthquakes of the four types. As the result, it was made clear that the epicentral area of the B type earthquakes, the explosion earthquakes and the volcanic pulsation agree with the locality of the active crater on the summit, and the A type earth quakes are originated from deeper part of the volcano. From the illustration of seismic activity and explosive eruption in Fig. 5, it seems that frequency of the B type earthquakes harmonizes with the eruptive activity. After precise examination of the development of the mentioned phenomena, we find that marked swarms of the B type earthquakes preceed in many cases to respective swarms of explosive eruption by ten or fifteen days. However, the problem is not always simple. It is neccessary to continue the observation not only in active state of the volcano but also in its calm state.