STUDY OF ERUPTIONS AND EARTHQUAKES ORIGINATING FROM VOLCANOES (PART 3 OF 3)

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Abstract
Investigations using various seismographs to determine positions of volcanogenic earthquakes have been underway at Asama for 25 years. The types of earthquake observed here through this period are mainly very shallow (B-type). Measurements taken with instruments spotted over the mountain made it possible to locate the epicenters with accuracy. Very few were deeper than 1 km. While the B-type earthquake prevailed throughout both quiescent and pre-active stages, the concurrent deeper A-type quakes showed a sharp decrease in the month just prior to the October 3 explosive eruption, while the tremors of shallow origin increased in intensity and frequency. After the December series of the 1958 eruptive period, earthquakes were of much deeper origin prior to and between the major explosions. Comparison is drawn to the 1943-1945 activity of Usu, where A-type earthquakes predominated and B-type earthquakes appeared only just prior to the eruptions. The writers suggest that depth of hypocenter as well as frequency of such earthquakes can be used to make eruption predictions. — A. Eustus