The Study of Eruptions and Earthquakes originating from Volcanoes : Part III. Relation Between Depth of Volcanic Earthquakes and Subsequent Volcanic Phenomena

Abstract

In order to determine the positions of earthquakes originating from volcanoes, we have conducted investigations by means of various seismographs at Asama for the last 25 years. As a result, it was made clear that earthquakes observed at the volcano during the period consist mainly of the B-type earthquakes, of which the epicentres concentrate near the summit crater. The depth of these earthquakes was studied on the basis of the arrival time of the initial motion, the distribution of the maximum amplitude of the respective earthquakes, and sometimes the duration of preliminary tremor. Needless to say seismographs or transducers were set near the crater, on the mountain-side, and at the foot of the volcano so as to cover the whole epicentral area. An effective way to determine the depth of extremely shallow earthquakes such as the B-type ones is to compare the maximum amplitude of the respective earthquakes at various epicentral distances. On the other hand, a remakable feature of Volcano Asama is that earthquakes deeper than 1km in hypocentre rarely took place during the period, notwithstanding the fact that the B-type earthquakes were markedly predominant throughout the active and calm states of the volcano.