火山体の変形, 表面活動と火山性地震発生の関係(1)-微小地震の群発現象について-

Abstract

The swarms of volcanic micro-earthquakes called as B-type earthquakes have been themost typical forerunner for explosive eruptions at Sakurajima Volcano since 1955. Thephysical meanings of B-type earthquake swarm was examined in relation to ground deforma-tion and eruptive activity. B-type earthquakes were categorized into two types by theirpredominant frequency in this study. The one is named as a BL-type earthquake (lowfrequency:1-3 Hz) and originates at a part shallower than 3 km beneath the summit crater.The other is a BH-type earthquake (high frequency: 5-8 Hz) and the hypocenter ranges from2 to 3.5 km. The epicenters of both type earthquakes are located in the region of the summitwith an extent smaller than the size of the crater.BH swarms were observed mainly during inflation-stage with no significant changes oferuptive activity. On the contrary, BL swarms originates during the period of rapid deflationof the summit area with the successive emission of volcanic ash and gases, and they weresometimes accompanied with the intermittent ejection of volcanic bombs similar to Strom-bolian eruption and the emission of weak infrasonic waves.These results suggest that BH-type earthquakes may reflect the intrusion of magma inthe conduit, and that BL-type earthquakes are generated closely related with the degassingof magma extruded up to the crater bottom.