火山体内部の貯溜水における『高周波水中振動』と火山活動との関係について

Abstract

A borehole with the depth of 405m was drilled in the lava dome, which is 2.8km northwestof the active crater of Minami-dake, Sakurajima Volcano. A hydrophone was installed inthe groundwater at the bottom of the borehole to observe acoustic waves associated with thevolcanic activity.The acoustic waves observed by the hydrophone are classified into two types by thewaveforms. One is a low-frequency acoustic wave of which dominant frequency is 8.8Hz.The acoustic waves of this type are inferred to be caused by seismic waves. The other isa high-frequency acoustic wave (HAW) of which dominant frequencies are both 25Hz and8.8Hz. No significant seismic events are observed at the time of the occurrence of HAW.Examining the relationship among the numbers of occurrence of HAW, seismic activityand the concentration of hydrogen gas monitored in Mochiki borehole-C which is 4.0kmsouthwest of the active crater, the following results were obtained.1) The increases in numbers of occurrence of HAW are observed after the significantincreases in the concentration of hydrogen gas.2) The increases in numbers of occurrence of HAW are observed at the almost same timewhen A-type earthquakes occurred beneath the active crater.3) The change of numbers of the occurrence of the HAW is similar to that of B-typeearthquakes. The increases in numbers of HAW are observed before B-type earthquakeswarms and eruptions occur.