Heat and Mass Transfer Processes After 1995 Phreatic Eruption of Kuju Volcano, Central Kyusyu, Japan

Abstract

Kuju volcano in central Kyushu, Japan began to erupt on 11 October,1995. The eruption was phreatic in nature, as there was no magmatic activity at the surface even though the vesiculated glass shards were detected from the ash. The heat discharge rate from the fumarolic field of Kuju volcano was about 100MW before the eruption. The heat discharge rate from new craters became higher than 2000MW during about two months after the eruption and subsequently decreased. However, it remained as high as several hundred MW several years after the eruption. We conducted dense repeated thermal and gravity measurements around the new craters in the interval of one week to several months following the eruption. As a result, we detected various thermal and hydrologic changes associated with the new craters and the pre-existing fumaroles. We also clarified that the rate of the groundwater vaporization by the uprising hot magmatic gas is gradually becoming equal to the rate of the water recharge from the surrounding regions, on the basis of the water mass balance determined from the repeat gravity measurements and estimates of the discharged volcanic steam rate. The water mass balance shows that the volcanic activity is gradually approaching a new stable state. These results show that the joint measurement of repeat gravity and water mass discharge rate is a very useful technique to clarify subsurface hydrological processes beneath volcanoes. This method will be also very effective in monitoring the underground hydrological processes when we extract heat from active volcanoes in the future.