Change in the Thermal State in a Volcanic Geothermal Reservoir beneath an Active Fumarolic Field after the 1995 Phreatic Eruption of Kuju Volcano, Japan

Abstract

Kuju volcano,which is a typical island arc andesitic volcano,is situated in central Kyushu,Japan. It has an active fumarolic field in the central part of the volcano and a two-phase volcanic geothermal reservoir beneath the fumarolic field is proposed based on numerical modeling. A phreatic eruption occurred near the fumarolic field in October 1995. Aftre that repeat geophysical measurements were conducted in and around the fumarolic field and the new craters. As a result, quick temperature decrease of the volcanic geothermal reservoir was deduced from repeat thermal and geomagnetic measurements. Repeat gravity measurements showed quick decrease around the new craters after the phreatic eruption and then gravity recovered gradually. Such temperature and gravity changes show recharge of a large amount of cold meteoric water to the volcanic geothermal reservoir. The meteoric water recharge was induced by the sudden decrease of pressure in the volcanic geothermal reservoir accompanyied by the phreatic eruption. Numerical modeling of the thermal process in the volcanic geothermal reservoir after the phreatic eruption simulates the observed cooling of the geothermal reservoir very well. Such a process is very similar to the production of fluid from the geothermal reservoir without reinjection.