HR: 1340h
AN: T23B-0543    [Abstracts]
TI: Change in the Thermal Process in a Volcanic Geothermal Reservoir Beneath an Active Fumarolic Field After the 1995 Phreatic Eruption of Kuju volcano, Japan
AU: * Ehara, S
EM: ehara@mine.kyushu-u.ac.jp
AF: Kyushu University, 1-6-1, Hakozaki, Higashiku, Fukuoka, 812-8581 Japan
AU: Fujimitsu, Y
EM: fujimitsu@mine.kyushu-u.ac.jp
AF: Kyushu University, 1-6-1, Hakozaki, Higashiku, Fukuoka, 812-8581 Japan
AU: Nishijima, J
EM: nisijima@mine.kyushu-u.ac.jp
AF: Kyushu University, 1-6-1, Hakozaki, Higashiku, Fukuoka, 812-8581 Japan
AU: Fukuoka, K
EM: fukuoka@mine.kyushu-u.ac.jp
AF: Kyushu University, 1-6-1, Hakozaki, Higashiku, Fukuoka, 812-8581 Japan
AU: Ozawa, M
T23B-0543 AF: Kyushu University, 1-6-1, Hakozaki, Higashiku, Fukuoka, 812-8581 Japan
AB: Kuju volcano, which is composed of many lava domes, is a typical andesite island arc volcano. The main rock type is hornblend andesite. The volcanic activity started 0.15 Ma and the most recent big pyroclastic eruption occurred 0.05 Ma. Magmatic eruptions occurred at intervals from every 1000 to 2000 years in recent 15 kyrs. The most recent magmatic eruption is about 1700 years ago. Several phreatic eruptions occurred in historic times at intervals ranging from several tens to a hundred years. The fumarolic field in the central part of Kuju volcano is one of the most intense geothermal fields in Japan. The natural heat discharge rate was estimated at about 100 MW before the 1995 phreatic eruption and most of it is from steaming ground and fumaroles. Temperatures of fumaroles generally exceed 200 degrees C and the maximum observed temperature prior to the 1995 phreatic eruption was 508 degrees C. A two-phase volcanic geothermal reservoir beneath the fumarolic field is proposed based on numerical modeling (Ehara, 1992). Kuju volcano began to erupt on 11 October, 1995 from the new craters which are about 300 m south of the pre-existing fumarolic field (first eruption). The volume of ashes discharged by the eruption is about 20000 cubic meters. The subsequent eruption in mid-December produced about 5000 cubic meters ashes (second eruption). After these eruptions, a large amount of steam and heat started to be discharged from the new craters and the pre-existing fumarolic field. Such discharge is still continuing at present (August,2005). The eruption was considered to be a phreatic eruption, because there has been no magmatic activity at the surface. Several kinds of geophysical measurements such as thermal, gravimetric, geomagnetic, seismic, geodetic etc. have been conducted after the 1995 eruption. 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 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 accompanied 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 volcanic geothermal reservoir very well. Such a process is very similar to the production of geothermal fluids from the geothermal reservoir without reinjection.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Tectonophysics [T]
MN: Fall Meeting 2005