1998 Fiscal Year Final Research Report Summary
Study of Geothermal Structure and Processes of Hydrothermal Fluid Flow in the Central Kyushu, Japan
| Project/Area Number |
08454132
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Meteorology/Physical oceanography/Hydrology
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| Research Institution | KYOTO UNIVERSITY |
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Principal Investigator |
YUSA Yuki Kyoto University, Graduate School of SCIENCE, Professor, 理学研究科, 教授 (90025403)
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| Co-Investigator(Kenkyū-buntansha) |
TAKEMURA Keiji Kyoto University, Graduate School of SCIENCE, Associate Professor, 理学研究科, 助教授 (00201608)
TANAKA Yoshikazu Kyoto University, Graduate School of SCIENCE, Associate Professor, 理学研究科, 助教授 (00025420)
KITAOKA Koichi Kyoto University, Graduate School of SCIENCE, Associate Professor, 理学研究科, 助教授 (30093230)
OHSAWA Shinji Kyoto University, Graduate School of SCIENCE, Associate Professor, 理学研究科, 助教授 (30243009)
FUKUDA Yoichi Kyoto University, Graduate School of SCIENCE, Associate Professor, 理学研究科, 助教授 (30133854)
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| Project Period (FY) |
1996 – 1998
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| Keywords | Undergoing structure / Tectonics / Origin of materials in hydrothermal system / Circulation of geothermal fluid / Evolution of hydrothermal system / Volcanic / Geothermal gases / self-potential / Central Kyushu, Japan |
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| Research Abstract |
1. Development of the underground structure in the central Kyushu is as follows :
(1) Before 1.5 million years, the half-graben was formed along the Median Tectonic Line (MTL) by the North-South extension. (2) Between 1.5 and 0.7 million years, the basic pull-apart basin was formed by a right lateral slip of two fault lines. (3) Since then, the basin structure has changed to be the present state following the reduction of the active zone of MTL.
2. Volcanic gases in the central Kyushu and the Ogasawara-Iwojima contain less nitrogen than those in the North-East Japan and the southern Kyushu. This indicates that the nitrogen in magmatic gases is originated from the nitrogen compounds in pre-Tertiary basement rocks.
3. The age of the Beppu hydrothermal system is estimated to be about 50 thousand years based on geochemical and hydrological data. More than 90% of chlorine discharged during this period are brought from the deep.
4. Circulation and flow of fluids in the Kuju-Iwoyama Volcano are characterized as follows :
(1) The deep circulation of geothermal fluid is driven by magmatic heat, and the average residence times of meteoric waters are short for the vapor system (about 7 years) and long for the liquid system (over 100 years). (2) Amounts of chlorine and magmatic water discharged during calm period are 3.7-8.7 and 360-1400 ton/day respectively. (3) When the volcanic activity increases, the atmospheric air is entrained into the volcanic system through the subsurface formation.
5. Self-potential measurements were conducted in Mt. Garan (Beppu) and in the central cone of Mt. Aso, and anomalies of electric potential suggesting flow paths of geothermal fluids were detected.
6. A temperature decline in the Beppu hydrothermal system during the last hundred years has been suggested by the change in compositions of red sediments in a hot pond.
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