1999 Fiscal Year Final Research Report Summary
Investigations of volcanic structure and intruded magma by means of the geoelectrical method - Usu, Miyake-jima and Hokkaidou Komagatake -
| Project/Area Number |
09440155
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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 |
固体地球物理学
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| Research Institution | HOKKAIDO UNIVERSITY |
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Principal Investigator |
NISHIDA Yasunori Grad. School of Sci., Hokkaido Univ., Pro., 大学院・理学研究科, 教授 (60000874)
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| Co-Investigator(Kenkyū-buntansha) |
KAGIYAMA Tsuneomi Earthq. Res. Inst., Univ. Tokyo, Asso. Pro., 地震研究所, 助教授 (50126025)
TANAKA Yoshikazu Grad School of Sci., Kyoto Univ., Asso. Pro., 理学研究科, 教授 (00025420)
SASAI Yoichi Earthq. Res. Inst., Univ. Tokyo, Asso. Pro., 地震研究所, 助教授 (20012924)
OGAWA Yasuo Geol. Surv. Jpn., Geophysisist., 地震調査所, 主任研究官 (10334525)
HASHIMOTO Takeshi Grad School of Sci., Kyoto Univ., Inst., 理学研究科, 助手 (70283588)
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| Project Period (FY) |
1997 – 1999
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| Keywords | Magma prospecting / Magnetotellurics / Resistivity structure / Self-potential / Hydrothermal circulation / Usu volcano / Miyake-jima volcano / Hokkaido-komagatake |
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| Research Abstract |
Present study has aimed to elucidate the volcano-hydrological environment including the hydrothermal circulation system and to detect the intruded magma by using the geoelectrical methods. For these purposes, we made magnetotelluric observations and self-potential measurements on Usu (dacite magmatism). Miyake-jima (basaltic magmatism) and Hokkaido Komagatake (andesitic magmatism) volcanoes.
1) The common feature of the resistivity structure is seen beneath the three volcanic edifices: The first layer (100m or so) is resistive (about 1000 ohm-m) due to the volcanic rocks and sediments. The second layer is composed of the conductive rocks (several to several tens of ohm-m), suggesting the existence of the ionized water. This layer covers the relatively resistive third layer (several hundreds of ohm-m). The third layer is assumed to be the pre-Tertiary rocks.
2) Positive self-potential anomalies are commonly observed on summit craters (Usu, 400 mV; Miyake-jima, 800 mV; Hokkaido Komagatake, 600 mV). These anomalies can originate from the upwelling of hot groundwater warmed by the heat supplied from deep-seated magma. These findings of hydrothermal circulation are important to consider the magma cooling process.
3) Magnetotelluric soundings on Usu revealed the relatively highly resistive body (400 ohm-m or so) beneath the summit crater. Considering the relation between the resistivity of dacite rock and temperature, and the fumarolic temperature of Usu (500〜600℃, this body is identified as the intruded magma at the time of the 1977-1978 eruptions.
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