2001 Fiscal Year Final Research Report Summary
Elucidation of the Pressure Source Beneath the Active Crater of Aso Volcano
| Project/Area Number |
09304043
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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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 | University of Tokyo |
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Principal Investigator |
KAWAKATSU Hitoshi Earthquake Research Institute, University of Tokyo, Professor, 地震研究所, 教授 (60242153)
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| Co-Investigator(Kenkyū-buntansha) |
TSUTSUI Tomoki Akita University, Engineering, Assoc. Professor, 工学資源学部, 助教授 (70240819)
KANESHIMA Satoshi Tokyo Institute of Technology, Science, Assoc. Professor, 理工学研究科, 助教授 (80202018)
SUDO Yasuaki Kyoto University, Science, Assoc. Professor, 理学系研究科, 助教授 (40025466)
OHMINATO Takao Earthquake Research Institute, University of Tokyo, Research Associate, 地震研究所, 助手 (70322039)
HASHIMOTO Takeshi Kyoto University, Science, Research Associate, 理学系研究科, 助手 (70283588)
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| Project Period (FY) |
1997 – 2000
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| Keywords | Aso volcano / volcanic tremor / volcanic eruption / hydrothermal reaction / broadband seismometer |
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| Research Abstract |
Seismic wavefields observed near active volcanoes contain plenty of information about volcanic activities in the frequency range between 0.01 Hz and 1.0 Hz, where, until recently, conventional geophysical monitoring of volcanoes had riot paid much attention except for a few rare cases. The physics operating in a volcano is extremely complex; in different frequency bands, different physical processes dominate the wavefield, which makes broadband seismic observations essential to unravel the physical processes of volcanic activity using seismic data.
Deployment of a dense network of broadband seismometers in Aso revealed a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15s, even when there is no apparent surface activity at the crater. When phreatic eruptions are observed, broadband seismograms elucidate vividly the whole eruption process: gradual pressurization and long-period (about 20s) pulsations of the reservoir during the 100-200s before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The observed spatial variation of the signal amplitudes reveals that the source of 15s long period tremors (LPTs) consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The extension of the buried crack plane meets the crater chain, including the active fumarole at the surface, suggesting that the crack has played an important role in transporting steam (gasses) and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.
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[Publications] Yamamoto, M., H. Kawakatsu, S. Kaneshima, T. lidaka, J. Oikawa, S. Watada, Y. Morita, T. Mori, T. Tsutsui, Y. Sudo, M. Yoshikawa, T. Hashimoto, M. Nakabo: "ASOBOI97: Aso Seismic Observation with Broadband Instruments in 1997"Bull. Earthquake Res. Inst.. 74. 267-285 (1999)
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[Publications] Kawakatsu, H., S. Kaneshima, H. Matsubayashi, T. Ohminato, Y. Sudo, T. Tutui, 」. Uhira, H. Yamasato, H. Ito, D. Legrand: "Aso-94: Aso seismic observation with broadband instruments"J. Vol. Geothermal Res.. 101. 129-154 (2000)
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