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北海道大学地球物理学研究報告 = Geophysical bulletin of Hokkaido University >
第73号 >

十勝岳の全磁力変化と蓄熱過程(2008 - 2009年)

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Title: 十勝岳の全磁力変化と蓄熱過程(2008 - 2009年)
Other Titles: Heat accumulation beneath Tokachidake volcano as inferred from magnetic changes
Authors: 橋本, 武志1 Browse this author
西村, 三治2 Browse this author
有田, 真3 Browse this author
山本, 輝明4 Browse this author
小木曽, 仁5 Browse this author
重野, 伸昭6 Browse this author
岡崎, 紀俊7 Browse this author
茂木, 透8 Browse this author
Authors(alt): HASHIMOTO, Takeshi1
NISHIMURA, Mitsuharu2
ARITA, Shin3
YAMAMOTO, Teruaki4
OGISO, Masashi5
SHIGENO, Nobuaki6
OKAZAKI, Noritoshi7
MOGI, Tohru8
Issue Date: 19-Mar-2010
Publisher: 北海道大学大学院理学研究院自然史科学部門(地球物理学)
Journal Title: 北海道大学地球物理学研究報告
Journal Title(alt): Geophysical bulletin of Hokkaido University
Volume: 73
Start Page: 269
End Page: 280
Abstract: We report the recent results of geomagnetic repeat surveys aiming for the detection of subsurface thermal changes at Tokachidake volcano, central Hokkaido, northern Japan. We conducted the first survey in October, 2008. The same pegs and marks were reoccupied in July and September, 2009. During this period a clear dipolar pattern of magnetic changes was detected around the 62-II crater, the present main vent of Tokachidake. This change is quite consistent with the pattern due to a demagnetized sphere placed at a depth of 150 m(1,600 m a.s.l.)beneath the 62-II crater. The change of magnetic moment amounts to 1 - 2×106 Am2. The maximum magnetic change is about 35 nT, overwhelming the errors due to extra-terrestrial changes, secular changes, or seasonal variations. The most likely explanation of this demagnetization is the thermomagnetic effect due to heating beneath the crater. Considering the moderate fumarolic temperature in the 62-II crater, the thermal demagnetization is not caused directly by magma intrusion. We here propose a model that water vapor supplied from depths is releasing latent heat through condensation at the source depth. The heating rate corresponding to the observed magnetic changes is an order of 10 MW. A more speculative suggestion is that water vapor of 107~108 kg/yr is required to realize this heating rate. Considering the fact that fumarolic activity of 62-II crater has somewhat declined since 2006, the ongoing demagnetization may be attributed to the decrease of heat discharge rate from the crater. If this is the case, elevation of heat supply from depths is not necessarily required.
Type: bulletin (article)
Appears in Collections:北海道大学地球物理学研究報告 = Geophysical bulletin of Hokkaido University > 第73号

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