電磁気観測で探る火山浅部の浸透率構造と地下水流動

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(3) Groundwater Flow beneath Volcanoes Inferred from Electric Self-Potential and Magnetotellurics Koki AIZAWA. + . "#$%&'( ( )*+*,. )05&*234/ %&'(63. /

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(6) ++-ῌ**-, +ῌ+ῌ+ Earthquake Research Institute, University of Tokyo, Yayoi +ῌ+ῌ+, Bunkyo-ku, Toky, ++-ῌ**-,, Japan.. Corresponding author : Koki Aizawa e-mail : [email protected].

(7) ;<=. 252. Fig. +. (a) Microscopic view of convection current generation by electrokinetic phenomenon (b) Schematic illustration of negative correlation between self potential and elevation. In the porous rock, electric charge is conveyed by convection (convection current) ; however no charge is conveyed outside the porous media. Therefore, the charge accumulation occurs on the recharge and discharge zone. The conduction current (Ohmic current) flows to cancel this charge separation.. 567

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(14) EF0. 253. Fig. ,. (a) Self-potential distribution around the summit of Mt. Fuji volcano. Contour interval is *.,/ V. Star shows the reference point (* V in the map). Thick lines indicate the locations of the summit crater and the Hoei crater. (b) The self-potential data for the long profile (site locations are shown in Aizawa, ,**.). Horizontal axis of the figure is the horizontal distance from the center of the summit crater. (c) Topographic profile for the data shown in Fig. ,b.. G4H9I

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(46) . 254 Table +.. Relationship between SP profile, resistivity structure and surface features.. Fig. -. Resistivity models and corresponding SP profiles for the Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane volcanoes, respectively (Aizawa et al., ,**3a). Inverted triangles indicate MT measurement sites. The horizontal axes are set as the projected distances along the MT survey lines. Surface geothermal activity is shown by blue arrows (solid : hot spring, dashed : fumarole). Pronounced SP minima on the flanks of Type B (Fig. .) are marked by dashed vertical lines..

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(62) . 255. Fig. .. Schematic model of a groundwater flow within a volcanic edifice. Solid and dashed arrows represent the flow directions of liquid and vapor, respectively. The top of the hydrothermal system is sealed by a low permeability and electrically conductive clay-rich layer, which is shown as thick gray line in the figure. Conductive clay minerals (smectite) exist in the gray zone, whose bottom corresponds to the ,** isotherm. The hydrothermal zone below ,** within the volcanic edifice is largely constrained to the volcano side of the SP minimum. The hydrothermal zone is extensive beneath the flat part of the SP profile. The two-phase zone may occur around the active conduit, but is not needed to explain the observations. Note that the deep hydrothermal system, which may exist beneath the volcanic basement, is not determined by Aizawa et al. ,**3a.. . !"

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(77) . 257. Fig. 0. Schematic model of groundwater flow (Fournier, +333). Self sealing by silica precipitation occur at about -1* to .**, which correspond to the brittle-ductile transition zone. Note that the sealing zone by clay minerals (Aizawa et al., ,**3) occur at shallower level.. !"#$ %&'()!*+, -.# /0123 4 5(!67 , 8 Sealing zone 9#:;*#)<= >?@A)BCD E,- DF GH I1JKLMNO PQ*#RSTR=UV WX YZ[ K,-.# \]^ I_ àb

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(79) s*#t KKC uvwxy z{tK 23 uv|.}.#]~)5(!6 'tKD |7'#wK ῐ. ῏. 1 ,+ X buv) )K]7 p uv)' tKF -#wKY[qw p'] 9 % ¡¢ - 1 /0G!wuvL)"-t p *.uv4)£#¤¥'t'] ¦ 'tW§K]' ¨©ª1 % ¡$« )¬'t'] ] , %&®¯ ° '$ « ±($« |.²³´@1 µ¶) *·'] ¸¹uvºBKt»buv+, ¼½ -uv¯+ ¥¼)'] ¦'t W§K]'. ῎ ῍ ῑ ῌ Aizawa, K. (,**.) A large self-potential anomaly and its changes on the quiet Mt. Fuji, Japan. Geophys. Res. Lett., -+, doi : L*/0+,+*.+*,3/,**.gl*+3.0,. Aizawa, K., et al. (,**/) Hydrothermal system beneath Mt. Fuji volcano inferred from magnetotellurics and electric self-potential. Earth Planet. Sci. Lett., ,-/, -.-ῌ -//, doi : +*.+*+0/j.epsl.,**/.*-.*,-. Aizawa, K. (,**2) Classification of self-potential anomalies on volcanoes and possible interpretations for their subsurface structure. J. Volcanol. Geotherm. Res., +1/, ,/-ῌ ,02, doi : +*.+*+0/j.jvolgeores.,**2.*-.*++. Aizawa, K., Ogawa, Y., Hashimoto, T., Koyama, T., Kanda, W., Yamaya, Y., Mishina, M. and Kagiyama, T. (,**2a) Shallow resistivity structure of Asama Volcano and its implications for magma ascent process in the ,**. eruption. J. Volcanol. Geotherm. Res., +1-, +0/ῌ+11, doi : +*.+*+0/j.jvolgeores.,**2.*+.*+0. Aizawa, K., Uyeshima, M. and Nogami, K. (,**2b) Zeta potential estimation of volcanic rocks on ++ island arctype volcanoes in Japan : Implication for the generation of local self-potential anomalies. J. Geophys. Res., ++-, doi : B*,,*++*.+*,3/,**1jb**/*/2. Aizawa, K., Ogawa, Y. and Ishido, T. (,**3a) Groundwater flow and hydrothermal systems within volcanic edifices : Delineation by electric self-potential and magnetotellurics. J. Geophys. Res., ++., doi : B*+,*2+*.+*,3/,**2jb**/3+*. Aizawa, K., Ogawa, Y., Mishina, M., Takahashi, K., Nagaoka, S., Takagi, N., Sakanaka, S. and Miura, T. (,**3b) Structural controls on the +332 volcanic unrest.

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(82) by Bayesian decomposition of strain, tilt and positioning data. J. Volcanol. Geotherm. Res., +//, ,..ῌ,0,, doi : +*.+*+0/j.jvolgeores.,**0.*-.*-+. Sibson, R.H. (+33,) Implications of fault-valve behavior for rupture nucleation and recurrence. Tectonophysics, ,++, ,2-ῌ,3-. Sill, W.R. (+32-) Self-potential modeling from primary flows. Geophysics, .2, 10ῌ20. Tosha, T., Matsushima, N. and Ishido, T. (,**-) Zeta potential measured for an intact granite sample at temperatures to ,** degrees C. Geophys. Res. Lett., -*, doi : +,3/+*.+*,3/,**,gl*+00*2. Wannamaker, P.E., Caldwell, T.G., Jiracek, G.R., Maris, V., Hill, G. J., Ogawa, Y., Bibby, H.M., Bennie, S.L. and Heise, W. (,**3) Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand. Nature, .0*, 1---U13*, doi : +*.+*-2/nature*2,*.. Zablocki, C. J., Tilling, R.I., Peterson, D.W. and Christiansen, R.L (+31.) : A deep research drill hole at the summit of an active volcano, Kilauea, Hawaii., Geophys. Res. Lett., +, -,-ῌ-,0. , ¡6 ¢ £FA$ ¤¥¦§ ¨©ª« <=&'¬® ¯° ±8²³ l f:»¼];( ,*+* <>^ =½ ,*+*. /. 259. (,**/) !" ! " #$%!&'$()*+, - . (,**3) MT /0 1 234 5 0+ S,,/ῌS,-2.

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