Groundwater Flow beneath Volcanoes Inferred from Electric Self-Potential and Magnetotellurics

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1 // (,*+*) 0,/+,/3 Groundwater Flow beneath Volcanoes Inferred from Electric Self-Potential and Magnetotellurics Koki A IZAWA + / (Kagiyama et al, +333 ; Hurwitz et al,,**1) (Lopez SiO -H O and Williams, +33- ; Reid,,**) OH Si-O +33/ ; Revil et al, +333,+ H K Na (Fig + a), - / 0 (convection current) (conduction current) (slipping plane) z, z,+ SP: Self Potential Spontaneous Potential,, ++- **-, Earthquake Research Institute, University of Tokyo, Yayoi + + +, Bunkyo-ku, Toky, ++- **-,, Japan Corresponding author: Koki Aizawa a-zawa@eriu-tokyoacjp

2 252 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 ph,, MT (Ishido and Mizutani, +32+ ; Hase et al,,**- ; Aizawa et al,,**2b) (Morgan et al, +323 ; Tosha et al,,**-) + +* z ( -m),*** (Gaillard and Marziano,,**/) (Sill, +32-) (Fig + a) (Fig + b),*** MT MT (Fig,) (Aizawa,,**) MT (Michel (,**3) and Zlotnicki, +332 ; Aizawa et al,,**/ ; Revil et al,,**2) MT (Static shift) (Ishido,,** ; Aizawa,,**2 ; Aizawa et al,,**3a) Static shift

3 253 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 A B Seal- (Ogawa and Uchida, +330 ; Sasaki,,**) ing zone,** - (Hyndman et al, +331 ; Lee et al,,*+*) Aizawa et al (,**3 a),** / + km Sealing Cap (Fig - and Table + ) rock ( + +* -m) +** m A +* +* -m,** B A +323 ; Pellerin,,**, ( + +* -m) Aizawa et al (,**3a) Sealing zone Fig - Fig (Fig ) + +* ( -m),** + km,**,** Sealing zone (Zablocki Sealing zone et al, +31 ; Fitterman et al, +322 ; Sakuma et al,,**2 ; Sealing zone Hurwitz et al,,*+*),** Sealing

4 254 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 Sealing zone B,**/ ; +313 (Fig,),3** Aizawa et al (,**3a) A B +322 A (Lopez and Williams, +33- ; Finn et B al,,**+) A B Fig -

5 255 Aizawa et al (,**3b) / MT km, Fig (Fig /) (Fig,) (Fig -a) (Aizawa et al, - He He,**2 a) ,**0 ;,*+* Sealing zone Aizawa et al (,*+*) + MT 0 +** +*** (m/day) Aizawa et al (,**3 a) + km Sealing zone Fournier ( +333) 2* Fracture ** Brittle- 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

6 256 Fig / Resistivity structure of Iwate volcano combined with seismicity and geodetic model (Aizawa et al,,**3b) Black circles and white stars represent volcano-tectonic and low-frequency earthquakes, respectively Open rectangles show the temporal evolution of the location of dike-shaped pressures source (Sato and Hamaguchi,,**0) The numbers ( ) shown at the center of each rectangle (dike) represent the order of the estimated periods Ductile transition zone MT Sealing zone (Fig 0) ( / km) + +* ( -m) Sealing,** Sealing ** - He He, Sealing zone Fournier ( +333) Aizawa et al (,**3a) Fournier ( +333 ) ** Brittle- Ductile transition zone Sealing (Ohba et al,,**2) MT Sealing Reid (,**) Hurwitz et al (,**1) (Ogawa et al,,**+ ; Wannamaker et al,,**3) (Etheridge et al, +32- ; Jones, +321) Sealing Sealing Sealing Sibson, Becken et al (,**2) San-Andreas +33, ;,**-

7 257 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, Sealing zone,+ -, Aizawa, K (,**) A large self-potential anomaly and its changes on the quiet Mt Fuji, Japan Geophys Res Lett, -+,doi:l*/0+,+* +*,3 /,** gl *+30, Aizawa, K, et al (,**/) Hydrothermal system beneath Mt Fuji volcano inferred from magnetotellurics and electric self-potential Earth Planet Sci Lett,,-/, -- -//, doi: +* +*+0 /jepsl,**/*-*,- Aizawa, K (,**2) Classification of self-potential anomalies on volcanoes and possible interpretations for their subsurface structure J Volcanol Geotherm Res, +1/,,/-,02, doi: +* +*+0 /jjvolgeores,**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/ +11, doi : +* +*+0 /jjvolgeores,**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 /,**1 jb **/*/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 /,**2 jb **/3+* Aizawa, K, Ogawa, Y, Mishina, M, Takahashi, K, Nagaoka, S, Takagi, N, Sakanaka, S and Miura, T (,**3 b) Structural controls on the +332 volcanic unrest

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