Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi: /2008jb005910, 2009 Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics Koki Aizawa, 1,2 Yasuo Ogawa, 1 and Tsuneo Ishido 3 Received 2 July 2008; revised 6 November 2008; accepted 18 November 2008; published 30 January [1] The imaging of hydrothermal systems within volcanoes is critical in evaluating the nature and likelihood of future volcanic activity and hazard assessment. In this study, we present a conceptual model of the hydrothermal system in a volcanic edifice, as deduced from the relationship between electric self-potential (SP) and high-resolution resistivity structures. In order to develop a comprehensive model of water flow in volcanoes, we conducted the audiofrequency (10, Hz) magnetotelluric surveys in five large stratovolcanoes (Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane) in Japan and found that the obtained 2-D resistivity profiles have a close relationship to the previously reported SP data: good extensive conductors occur beneath areas without SP anomalies, whereas good localized conductors only occur beneath large spatial wavelength SP anomalies on the volcano side of the SP minimum. Also taking into account the locations of surface geothermal activity, the good conductors roughly correspond to the hydrothermal zone, whose upper limit is sealed by a low-permeability clay layer. The sealing layer separates an upper groundwater flow from a lower hydrothermal flow in the subsurface and controls the geothermal manifestations and river locations on the surface. We confirmed the feasibility of the proposed model based on numerical simulations of a hydrothermal system. The horizontal extent of the hydrothermal zone is highly heterogeneous even in a volcanic edifice. This heterogeneity can reflect the geological age of flanks that may be related to the occurrence of a previous large sector collapse. Citation: Aizawa, K., Y. Ogawa, and T. Ishido (2009), Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics, J. Geophys. Res., 114,, doi: /2008jb Introduction [2] Volcanic edifices, which mainly consist of volcanoclastic rock and lava, generally possess a highly permeable structure that is readily infiltrated by meteoric water. Hydrothermal systems develop in such systems where descending meteoric water encounters deep magmatic fluids. Leading up to eruptions, magmatic fluids interact with water at shallow depths, and various precursory phenomena are expected to occur in the hydrothermal system. For example, lowfrequency earthquakes [e.g., Chouet, 1996; Kaneshima et al., 1996; Kawakatsu et al., 2000; Nishimura et al., 2000], ground deformation [e.g., Kaneshima et al., 1996; Aoyama and Oshima, 2008; Iguchi et al., 2008], and demagnetization [Yukutake et al., 1990] occur in the upper few kilometers of volcanoes where hydrothermal systems are likely to exist. With increasing volcanic activity, the risk of catastrophic flank failure increases owing to elevated 1 Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan. 2 Now at Sakurajima Volcano Research Center, DPRI, Kyoto University, Kagoshima, Japan. 3 National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, Tsukuba, Japan. Copyright 2009 by the American Geophysical Union /09/2008JB005910$09.00 pore pressure [Reid, 2004]. Flank instability is also promoted by the long-lived hydrothermally alteration [e.g., López and Williams, 1993; Finn et al., 2001; Opfergelt et al., 2006]. In addition, it is possible that the eruption type is controlled by the relative locations of the ascending magma and the shallow hydrothermal system [Kagiyama et al., 1999].To understand ongoing volcanic activity and conduct a reliable hazard assessment, it is important to image the hydrothermal system and alteration zone in a volcano prior to the initiation of volcanic unrest. [3] The nature of hydrodynamics beneath stratovolcanoes has been discussed previously on the basis of gas composition, spring chemistry, and drilling data [e.g., Oki and Hirano, 1970; Giggenbach, 1988; Hedenquist and Lowenstern, 1994; Fournier, 1999]. However, a general lack of drilling data makes it difficult to accurately model the flux, composition, and location of subsurface water flow. Despite the importance of water flow and the corresponding permeability structure, they are rarely modeled, especially within volcanic edifices. Although the structure within volcanic edifices can be deduced by seismicity-based investigations [e.g., Tanaka et al., 2002; Yamawaki et al., 2004] or helicopter-borne electromagnetic surveys [Finn et al., 2001, 2007], it is generally difficult to obtain information on subsurface water flow. [4] Electrical resistivity and self-potential (SP) are the most useful physical parameters in studying water within volcanic edifices. The resistivity structure provides fundamental 1of12

2 Figure 1. Topographic maps showing the locations of self-potential (SP) and magnetotelluric (MT) observation sites. (a) Iwate volcano, (b) Iwaki volcano, (c) Nasu volcano, (d) Nantai volcano and (e) Nikko- Shirane volcano. Small black dots and stars represent the sites of electric SP measurements and the reference site, respectively. Blue triangles represent MT observation sites. Red areas indicate geothermal activity such as fumaroles and hot springs. Rivers are shown as blue lines. Topographic contours are drawn at intervals of 200 m. The components (temperature, ph, flux) of major hot springs are presented. information in determining the presence of a hydrothermal system because high-salinity water (around 1000 mg/l dissolved content) and altered clay minerals are highly conductive. In turn, SP data for volcanoes provide unique information on groundwater dynamics on the basis of the electrokinetic mechanism (i.e., water flow through a porous media generates an electric current [e.g., Ishido and Mizutani, 1981; Revil et al., 1999]). Because the SP pattern is closely related to both the presence and dynamics of water, and because it is inevitably affected by the resistivity structure, a combined analysis of SP and resistivity data is useful in gaining an understanding of the overall hydrothermal system. [5] Joint studies of SP and resistivity structure have previously been conducted at several active volcanoes; however, the results have been insufficient in terms of discussing the bulk water flow within large stratovolcanoes, as the obtained resistivity structures were only approximate in nature [Lénat et al., 2000; Aizawa et al., 2005; Hase et al., 2005] or were not extended to the deeper part [Zlotnicki et al., 2003; Revil et al., 2004; Finizola et al., 2006]. In Misti volcano in Peru, the resistivity structure to a depth of 500m was obtained by a dense array of magnetotelluric soundings (92 khz 10 Hz). Although the summit area was not imaged du to the lack of the data, it was suggested that the resistivity structure has a relation to high-resolution SP data [Finizola et al., 2004]. Recently dense multidisciplinary (SP, resistivity, CO2 flux, temperature) survey has been conducted at La Fossa cone at Vulcano Island, and its groundwater flow model was proposed [Revil et al., 2008]. However, as the size of a target volcanic cone is moderate (300 m tall and 3 km width) it is unclear whether or not the deduced water flow model generally applies to large stratovolcanoes. Furthermore, shallow water flow of inland volcanoes may be different from those of island volcanoes such as Stromboli and Vulcano owing to infiltrating seawater. [6] In this paper, we show the results of magnetotelluric surveys that were conducted in five large inland stratovolcanoes in Japan. Then, the relationship between shallow (1 km) resistivity structure and previously obtained SP data [Aizawa, 2008] are discussed. The detailed discussion concerning the local geology is not the focus of this study. By gathering data on many volcanoes, we try to develop the discussion to the comprehensive structures of large stratovolcanoes. Next, from the relationship between SP and resistivity structure, we propose the conceptual model of a hydrothermal system within the volcanic edifice. This model is tested using a simple hydrothermal simulation. Finally, we propose that heterogeneities in water flow within volcanic edifices may represent hidden geological structures. 2. Geological Setting [7] In order to develop a comprehensive model of water flow in volcanoes, we acquired SP and resistivity data from five volcanoes in northeast Japan: Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane (Figure 1). The studied volcanoes are 2of12

3 Table 1. Summary of the Feature of the Studied Volcanoes Volcano Height of Summit Above Volcano Base (m) Composition Geological Age (ka) Last Eruption (ka) Hydrothermal Activity at the Surface Recent Phreatic Eruption Shallow Seismicity Last Large Sector Collapse (ka) Iwate (east) 1700 basalt andesite only at the summit does not occur occurs 7, NE flank Iwate (west) 800 basalt andesite occurs occurs occurs does not occur Nantai 1200 andesite dacite 22 7 does not occur does not occur does not occur 13, north flank Nasu 1200 andesite occurs occurs occurs does not occur Iwaki 1400 andesite occurs occurs does not occur Unknown age, NE flank Nikko-Shirane 1000 andesite dacite occurs unknown occurs does not occur large ( km 3 ) stratovolcanoes in inland Honshu, rising m from the surrounding land. The geology of each volcano is briefly described below. [8] Iwate volcano (Figure 1a) is a large basaltic-andesitic stratovolcano, of which western part and eastern parts show a high contrast in character [Itoh and Doi, 2005]. West Iwate (noted in Figure 1a) consists of old rock which has been erupting during the last 300,000 years, while East Iwate is relatively young with an age of 30,000 years and its edifice is not as eroded. On West Iwate there are many geothermal manifestation, while on East Iwate geothermal manifestations cannot be seen except at the summit. In 1998, intense seismicity and crustal movements were observed, but no eruption occurred [Tanaka et al., 2002]. [9] Iwaki volcano (Figure 1b) is a symmetric andesitic stratovolcano approximately 33,000 years old. From 1597 to the last eruption in 1863, 12 phreatic eruptions around the summit have been recognized. Although hot springs exist in the valley on the southwestern flank, the present Iwaki volcano is inactive. In 1978, fumarolic activity emerged temporarily in the valley on the northeastern flank, but disappeared after several months [Japan Meteorological Agency, 2005]. [10] Nasu volcano (Figure 1c) is an andesitic and relatively young volcano which is approximately 16,000 years old. Since its last ejection of magma in 1410, eruption activity has been phreatic until the most recent eruption in Numerous geothermal activities exist around the volcano, suggesting the presence of an active hydrothermal system. [11] Nantai volcano (Figure 1d) is a 22,000-year-old andesitic-dacitic stratovolcano, and its last eruption occurred about 7000 years ago [Ishizaki and Oikawa, 2008]. The present Nantai is quiet without anomalous volcanic activities. [12] Nikko-Shirane volcano (Figure 1e) is an andesiticdacitic volcano whose basement is located at an altitude of 2000 m. The main edifice of Nikko-Shirane is composed of a small lava dome, which is approximately 300 m in height and 1000 m in diameter. The present Nikko-Shirane has only a hot spring on the eastern flank 4 km away from the lava dome, and the last eruption was in Table 1 summarizes the features of each volcano. 3. Magnetotelluric Survey and Analysis [13] In 2005, 2006, and 2007, we conducted the audiomagnetotelluric surveys, which measure the earth s impedance between naturally occurring electromagnetic waves and induced earth current in the frequency range between 10,000 to 0.3 Hz across the entirety of the edifices of the five volcanoes. Electrode spacing was set at around 20 m, and natural electric (Ex and Ey) and magnetic (Hx, Hy, and Hz) fields were measured at all observation sites by using Phoenix MTU5-A systems. The typical recording duration for each site was 2 to 12 h. The total number of measurements sites amounted to 92 on five volcanoes. [14] In conducting MT observations of a large stratovolcano, gaining coverage of the entire edifice is commonly difficult owing to poor access to measurement sites; therefore, 2-D analysis along a mountain trail is commonly undertaken. However, the assumption of 1- or 2-dimensionality is potentially invalid in highly heterogeneous volcanic zones. In this study, we used the TM-mode electromagnetic response (flow of electric current across the regional strike), which is sensitive to lateral discontinuities of the structure and is relatively insensitive to the 3-D structure [e.g., Wannamaker et al., 1984; Ledo et al., 1998; Ogawa, 2002]. TE-mode data (flow of electric current along with the regional strike) are included in the inversion only when the regional 2-D structure is validated from phase tensor analysis [Caldwell et al., 2004]. [15] Figure 2 shows histograms of the phase tensor at three different frequencies. Except for Iwaki volcano (Figure 2b), the data do not suggest the presence of strong 2-D regional strikes perpendicular to the survey lines; therefore, only TM data (flow of electric current along the survey line) were inverted. At Iwaki volcano, the axis of the phase tensor converges to N45E and N45W for all frequencies, suggesting the existence of strong 2-D regional structure around the survey line. Accordingly, we fixed the regional strike to N45W S45E and used both the TM and TE data. After fixing the regional strike orientation, TM and TE impedances were decomposed using site-dependent and frequencyindependent distortion parameters [Groom and Bailey, 1989]; we then inverted the decomposed apparent resistivity and the phase in both modes. On all volcanoes, the resistivity structure, taking into account topography, was determined using a 2-D inversion code [Ogawa and Uchida, 1996]. The root-mean-square (RMS) converged to 1.27, 1.07, 1.31, 1.19, and 1.11 in Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane, respectively with the error floor of 10% in apparent resistivity and its equivalent value in phase. Figure 3 show the fits of the responses calculated from the best fit models to the observed responses. The overall fit is good at all volcanoes. 4. Electric Structure 4.1. Resistivity Structure [16] Figure 4 shows the best-fit resistivity models for the uppermost 1 km of the analyzed volcanoes, and the corresponding SP profiles [Aizawa, 2008]. The relationship 3of12

4 Figure 2. Histograms of the regional 2-D strikes estimated from the phase tensor [Caldwell et al., 2004] for each volcano. The main axis of the phase tensor is plotted with 90 ambiguity. Figures 2a 2e show data for Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane volcanoes, respectively. Lowermost arrows show the directions of each survey line. Figure 3. Data fitted as pseudosections for observed (obs) and calculated (cal) data. Figures 3a 3e show data for the Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane volcanoes, respectively. The calculated data are derived from the best fit resistivity models shown in Figure 4. Black dots represent the data used in the inversion. Areas with no data are shown in gray. Distances along the horizontal axes correspond to horizontal distances along the resistivity profiles. Static shift, which is shown beneath the labels of App. Resistivity (cal), is corrected in each colored plot. The SP minima (as shown in Figure 4) are indicated by dashed vertical lines. 4of12

5 Figure 4. Best fit resistivity models and corresponding SP profiles for the five volcanoes analyzed in the present study. SP data are taken from Aizawa [2008]. The locations of survey lines and measurement sites are shown in Figure 1. 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). Thick blue arrows indicate the thermal features shown in Figure 1. Pronounced SP minima are marked by dashed vertical lines. between resistivity structure and SP is discussed in section 5. Broadly speaking, the interpreted resistivity profiles are characterized by two-layered structures. The upper layer is a resistive surface layer of around 100 to 10,000 ohm m, the thickness of which is highly variable. It is difficult to interpret the variations in thickness based simply on known geological structure such as lava flows or a buried collapsed caldera wall. The lower layer is a conductive zone of around 0.5 to 100 ohm m. This layer contains the dominantly good conductors around 0.5 to 20 ohm m Self-Potential Profile [17] In 2004, SP data has been taken along the same survey lines as MT surveys [Aizawa, 2008]. In Figure 4, the raw SP data are also shown. The reference site (= 0 mv) is set at the base of the mountains, and the relative voltage were plotted along the MT survey line. Neglecting the local fluctuations, we see that long spatial wavelength SP anomalies cover entirely of some flank. The amplitude of the anomaly ranges from 700 to 2000 mv, which is consistent with those found in the literature [e.g., Zlotnicki and Nishida, 2003]. The SP profile is highly heterogeneous even in a volcano, and there also be flanks that show no SP anomaly Tentative Interpretation of Highly Conductive Zones in Volcanic Edifices [18] In volcanic areas, a good conductor (0.5 to 20 ohm m) can be attributed to either the presence of hydrothermal water or altered clay (smectite) minerals [Ussher et al., 2000; Nurhasan et al., 2006], but these two scenarios involve contrasting permeability structures (namely, altered clay minerals can act as a relatively low permeability zone, whereas fluids probably exist in fractures). Here it should be noted that we observed surface geothermal activities above the dominant high conductivity (Figures 1 and 4). These observations are inconsistent with the simple hypothesis that the highly conductive zones consist solely of altered clay, without hot saline fluids. Therefore, we interpret that the conductive zones largely correspond to active hydrothermal zones within which hot saline fluids and altered rocks coexist. Similar rough interpretations of a high conductivity zone in volcanoes are also found in the recent dense magnetotelluric observations [Aizawa et al., 2008a; Kanda et al., 2008]. The presence of saline hot water is also suggested by the occurrence of hot springs (thermal features shown in Figures 1 and 4) that contain dissolved ion contents of mg. In section 7, we 5of12

6 discuss the possible range of physical parameters (permeability and temperature) within this conductive zone. [19] In the field of geothermal exploration, which aims to obtain deep (2 km depth) high-temperature fluids, the coexistence of hot saline water and conductive clay minerals is not widely accepted [e.g., Pellerin et al., 1996]. Data from numerous deep drillholes reveal that conductive zones are low-permeability clay-dominated zones rather than fluiddominated zone [Jones and Dumas, 1993; Ussher et al., 2000]. The hot water exists in the underlying resistive zone characterized by high temperatures (in excess of 200 C, which leads to the breakdown of the conductive smectite) and low salinity [Ussher et al., 2000]. However, within the volcanic edifices, the infiltration of cold meteoric water means that the water temperature is expected to be low except near the active conduits. Indeed, recent drilling at Unzen volcano [Nakada et al., 2005], which erupted in , revealed that the temperature remains below 200 C to a depth of 2000 m [Sakuma et al., 2008]. Moreover, active volcanoes are geologically young, and hydrothermal alteration (argillation) is generally at an early stage of development with certain amounts of hot water within the volcano. Furthermore, high salinity is expected in active volcanoes owing to the influence of saline fluids derived from crystallizing or ascending magma. Accordingly, the highly conductive zone in a volcanic edifice is considered to largely correspond to the zone of coexisting clay and hot water, with temperatures below 200 C. In fact, this interpretation is consistent with existing drillhole data obtained for the Newberry caldera, which demonstrated that altered clay minerals (smectite) and hot (80 C) saline water coexist at a depth range of m [Fitterman et al., 1988]. 5. Relationship Between Resistivity Structure and Self-Potential [20] On the basis of the results of SP surveys of 10 volcanoes in Japan, Aizawa [2008] classified large spatial wavelength SP anomalies into three types: (1) no anomaly (flat SP profile), (2) W-shaped SP profile, and (3) V-shaped SP profile. The SP profile in Figure 4c is a flat-type anomaly, and those in Figures 4a 4b and 4d 4e show coexisting flat SP profiles and partial W-shaped profiles. On the basis of possible explanations of flat SP profiles, Aizawa [2008] suggested the presence of hydrothermal systems beneath areas devoid of an SP anomaly. However, this preliminary interpretation did not quantitatively account for subsurface water flow. In the present study, we seek to model water flow within a volcanic edifice by investigating the relationship between resistivity structure and the SP profile. [21] Figure 4 shows that there are main good conductors (0.5 to 20 ohm m) in the conductive layer in all volcanoes. These good conductors show a good correlation with corresponding SP data. The SP signal is approximately flat beneath the flank where the good conductor is extensive. In contrast, the SP profile shows a distinct negative anomaly (half of W-shaped) with a strength in the order of 1 (volt) (marked by dashed lines in Figure 4) beneath the flank on which the good conductor is truncated. In other words, the good conductors are confined to the volcano side of the SP minimum. In agreement with this trend, the surface resistive layer shows a sudden increase in thickness across the SP minima. A similar relationship between the surface resistive zone and the SP minimum was reported on Misti volcano [Finizola et al., 2004] though the structure around the summit and deep zone were not revealed. On Iwaki volcano (Figure 3b), a temporary fumarole activity in 1976 is located near the SP minimum. [22] Although resistivity contrast can affect spatial trends in SP, polarity change in the SP profile cannot be explained by a resistivity contrast alone. It has also been shown that long spatial wavelength SP anomalies are not related to the physical properties of fresh rock [Aizawa et al., 2008b; Aizawa, 2008]. The relationship between resistivity structures and SP profiles probably indicates a drastic change in water flow around the SP minimum. The extent of the good conductor, thickening of the surface resistive layer, and location of the SP minimum all presumably reflect the heterogeneous nature of the hydraulic structure and should therefore be interpreted together. 6. Conceptual Model of a Hydrothermal System Within a Volcanic Edifice 6.1. Sealing Zone at the Top of the Highly Conductive Zone [23] In section 4.3, we tentatively interpreted the highly conductive zone as a volume of coexisting altered rock and hot saline water. In this section, we modify this initial proposal to explain the relation between SP and resistivity structure. Here we introduce the idea that the upper layer of good conductors is largely sealed by clay-rich material that acts as a relatively low-permeability cap. Indeed, the self sealing of hydrothermal systems is a common phenomenon [Facca and Tonani, 1967; Ingebritsen and Sorey, 1988; Lowell et al., 1993], and the relationship between soil CO 2 emanations and SP at other volcanoes [Finizola et al., 2002, 2004] is consistent with the proposal that the upper surfaces of hydrothermal systems are largely sealed by highly altered clay-rich material. The sealing of hydrothermal systems is also supported by the fact that surface geothermal activity is generally highly localized, despite the fact that the conductive zone is extensive (Figures 4a and 4c) Conceptual Model of Water Flow [24] On the basis of the relationship between SP and resistivity structure, as well as the observations and interpretations described above, we propose a conceptual model for a hydrothermal system within a volcanic edifice, as schematically illustrated in Figure 5. The top layer of the hydrothermal system is a self-sealed, low-permeability, clay-rich cap that acts as both the base of the zone of infiltrated meteoric groundwater and the roof of the zone of hydrothermal fluids. The two-phase zone that exists in the hydrothermal system around the active conduit is not necessarily present in all active volcanoes. [25] Beneath the volcano flank with a distinct SP minimum, groundwater that flows above the low-permeability clay layer descends to great depth, resulting in the large unsaturated layer that lies outside of the SP minimum. The resistive upper layers observed beneath the five volcanoes analyzed in the present study are interpreted to represent this unsaturated zone, arising because of the high permeability of volcanoclastic rocks. The thickness of the unsaturated zone is 6of12

7 Figure 5. Schematic model of a hydrothermal system 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. The hydrothermal zone 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 occurs around the active conduit but is not necessarily needed to explain the observations. Note that the deep hydrothermal system, which may exist beneath the volcanic basement, is not determined from SP or audio-frequency magnetotelluric (AMT) methods. controlled by the balance between permeability and precipitation rate [Hurwitz et al., 2003]. The SP gradient outside of the SP minimum is explained by electrokinetic theory, which describes a negative correlation between SP and elevation due to gravity-driven flow [Zlotnicki and Nishida, 2003], although the detailed mechanisms of SP generation remains unclear. There exists a cold groundwater-saturated zone beneath the unsaturated zone. Small amounts of volatiles degassed from the magma are possibly dissolved in this groundwater. [26] Beneath the flank with a flat SP profile, the lowpermeability sealing layer and underlying hydrothermal zone are extensively present at a shallow depth. The flat SP pattern is obtained because the high conductivity acts to reduce the voltage difference [Aizawa, 2008]. Furthermore, the electrokinetic effect is reduced with decreasing zeta potential owing to low ph and/or high salinity [Ishido and Mizutani, 1981; Aizawa, 2008]. Zeta potential is a key parameter that describes the electrokinetic effect. Because a sealing zone exists close to the surface beneath the flat SP anomaly, laterally flowing groundwater tends to emerge at springs, forming rivers. Indeed, many such rivers are found on those flanks with flat SP profiles (Figure 1; West Iwate, Nasu, and southwestern flank of Iwaki). At the foot of the flanks with flat SP profiles, hot springs are formed by the mixing of hydrothermal water and laterally flowing meteoric water. If the self-sealing layer becomes thinned or fractured, the hydrothermal water and meteoric water mix to form a fumarolic zone at the surface. The temporal occurrence of fumaroles at Iwaki volcano (Figures 1b and 2b) may be related to temporal fracturing of the sealing zone. [27] Natural systems in real volcanoes are more complex than Figure 5, in which we simply illustrate the case of two typical flank scenarios coexisting on a volcano. We note that the hydrothermal system within a volcanic edifice can sometimes be a complex blend of the two situation shown in Figure 5, that is, the laterally extensive (left half of Figure 5) and the laterally confined (right half of Figure 5) zones of hot water. We also note that the deep hydrothermal system, which exists beneath the conductor, cannot to be determined by AMT (10, ) measurements owing to the limitation of the penetration depth of EM wave. 7. Numerical Model 7.1. Method Employed in the Hydrothermal Simulation [28] To test the feasibility of the proposed model, we conducted a numerical simulation using the finite difference method, calculating the spatial and temporal underground conditions (pressure, temperature, fluid flux, etc) and resulting distributions of resistivity and self-potential. The simulation was carried out by using the STAR code [Pritchett, 1995] and EKP postprocesser [Ishido et al., 1997; Ishido and Pritchett, 1999; Ishido, 2004]. The simulation consisted of two steps. The first involved transient simulation of the 7of12

8 Figure 6. Details of the 2-D axisymmetric grid used in the simulations. The actual computation area extends to 16,700 m horizontally and 3600 m vertically. Rock parameters for each zone are provided in the lower table. Figure 6a represents the volcano flank with no SP anomaly. Figure 6b represents the flank with a large spatial wavelength SP anomaly. subsurface conditions (pressure, temperature, salinity, liquidphase saturation, etc.) based on a given distribution of rock types and fluid sources. This calculation was conducted by using the STAR code and BRNGAS equation of state [Pritchett, 1995], which is applicable to three pore components (H2O, NaCl, and air) and three pore phases (liquid, gas/ vapor, and solid halite precipitation up to the temperature of 350 C and pressure of 200 MPa). In the second step, the SP and resistivity distribution were calculated on the basis of the results of the first step. In the SP calculation, the strength, polarity, and distribution of conduction current source (SP source) are first calculated by the divergence of convection currents [Sill, 1983]. This formulation has the advantage in obtaining the physical meaning of SP sources (SP source generate where there are gradients of subsurface parameters parallel to the fluid flow or divergence of fluid flow). [29] The simulation especially focused on explaining the relationship between resistivity structure and self potential, which is main finding of this study. For simplicity, an axisymmetric spatial grid was used to represent a 3-D volcano. The rock unit is set as in Figure 6. Here we describe two hydrothermal computations: one that represents the volcano flank with no SP anomaly (Figure 6a), and another that represents the flank with a large spatial wavelength SP anomaly (Figure 6b). A relatively low-permeability and conductive-sealing cap layer was imposed within the highpermeability rock with caps of varying lengths employed in the different simulations. In this permeability and porosity structure, meteoric water recharge is imposed by dilute water sources (H 2 O with mass fraction of NaCl, corresponding to 13.8 ohm m at 21 C) equivalent to 0.4 m of rainfall recharge per year. To induce hydrothermal flow, we set a deep hot-water source (200 C H 2 O with mass fraction of NaCl, corresponding to 0.17 ohm m at 21 C) of 10,000 (ton/day) at the deepest area located close to the axis of the computation area. All exterior boundaries except the top surface are set to be impermeable. Pressure and temperature along the top surface boundary are maintained at 1 bar and 21 C, respectively (any fluid flowing into the computational grid from the top surface is air at 1 bar and 21 C). The SP and resistivity calculation is conducted for the computation area, which is slightly larger than the area of the step 1 hydrothermal simulation. The resistivity is computed according to the modified Archie s law [Glover et al., 2000], in which the exponent m is simply assumed to be 2. The selfpotential calculation is conducted using the mesh discretization of Dey and Morrison [1979] in the 2-D axisymmetric grid. The air is given resistivity of (ohm m), and the boundary condition is set to zero electric current across the central axis and zero electric potential at other distant 8of12

9 Figure 7. Results of numerical simulations using a 2-D axisymmetric grid. (a) Simulation of a volcano in which the hydrothermal zone is widely developed. (b) Simulation of a volcano in which the hydrothermal zone is not widely developed. Top shows the calculated electric self-potential (SP). Middle shows the calculated resistivity and electric current source (white is positive; black is negative) distributions. The color scale of the resistivity structure is the same as that in Figure 4. White contours show the isoelectric potential lines at intervals of 200 mv. Bottom shows the rock units (refer to Figure 6) and calculated water mass flux. Red arrows represent the vapor phase. Note that discharge from the summit only occurs in the vapor phase. White contours show the temperature distribution (contour interval, 50 C). planes. Our simulation differs from the previous studies [Ishido, 2004; Revil et al., 2008] in the point that we try to reproduce not only SP data but also resistivity structure by using the information of calculated water content, temperature, and porosity Results of Hydrothermal Simulation [30] Figure 7 shows the induced fluid flow pattern, temperature, and corresponding SP and resistivity structure derived from the simulations. Figure 7a shows the quasisteady state after 50,000 years from the hydrostatic condition. The hydrostatic condition was calculated beforehand, with only meteoric recharge being imposed. Compared with the hydrostatic condition, the water table does not show a significant rise with the mass injection of a deep hot-water source, because the imposed deep source (10,000 ton/day) is small compared with the magnitude of the meteoric water source in a mountainous area (approximately 100,000 ton/ day). Figure 7b shows the transient state from the appropriate initial condition. The relationship between the structural gap and SP minimum in terms of Figure 5 are reproduced well. Zone 1R in Figure 6 contributes to the correspondence of the 9of12

10 SP minimum to the resistivity transition. Without zone 1R, the SP minimum would shift approximately 300 m to the base of the mountain. [31] In contrast to conventional thought that upward hydrothermal flow significantly contribute to the generation of relatively positive SP zone [Ishido and Mizutani, 1981; Zlotnicki et al., 1998, 2003; Lénat et al., 2000; Kanda and Mori, 2002; Finizola et al., 2002, 2004, 2006; Zlotnicki and Nishida, 2003; Revil et al., 2004; Aizawa et al., 2005; Hase et al., 2005; Bedrosian et al., 2007; Revil et al., 2008], the SP pattern is relatively insensitive to the hydrothermal upwelling in this model. Because the hydrothermal water has a low ability to transport the electric current owing to its low ph and/or high salinity [Ishido and Mizutani, 1981; Aizawa, 2008], remarkable SP sources are not generated even when strong hydrothermal upwelling exists. [32] The subsurface water flow generally carries a positive charge downstream via the electrokinetic effect, but its efficiency is relatively poor at low temperatures [e.g., Ishido and Mizutani, 1981; Aizawa et al., 2005] or under conditions of vaporization [e.g., Kanda and Mori, 2002; Hase et al., 2005]. On this basis, previous studies proposed that a positive electric charge accumulates at the top of the hydrothermal convection cell, where a reduction in temperature or vaporization may occur, thereby explaining the generation of the relatively positive SP zone. However, our conceptual model and corresponding numerical simulation differ from previous models in that the SP sources are mainly located immediately above the sealing zone, and at the water table. In this model, the SP pattern and its amplitude are mainly constrained by groundwater flow in the resistive unsaturated zone and underlying aquifer. This feature is similar to the model of Ishido [2004], in which the most important controlling factors are resistivity, permeability, and porosity in the zone of unsaturation. Although there are moderate electric current sources in the hydrothermal zone, these do not significantly contribute to the SP anomaly on the surface because the surrounding highly conductive zone attenuates the SP amplitude. [33] In Figure 7, there are gas discharges out of the summit crater. The simulated discharge rate from the summit and the extent of the two-phase zone (coexisting vapor and hot water) can be controlled by permeability and temperature, but the calculated SP distribution and resistivity structure are relatively insensitive to these parameters. Even when vaporization occurs, shallow groundwater flow is free from the influence of the deep, hot region owing to the presence of the low-permeability sealing zone Uniqueness of the Hydrothermal Model [34] It is difficult to determine subsurface parameters such as permeability and porosity. In stratovolcanoes, permeability and its anisotropy are somewhat unknown parameters that can vary by several orders of magnitude. It was previously shown that small changes in permeability can result in large changes in the position of the water table [Hurwitz et al., 2003]. Here, we briefly discuss how we selected the parameters shown in Figure 6. [35] The position of the water table (i.e., the bottom of the thick resistive layer) is determined from the resistivity structure. Because we assumed 0.4 m/year of water recharge from the surface (infiltration of approximately 20% of annual rainfall), the permeability of fresh surface rock (unit 1) is first estimated to fit the observations. We assigned an anisotropic permeability to zone 1 by considering that the shallowest part consists of interlayered fresh lava and pyroclastic rocks (we obtained similar results for both isotropic and anisotropic permeability in zone 1). [36] The permeability of the other zones is poorly constrained. In this study, the permeability of basement (zone 3) and the sealing zone (zone 4) are set to be lower than that for zone 1. The conduit (zone 5) is set to be high permeability. The possible range of permeability of the hydrothermal zone (zone 2), which is electrically conductive, is required in performing the hydrothermal simulation. If a high permeability (such as that for zone 1) is applied, zone 2 becomes a cold, unsaturated zone, and no hydrothermal system is developed. The permeability of zone 2 should therefore be lower than that of zone 1. Given the known occurrence of hydrothermal alteration to clay minerals, a low permeability for the hydrothermal zone may be reasonable in a volcanic edifice. [37] We used hydrothermal simulations to test the feasibility of the conceptual model. However, the nonuniqueness of subsurface parameters (especially permeability) means that other models might also explain the resistivity and SP data. Given the difficulty involved in determining a unique model of volcano hydrodynamics, additional observational constraints are required. We emphasize that SP at the base and the summit is approximately equipotential in all volcanoes (Figure 4). We propose that rather than being a positive SP zone, the summit is a normal SP zone surrounded by a negative SP zone. This interpretation is consistent with our model, in which a negative SP zone is generated within the thick, resistive, unsaturated zone. 8. Implications for Geological Structure [38] As shown in Figures 4, 5, and 7, the horizontal extent of the hydrothermal zone in volcanic edifices can be highly variable. This heterogeneity possibly reflects the age of the volcano flank. In general, the hydrothermal system evolves from the center of the volcano to the base, meaning that the structure shown in Figure 7b is expected to progressively change to that in Figure 7a. As the hydrothermal system evolves, hydrothermal fluids act to reduce the permeability of the fresh rock [Hurwitz et al., 2003], forming a sealing zone at the top of the system [Lowell et al., 1993]. The flank with an SP minimum and resistivity transition is interpreted to be the newly constructed flank. [39] We also propose that heterogeneities in the SP and resistivity data represent hidden geological structures. Flank failure tends to occur within highly altered flanks such as that shown in Figure 7a [López and Williams, 1993; Opfergelt et al., 2006; Nicollin et al., 2006], resulting in a large amphitheater. If subsequent fresh effusions cover the amphitheater accompanied by moderate growth of the hydrothermal system and meteoric water infiltration, a subsurface structure develops similar to that shown in Figure 7b. Indeed, large sector collapses have occurred in the past on the northeastern flanks of Iwate (around 7000 years ago) [Itoh and Doi, 2005] and Iwaki volcanoes [Hashimoto et al., 1979], where the dominant SP minima presently exist (Figure 4). The dominant SP minima at Mt. Fuji volcano also exist 10 of 12

11 immediately above the likely locations of previous largescale flank failures [Miyaji et al., 2004; Aizawa et al., 2005]. In summary, the dominant SP minimum and structural gap may indicate the site of a previous sector collapse, while a flat SP profile and the extensive good conductor may indicate a high potential for future sector collapse. 9. Concluding Remarks [40] This study demonstrated that a combination of SP and resistivity structures is effective in investigating the water flow within volcanic edifices. We found that good laterally extensive conductors occur beneath areas without SP anomalies, whereas good conductors only occur beneath large spatial wavelength SP anomalies on the volcano side of the SP minimum. Also taking into account the locations of surface geothermal activity, we interpret that the good conductors roughly correspond to the hydrothermal zone, whose upper limit is sealed by a low-permeability clay layer. On the basis of the relationship between SP and resistivity structure and the results of hydrothermal simulations, we proposed the conceptual model that the sealing layer separates an upper groundwater flow from a lower hydrothermal flow. This model well accounts for the surface manifestations, such as fumaroles, hot springs and river locations. Although natural systems in real volcanoes are more complex than those considered here, we think that the hydrothermal systems within volcanic edifices can be roughly represented by a modified version of the proposed model. [41] In this study, the conceptual water flow model was proposed to explain the characteristic of SP and resistivity data. However, by concentrating on explaining the SP and resistivity structure in a specific volcano and in obtaining high-resolution data, a more quantitative model of water flow could be proposed. We believe that a quantitative image of the steady state hydrothermal system represents fundamental information of use in interpreting various volcanic activities. A future study that simulates magma intrusion into a steady state water flow within a specific volcano may further contribute to our understanding of volcanic activity. [42] Acknowledgments. We thank S. Nagaoka, A. Shito, and S. Mishima for the help in the MT field surveys. We thank H. Takano for providing the data on hot springs. Review of the earlier version of the manuscript by K. Yamazaki is greatly appreciated. The careful review and numerous constructive comments by S. Hurwitz, D. Fitterman, and an anonymous reviewer are greatly appreciated. The editorial work of the Associate Editor, A. Revil, is also appreciated. K.A. acknowledges the support of a JSPS fellowship. References Aizawa, K. (2008), Classification of self-potential anomalies on volcanoes and possible interpretations for their subsurface structure, J. 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