Imaging the hydrothermal system beneath the Jigokudani valley, Tateyama volcano, Japan: implications for structures controlling repeated phreatic eruptions from an audio-frequency magnetotelluric survey
© Seki et al.; licensee Springer. 2015
Received: 5 September 2014
Accepted: 9 December 2014
Published: 15 January 2015
This study focuses on the results of an audio-frequency magnetotelluric (AMT) survey across the Jigokudani valley, Tateyama volcano, Japan, to investigate the spatial relationship between the distribution of electrical resistivity and geothermal activity and to elucidate the geologic controls on both its phreatic eruption history and recent increase in phreatic activity. The AMT data were collected at eight locations across the Jigokudani valley in September 2013, with high quality data obtained from most sites, enabling the identification of an underground 2D resistivity structure from the transverse magnetic (TM) mode data. The data obtained during this study provided evidence of a large conductive region beneath the surface of the Jigokudani valley that is underlain by a resistive layer at depths below 500 m. The resistive layer is cut by a relatively conductive region that extends subvertically toward the shallow conductor. The shallow conductive region is divided into an uppermost slightly conductive section that is thought to be a lacustrine sediment layer of an extinct crater lake containing hydrothermal fluids and a lower section containing a mix of volcanic gases and hydrothermal fluids. The low permeability of the clay zone means that the uppermost clayey sediments allow the accumulation of gases in the lower section of the conductive region, suggesting the existence of a cap structure. The deep resistive layer likely consists of units similar to the granitic rocks that are widely exposed throughout the Jigokudani valley. We suggest that the relatively conductive zone that separates these granitic rocks represents a high-temperature volcanic gas conduit, given that the most active fumarole in the Jigokudani valley lies directly along the trajectory of this path.
Phreatic eruptions, or hydrothermal eruptions, occur through the intense heating of groundwater by a high-temperature material of the magma origin such as volcanic gases and may occur in any volcanic system (e.g., Barberi et al. 1992; Germanovich and Lowell 1995). Phreatic eruptions are intrinsically connected to the hydrothermal systems present within volcanic edifices, given that these eruptions often contain ash consisting of hydrothermally altered minerals (e.g., Ohba and Kitade 2005). In addition, volcanic hydrothermal systems are often associated with low permeable clayey layers that consist of hydrothermally altered minerals and commonly develop within the shallow levels of volcanic edifices (e.g., Wright et al. 1985; Pellerin et al. 1996). The accumulation of upwelling volcanic gas beneath this low permeable layer can lead to an increase in temperature and pressure that can potentially cause hydrothermal eruptions. This low permeable clayey layer is conductive and can be imaged using magnetotelluric (MT) methods (e.g., Ussher et al. 2000; Aizawa et al. 2013). The fact that gases are not as conductive as fluids or clays means that these low permeable clayey caps are often underlain by slightly resistive regions. A number of volcanoes that have multiple recorded phreatic eruptions have been investigated by magnetotelluric surveys, including Kusatsu–Shirane volcano in Japan (Nurhasan et al. 2006) and Taal volcano in the Philippines (Yamaya et al. 2013). A significant number of these volcanoes are associated with near-surface conductive anomalies that have been interpreted to be low permeable clayey layers.
Several recent anomalous events have been repeatedly observed in the study area, including a sulfur outflow in 2010 (Japan Meteorological Agency2013a) and fumaroles with temperatures that exceed the boiling point of water, reaching a maximum of about 140°C (Japan Meteorological Agency2013b). These fumarole vent gases have high HCl/SO2 ratios, indicating that these gases were derived from high-temperature sources (Tokyo Institute of Technology et al.2013). These events thus provide evidence of an increase in volcanic activity. However, only a few geophysical studies have been conducted in the Jigokudani valley to date, meaning that there is currently little information available on the shallow subsurface structure of this area. Here, we present the results of an audio-frequency magnetotelluric (AMT) survey within the Jigokudani valley, an area that has experienced multiple phreatic eruption events. These data provide information on the distribution of hydrothermal fluids and vapor reservoirs in this area and provide new constraints on the spatial relationship between subsurface structures and fumarolic activity, including the presence of cap structures that can store upwelling volcanic gases.
We carried out an AMT survey across the Jigokudani valley in September 2013. This method uses magnetotelluric responses derived from variations in the natural audio-frequency band (approximately 1 to 10x 4 Hz) of the electromagnetic field to estimate the resistivity structure in an area down to a depth of a few kilometers. The AMT data were collected at eight locations at approximately 200-m intervals along an ENE-WSW survey line crossing the Jigokudani valley (Figure 1). Two components of the electric field and three components of the magnetic field were recorded for 10 to 12 h overnight using a Phoenix Geophysics MTU-5A system (Phoenix Geophysics Ltd., Toronto, ON, Canada), barring site 07, where electromagnetic fields were only measured for 3 h during the daytime. The AMT responses were estimated using mutual local references to reduce the influence of noise within the data, except for site 07, where single-site processing was used due to the lack of available reference sites. The main noise sources are a trolley bus operated using a DC current that departs from the underground Murodo terminal (at the southern edge of Figure 1), and commercial 60 Hz frequency power supplied to lodges in this area. AC power is also supplied to the Jigokudani valley to monitor volcanic gases in this area.
Dimensionality of structures in the study area
The 2D nature of the structures in the study area is also constrained by the shapes of phase-tensor ellipses and induction arrows (Figure 3). The major axes of the phase-tensor ellipses obtained at 9.4 Hz are roughly parallel to the survey line, and the ratios of major to minor axes lengths are generally large (Figure 3a). In contrast, these ellipses become more circular at higher frequencies (see 1,100 Hz in Figure 3c). In addition, real induction arrows (Parkinson1962; Figure 3), which intrinsically tend to point toward lower resistivity areas, do not appear to have any preferential orientation or length at high frequencies (1,100 Hz; Figure 3c). In comparison, the arrows obtained for lower frequency data (97 Hz and 9.4 Hz; Figure 3a,b) point toward the central part of the study area. This suggests that the resistivity structure has lateral variations parallel to the survey line at depths corresponding to the low frequency data obtained during this study.
Three characteristic features are present in the final model (Figure 5); a conductive body (less than 10 Ωm) beneath the Jigokudani valley (between sites 04 and 06) that seems to be divided into a more conductive upper part (C1 in Figure 5) and a less conductive lower part (C2), a resistive layer that has resistivity values of several hundreds of ohm meters at depths greater than 500 m (R1) with a similar resistive zone at shallower depths in the northeast of the study area (R2), and a relatively conductive zone (C3) that seems to separate the two resistive zones (R1 and R2) and extends subvertically from these portions to the shallow conductive body. These features are also present within the pseudo-sections for the study area (Figure 6). The shallow conductive region around the Jigokudani valley corresponds to the low apparent resistivity zone that extends from an area with high frequencies to an area with lower frequencies around the middle sites within the pseudo-section. The deep resistive layer is indicated by lower phases within the low frequency data. The relatively conductive portion is also present as relatively high values around sites 03 and 04 within the phase pseudo-section.
Origin of the shallow conductor
As described in the previous section, a total of three features were identified in the resistivity distribution for the study area. A highly conductive region with resistivity values less than 10 Ωm extends beneath a significant part of the Jigokudani valley (C1 and C2 in Figure 5); this near-surface conductor is likely separated into upper and lower sections, with the former having a convex shape and is more conductive than the lower part.
The northeastern part of the Jigokudani valley contains laminated lacustrine sediments that are located at the base of an extinct crater lake (Kusakabe et al. 1983; 1986), whereas the southern area contains an active solfatara field, a hot water pool, and andesitic tuff breccias that are exposed on the hill side of the valley and has a valley side where these breccias are overlain by laminated lacustrine sediments (Kouno 1988). The distribution of solfataras in this area indicates that high-temperature volcanic gases are ascending beneath the southern area of the Jigokudani valley (Kouno 1988). The high temperature and acidic nature (pH = 1.7 to 3.6; Mizutani et al. 2000) of these fluids has led to the development of hydrothermal kaolinite (Kusakabe et al. 1983). Although kaolinite itself possesses a higher resistivity, the presence of hot acidic fluids in areas with kaolinite can lead to a significant decrease in bulk resistivity (Takakura 2009). This indicates that the more conductive upper part (C1) of the conductive body most likely represents clayey sediments containing acidic hydrothermal fluids (Figure 5).
The low permeable nature of clays means that these upper clayey sediments partly cap the upwelling volcanic gases and that the high-temperature gases in the lower part of the near-surface conductor (C2) appear slightly more resistive than the upper part of the conductor (C1). The temperature of volcanic gases discharged from several fumaroles exceeds the boiling point of water in this area (Japan Meteorological Agency 2013b), which indicates the existence of the superheated vapor beneath the Jigokudani valley. The apparent equilibrium temperature of fumarolic gases estimated from the sulfur reaction suggests that the volcanic gases at shallow levels under the Jigokudani valley are present at temperatures of 200°C to 340°C (Mizutani et al. 2000). The capping of these gases could cause an increase in pressure as the gases accumulate, creating the ideal setting for a phreatic explosion, although there are currently no geodetic data that would either support or disprove this hypothesis. However, it should be noted that the active fumaroles discharging superheated steam in the Jigokudani valley are located directly over the shallow conductor, as is seen in Figure 5.
High resistivity layer and relatively conductive body
Next, we consider the resistive layer present at depths below 500 m in the study area (R1 and R2 in Figure 5). These resistive zones are most likely composed of granitic basement material that is similar to the Tateyama volcanic edifice, which is dominated by granitic rocks. These granites and their metamorphosed equivalents are widespread throughout the Jigokudani valley (Harayama et al. 2000), including the granitic mountains to the northeast of the Jigokudani valley (Figure 1), that may be linked to the shallowing of the R2 resistive layer in this area.
This interpretation is supported from geochemical studies. The fumarolic gases within the Jigokudani valley have high 3He/ 4He ratios that are indicative of a dominantly magmatic origin (Nagao et al. 1981). Mizutani et al. (2000) reported that the magmatic gases contributed significantly to the fumarolic gases in this area and suggested that they were not contaminated by interaction with surface materials due to shallow basement rocks. These studies on fumarolic gases infer that the granitic basement is located at shallow levels, which is consistent with our interpretation of the high resistivity layer.
The relatively conductive body that extends subvertically from the shallow conductor is present within the deep resistive layer (C3 in Figure 5) and appears to separate the resistive granitic basement into two zones. Kobayashi and Hatano (1989) discussed the relationship between faulting and phreatic eruptions in the Jigokudani valley, which is located at the intersection of two faults. One of the two faults lies in a WSW-ENE direction from the Midagahara Plateau to the Jigokudani valley and has a length of approximately 7 km. The other is a 9-km-long fault extending nearly south-southeastward from around the Jigokudani valley. These two faults are orthogonal to each other and form a conjugate fault system. The explosion craters within the Jigokudani valley define a NW-SE oriented trend (Figure 1), indicating that these faults and their associated eruptive activities were located along a NW-SE oriented principal compression axis in this area (Kobayashi and Hatano 1989). This model suggests that the area around the Jigokudani valley underwent NW-SE-directed compression that fractured the rocks in this area as a result of strain accumulation between faulting events. These fractures then led to interaction between near-surface waters and heat sources, leading to numerous repeated phreatic eruptions within the Jigokudani valley (Kobayashi and Hatano 1989). This suggests that the relatively conductive body (C3) acts as a conduit for magmatic gases and most likely formed as a result of the fracturing outlined above. The heat source, and hence the source of the magmatic gases in this area, has been located at a depth of 2 to 4 km using seismic tomography (Matsubara et al. 2000), a depth that is greater than the investigation depth of the AMT study discussed here, indicating that longer period MT data would be required to detect this possible magma chamber.
Data obtained from an AMT survey undertaken along an ENE-WSW survey line that crossed the Jigokudani valley were used to determine the underground structure of an area underlying a zone of repeated phreatic eruptions. The 2D resistivity modeling undertaken during this study identified the following three features:
(1) A highly conductive region that extends to shallow levels beneath the Jigokudani valley. This conductor is divided into a conductive upper part and a less conductive lower part. The upper part most likely represents clayey sediments containing hydrothermal acid fluids, whereas the lower part is probably associated with the presence of rather resistive volcanic gases. The less permeable nature of the clay in this area means that the upper clayey sediments act as a cap that stores the magmatic gases in the lower part of this zone.
(2) A deep resistive layer that is present at depths below 500 m and is interpreted to represent granitic rocks that are widespread throughout the Jigokudani valley.
(3) A relatively conductive region that extends subvertically toward the shallow conductor and separates the deep resistive granitic layer into two sections. This zone most likely represents a conduit for magmatic gases, particularly since it underlies the most active fumarole in the Jigokudani valley.
The three features of the resistivity structure across the study area define a model where magmatic gases are supplied from a deep source to the Jigokudani valley, an area that has undergone numerous phreatic eruptions. Further research, including 3D modeling with denser AMT sites, is required to confirm this preliminary model.
We are deeply grateful to the Tateyama Caldera Sabo Museum and the Tateyama Nature Conservation Center for their cooperation during our research. The Chubu Region Environmental Office, the Toyama District Forest Office, and Toyama Prefecture granted permission for the AMT survey undertaken within the Jigokudani valley during this study. We thank the staff at the Volcanic Fluid Research Center of Tokyo Institute of Technology for useful discussions. The constructive comments provided by the two anonymous referees and the Editor-in-Charge (H. Toh) were highly helpful to improve the manuscript. All of the figures used in this paper were prepared using Generic Mapping Tools (Wessel and Smith 1998). This study was supported by JSPS KAKENHI Grant Number 25350494.
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