Magmatic hydrothermal system inferred from the resistivity structure of Kusatsu-Shirane Volcano

Journal Pre-proof Magmatic hydrothermal system inferred from the resistivity structure of Kusatsu-Shirane Volcano

Yasuo Matsunaga, Wataru Kanda, Shinichi Takakura, Takao Koyama, Zenshiro Saito, Kaori Seki, Atsushi Suzuki, Takahiro Kishita, Yusuke Kinoshita, Yasuo Ogawa PII:

S0377-0273(19)30331-2

DOI:

https://doi.org/10.1016/j.jvolgeores.2019.106742

Reference:

VOLGEO 106742

To appear in:

Journal of Volcanology and Geothermal Research

Received date:

12 June 2019

Revised date:

19 November 2019

Accepted date:

19 November 2019

Please cite this article as: Y. Matsunaga, W. Kanda, S. Takakura, et al., Magmatic hydrothermal system inferred from the resistivity structure of Kusatsu-Shirane Volcano, Journal of Volcanology and Geothermal Research(2019), https://doi.org/10.1016/ j.jvolgeores.2019.106742

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© 2019 Published by Elsevier.

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Magmatic hydrothermal system inferred from the resistivity structure of Kusatsu-Shirane Volcano

Yasuo Matsunaga(a), Wataru Kanda(a), Shinichi Takakura(b), Takao Koyama(c), Zenshiro Saito(a, d), Kaori Seki(a, b), Atsushi Suzuki(a), Takahiro Kishita(a, d), Yusuke Kinoshita(a, e), Yasuo

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Ogawa(a)

(a) School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551,

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Japan

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(b) Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology

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(AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan (c) Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032,

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Japan

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(d) Present address: Oyo Co., 7 Kanda-Mitoshiro-cho, Chiyoda, Tokyo 101-8486, Japan

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(e) Present address: MUFG Bank, Ltd., 2-7-1 Marunouchi, Chiyoda, Tokyo 100-8388, Japan

Corresponding author:

Yasuo Matsunaga, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan Email: [email protected]

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Abstract The Kusatsu-Shirane volcano is a Quaternary andesitic-to-dacitic active volcano located on the central Honshu Arc. The volcano is known to have had repeated phreatic eruptions in last 200 years. A number of geochemical and geophysical studies have been conducted around the Yugama crater lake, the focus of current volcanic activity. However, the 2018 unexpected phreatic eruption occurred at Mt. Motoshirane, a different pyroclastic cone from that which hosts Yugama. There were

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also frequent magmatic eruptions until 1500 years ago at this cone and the magma produced at that time has not yet cooled and solidified. A better understanding of the magmatic hydrothermal system

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of this volcano requires structural information gathered over a larger area, and to greater depths, than

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has been achieved to date. Here, we describe the three-dimensional (3-D) electrical resistivity

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structure around Mt. Motoshirane down to a depth of ~10 km using broadband magnetotelluric (MT) data (320−0.0005 Hz) collected in 2015 and 2016. The 3-D resistivity structure model shows an

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extensive low-resistivity layer at depths of 1–3.5 km beneath the summit. However, structures

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characteristic of the presence of magma are not observed beneath this layer. It could be too deep or

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too small to be detected. Combining the new data with results of previous geochemical and geophysical studies, we interpret the conductor as a hydrothermal fluid reservoir that supplies fluids to the crater lake and to hot springs on the eastern and western flanks of the volcano. The existence of a fluid reservoir that extends from the vicinity of the Yugama crater lake to the subsurface beneath an inactive pyroclastic cone that has produced repeated magmatic eruptions in the past suggests that a large-scale magmatic hydrothermal system is developing beneath the volcano.

Keywords: Kusatsu-Shirane volcano

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magnetotellurics resistivity structure hydrothermal system Mt. Motoshirane

1. Introduction

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The geometry of magmatic hydrothermal systems provides information on the mechanisms that drive volcanic eruptions, as eruptions and other volcanic activity are caused by ascending magma and by

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fluid flow driven by the associated heat. Aqueous hydrothermal fluids and magmatic melts (e.g.

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Nesbitt, 1993; Tyburczy and Waff, 1983; Gaillard and Marziano, 2005; Guo et al., 2017) are

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electrically conductive, meaning that electromagnetic exploration techniques can detect structures that contain fluids. Magnetotelluric (MT) sounding methods utilize disturbances in Earth’s natural

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electromagnetic field, and are commonly used to investigate structures at depths of up to several

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kilometers (e.g. Simpson and Bahr, 2005). Sub-vertical elongated conductive bodies have been

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found in a number of volcanoes using MT surveys and interpreted as paths for volcanic fluids or magma (Heise et al., 2007; Ingham et al., 2009; Hill et al., 2009; Bertrand et al., 2012; Kelbert et al., 2012; Bertrand et al., 2013; Aizawa et al., 2014; Ogawa et al., 2014; Comeau et al., 2015; Diaz et al., 2015; Hill et al., 2015; Peacock et al., 2015; Bedrosian et al., 2018). In this study, we conducted broadband MT surveys (320−0.0005 Hz) in 2015 and 2016 around Mt. Motoshirane, one of the central cones that forms the Kusatsu-Shirane volcano, and used these data to model the resistivity structure of the volcano.

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Kusatsu-Shirane is an active Quaternary volcano located on the central Honshu Arc, Japan. Eruptions of andesitic to dacitic magma began ~0.6 Ma on the eastern flank of the volcanic edifice of the older Matsuwozawa volcano (Uto et al., 1983; Hayakawa and Yui, 1989; Fig. 1). The summit area consists of three conspicuous pyroclastic cones. From north to south, these are Mt. Shirane, Mt. Ainomine, and Mt. Motoshirane. Mt. Shirane, the volcanic center with recent activity with the exception of the 2018 eruption, has three main craters. Yugama crater is the largest and is filled with

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an acidic (pH ~1.0) crater lake (Ohba et al., 2000) and vegetation is rare in the vicinity. There have been repeated phreatic eruptions within and around the Yugama crater over the last two hundred

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years. No eruptions have been observed at Mt. Shirane since the last phreatic activity at the Yugama

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and Karegama craters in 1982 and 1983, although earthquakes during the early 1990s were

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associated with marked changes in the total geomagnetic intensity (Takahashi and Fujii, 2014). These earthquakes were followed by a period of relative inactivity. In 2014, an increase in the

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frequency of earthquakes in the summit area was accompanied by changes in the tilt, the total

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geomagnetic intensity, and the composition of fumarolic gases. Subsequently, activity around the

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Yugama crater decreased. The next event was a small phreatic eruption on 23 January 2018 from Mt. Motoshirane, which is located about 2 km south of the Yugama crater. The mass of erupted materials was estimated at ~36,000 tons (Ogawa et al., 2018). This was the first historically observed eruption of the Kusatsu-Shirane volcano other than those from Mt. Shirane.

These activities have motivated studies to be conducted at the Kusatsu-Shirane volcano, such as geochemical analysis of lake water and fumarolic gases, and geophysical exploration and monitoring. The hydrothermal system within the volcano has been investigated using geochemical studies. For example, Ohba et al. (2000) reported the chemical and isotopic compositions of hot spring waters

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and fumarolic gases in this area. Those authors concluded that hot springs such as Kusatsu and Bandaiko on the eastern flank of the volcano discharge a mixture of high-temperature magmatic gases and local meteoric water. In contrast, fluids discharged from fumaroles and hot springs within the summit area, including the Yugama crater lake, are supplied by a hypothesized two-phase hydrothermal reservoir beneath the craters (Ohba et al., 2000). Ohba et al. (2008) proposed the existence of a low-permeability seal at a depth of ~2 km beneath Yugama, formed in the upper part

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of the high-temperature zone surrounding the deep-seated degassing magma, based on a record of changes in the composition of the lake water over several decades. Those authors concluded that the

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seal prevents release of high-temperature magmatic gases when there is little volcanic activity and

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that increased volcanic activity, including phreatic events, occurs when the seal is breached and the

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fluids are released.

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Some geophysical exploration has been carried out at the Kusatsu-Shirane volcano. An

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audio-frequency (10 kHz – 1 Hz) magnetotelluric (AMT) survey conducted along an E–W profile of

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the volcano through Mt. Shirane detected a 300–1000 m thick conductive layer near surface beneath the eastern slope (Nurhasan et al., 2006). This conductor was interpreted as a smectite-rich layer within Pliocene rocks, which acts as a low-permeability cap and separates the fluids that feed the hot springs and fumaroles within the summit area from the fluids that feed the hot springs on the eastern flank of the volcano. This interpretation is consistent with the geochemical model of the hydrothermal system proposed by Ohba et al. (2000).

In contrast to Mt. Shirane, there has been no historical activity recorded at Mt. Motoshirane, which is located about 2 km south of the Yugama crater, prior to its eruption in 2018. As a consequence, the

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summit area is covered with vegetation. However, there are craters in the summit area, so it is likely that this cone was active in the past. The last magmatic eruption occurred about 1500 years ago (Kametani et al., 2015), and the Sessho lava effused from the summit area 3000 years ago (Hayakawa and Yui, 1989). The Sessho lava flow, and other similar flows, form topographic undulations on the slopes of Mt. Motoshirane. These lava flows are larger in scale and younger in age than lavas from Mt. Shirane (Uto et al., 1983; Fig. 1). In addition, there are two major volcanic

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hot springs, Kusatsu and Bandaiko, on the eastern flank of Mt. Motoshirane. These springs are

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characterized by both high temperatures and high discharge rates. Details are shown in Table 1.

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These aforementioned features are consistent with the existence of a shallow heat source beneath Mt.

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Motoshirane. However, there has been little investigation of Mt. Motoshirane, with the exception of some geological studies (Uto et al., 1983; Hayakawa and Yui, 1989; Kametani et al., 2015), because

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there was no volcanic activity at the surface prior to the 2018 eruption. Existing studies at Mt.

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Shirane have focused on imaging the structure of the shallow volcanic edifice to a depth of ~3 km

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from the AMT survey (Nurhasan et al., 2006), or on the origin of hot spring waters and fumarolic gases, based on mixing relationships between meteoric water and magmatic fluids (Ohba et al., 2000, 2008). Since these previous studies did not clarify the extent and geometry of the magmatic hydrothermal system beneath the volcano and the pathways used by deeply sourced hydrothermal fluids, the eruption risk at currently inactive pyroclastic cones such as Mt. Motoshirane had not been recognized. The objective of this study was to determine the structure of the magmatic hydrothermal system of the Kusatsu-Shirane volcano in the vicinity of Mt. Motoshirane and thereby provide insights into current volcanic and hydrothermal activity of the entire volcano.

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2. Data 2.1 Methods The magnetotelluric (MT) method utilizes the natural fluctuation of the electromagnetic fields to infer the subsurface electrical resistivity structure. The electrical resistivity can be determined from the ratio of the magnetic and electric fields measured on the surface in the frequency domain. In this study, we used two complex transfer function for the analysis. The first is the transfer function

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between the magnetic and electric fields in horizontal two directions, which is called the impedance tensor (𝒁) and is defined as a second-rank tensor as follows:

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𝑬 = 𝒁𝑯

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The amplitude of each element of 𝒁 is expressed as an apparent resistivity 𝝆𝒂 given by: 2

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𝝆𝒂 𝑖𝑗

|𝑍𝑖𝑗 | = 2𝜋𝑓𝜇0

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where 𝑓 is the frequency and 𝜇0 is the magnetic permeability in the vacuum. Phase 𝝓 is defined as the phase difference between the electric and magnetic field components:

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𝜙𝑖𝑗 = arg(𝑍𝑖𝑗 )

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The second is the magnetic transfer function 𝑻 defined as: 𝐻𝑥 𝐻𝑧 = (𝑇𝑥 , 𝑇𝑦 ) ( ) 𝐻𝑦

Induction vector is a graphical representation of 𝑻 as a vector and is used to evaluate the lateral gradient of resistivity distribution. The vector will point towards conductors for the Parkinson’s convention (Parkinson, 1962).

2.2 Data acquisition and processing

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Magnetotelluric measurements were conducted at 23 sites around Mt. Motoshirane in 2015 and 2016 (Fig. 1). Time-series data consisting of two horizontal electric field components and three magnetic field components were recorded with 24-bit resolution at each site using two pairs of Pb–PbCl2 electrodes and three induction coils, respectively. All sensors were connected to an MTU-5A system (Phoenix Geophysics) with cables. Data were recorded over 2–20 days for each site.

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The time-series data were converted to the frequency domain using SSMT2000 software (Phoenix Geophysics). A remote reference technique (Gamble et al., 1979; Jones et al., 1989) was applied to

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reduce the influence of local noise using horizontal magnetic field data (provided by Nittetsu Mining

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Consultants) measured at the Sawauchi MT station in the Iwate prefecture about 370 km northeast of

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Kusatsu-Shirane. After removal of outliers, the apparent resistivity and phase of the MT response were estimated at each site for frequencies between 320 and 0.0005 Hz. Data below 0.01 Hz were of

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poor quality and were excluded from this study. The apparent resistivity and phase calculated from

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(Figs. S5−S7).

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𝒁, and the real and imaginary parts of 𝑻 at all sites are provided in the Supplementary Information

2.3 Characteristics of the data The induction vector (Parkinson, 1962) and the phase tensor (Caldwell et al., 2004) were calculated at each site to characterize the overall trends of the data. The phase tensor 𝚽 is robust to the distortion of regional electromagnetic responses caused by near-surface heterogeneity and is defined as: 𝚽 = 𝑿−1 𝒀

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where 𝐗 = Re(𝐙) and 𝐘 = Im(𝐙). The second rank tensor 𝚽 can be graphically represented as an ellipse where the major and minor axes indicate the orientation of lateral resistivity gradients (Caldwell et al., 2004). Skew angle |𝛽| is one of the rotational invariants of 𝚽 and can be used as an indicator of dimensionality of resistivity structure. 𝛽 is represented as: 𝛽=

1 sk(𝚽) tan−1 2 tr(𝚽)

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where sk(𝚽) and tr(𝚽) is the skew and trace of 𝚽, respectively. The variation in these parameters was plotted for five selected frequencies using ellipses for the

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phase tensor and arrows in Parkinson’s convention for the real induction vector (Fig. 2).

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At higher frequencies (typically 97 Hz), the induction vectors commonly point towards topographic

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highs because ground resistivity is much lower than air resistivity. Between 11 and 1.4 Hz, the

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orientations of the major axes of phase-tensor ellipses on the western flank of Mt. Motoshirane differ by ~90° from those on the eastern flank. This suggests the presence of a structural boundary beneath

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the summit area at depths that correspond to these frequencies. Such resistivity boundaries beneath

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the summit area have been reported by previous workers in other volcanoes (e. g. Hill et al., 2015; Bedrosian et al, 2018). At lower frequencies, the induction vectors point to the east of the summit, which suggests a conductive anomaly in that region. The phase tensor (Caldwell et al., 2004) shows large |β| values at lower frequencies would indicate deep three-dimensional (3-D) structures (Booker, 2014). However, we cannot rule out another possibility that large |β| values were due to the data affected by artificial polarized noise. Kusatsu town is located 5 km east from the summit of Mt. Motoshirane, and the DC railway runs in E-W direction around 10 km south from the summit. The low-frequency data could be affected by those artificial noise sources.

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The distribution of apparent resistivity was calculated from the rotational invariant of the impedance tensor, det(Re(Z)) (Fig. 3). The determinant of Re(Z) is a rotational invariant and can be used to delineate 3-D conductors (Szarka and Menvielle, 1997). The data at lower frequencies from the northern sites suggest the presence of a highly conductive feature (Fig. 3). Hydrothermally altered zones around the Yugama crater lake, the Sessho fumarolic area, and the Bandaiko hot spring also show low apparent resistivities at most frequencies. The higher-frequency resistivity data are

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consistent with the presence of a highly resistive feature west of Mt. Motoshirane.

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3. Resistivity model

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3.1 Three-dimensional modeling

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The results of the phase-tensor analysis suggest the presence of 3-D features within the deep subsurface, so 3-D analysis of the data is required to understand the structure of the entire volcano.

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The data from all 23 sites (Fig. 1) were inverted using the WSINV3DMT finite difference 3-D

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inversion code developed by Siripunvaraporn and Egbert (2009). The following functional was minimized during the inversion (Siripunvaraporn et al., 2005):

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−𝟏 𝑇 −1 𝑊𝜆 (𝒎) = (𝒎 − 𝒎0 )𝑇 𝑪−1 𝑚 (𝒎 − 𝒎0 ) + 𝜆 {(𝒅 − 𝑭[𝒎]) 𝑪𝑑 (𝒅 − 𝑭[𝒎])}

where 𝒅, 𝒎, and 𝒎0 represent the observed data, the resistivity model, and the prior model, respectively. 𝑪𝑚 and 𝑪𝑑 are the model and data covariance matrices, respectively. F[m] denotes the model response. 𝜆−𝟏 is a Lagrange multiplier that defines a trade-off between data misfit and model smoothness. Because of the nonlinear problem, minimizing this equation requires an iterative process. It starts with a given initial resistivity model and searches for the smoothest model until the misfit between the model response and the data is reduced to the desired level.

Journal Pre-proof Eight components of impedance tensor (the real and imaginary parts of 𝑍𝑥𝑥 , 𝑍𝑥𝑦 , 𝑍𝑦𝑥 , and 𝑍𝑦𝑦 ) and four components of induction vector (the real and imaginary parts of 𝑇𝑥 and 𝑇𝑦 ) were inverted in this study, which provided detailed structural information. The calculation used fifteen frequencies between 194 and 0.011 Hz, selected at approximately equal intervals on the logarithmic scale. Low-quality data that produced large uncertainties were excluded. The modeled area of 691 km × 691 km × 482 km was divided into a 48 (N–S) × 48 (E–W) × 56 (vertical) mesh. Topography was

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incorporated into the model by specifying a high resistivity value for the cells that corresponded to air. An area of 5.5 km × 5.5 km centered on the summit of Mt. Motoshirane was divided using a 250

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m horizontal mesh. The vertical mesh spacing was set to 25−50 m to provide an approximate

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representation of the topography. The resistivity of blocks corresponding to air was set to 108 Ωm.

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The initial model used a uniform half-space of 100 Ωm, with the resistivity of blocks corresponding to seawater being fixed at 0.33 Ωm. The error floors for components of the impedance tensor and

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the induction vector at each station and period were set to 5% of √|𝑍𝑥𝑦 𝑍𝑦𝑥 | and 10% of

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√𝑇𝑥2 + 𝑇𝑦2 , respectively (Siripunvaraporn and Egbert, 2009).

The inversion was performed in two steps because it was difficult to estimate the model reproducing the data for all frequencies in a single inversion. In the first step, only the data with frequencies smaller than 1 Hz were inverted. The root mean square (RMS) of the misfit of the model decreased to 2.05 after 13 iterations. This model was used as the prior model for the second step of the inversion, which was performed using all 15 frequencies. The first-step result is provided in the supplementary information (Figs. S8, S9, S10). The final model, with an RMS misfit of 2.04, was calculated after five iterations (Figs. 4 and 5). The final model contains three conductive (< 10 Ωm) features (C1, C2, and C3) and two resistive (> 100 Ωm) features (R1 and R2). Conductor C1 is a

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maximum of ~1-km-thick conductive layer located at near surface of the eastern flank. Conductor C2 is a large N–S-oriented conductive region that extends from 1 to 3.5 km deep from Mt. Shirane to Mt. Motoshirane. Conductor C3 lies 0.5−2.5 km beneath the eastern foot of Mt. Motoshirane. Conductors C2 and C3 lie at a similar depth but may be separated by a convex resistive body that lies below the Sessho fumarolic area (R1). Feature R2 lies at depths of 0.2–2 km below the MT sites on

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the western flank of the volcano.

3.2 Sensitivity tests

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The locations of two resistive bodies (R1 and R2) and of two deep features extending at depths of 3–

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8 km bsl (A and B in Fig. 6) were used to test the sensitivity of the model. In these tests, changes to

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the model in response to changes in the resistivity of the target structural feature were calculated and

Information (Figs. S1−S4).

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compared with the final model. Representative results of the tests are provided in the Supplementary

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In the first test, R1 was replaced with a uniform conductive structure. Resistivity values of 10 Ωm

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and 1 Ωm caused the value of the RMS misfit to increase from 2.04, the value for the final model, to 2.11 and 2.31, respectively. A similar test was performed on R2. Resistivity values of 100 Ωm and 10 Ωm caused the value of the RMS misfit to increase to 2.08 and 2.35, respectively. These results suggest that the resistivities of features R1 and R2 are indeed higher than those of their surroundings and that the data cannot be explained without these features.

To test the sensitivity of the modeled deep features to the modeling process, the resistivity of region A in Figure 6 was set first to 10 Ωm and then to 1 Ωm. These changes caused the RMS misfit to increase to 2.19 and 2.53, respectively. In contrast, when the resistivity of region B was set to

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100 Ωm and then to 1000 Ωm, the increase in the RMS misfit was small, with calculated values of 2.05 and 2.06, respectively. Thus, the evidence for a large modeled conductor with a resistivity of less than 10 Ωm covering most of summit area of Mt. Motoshirane at depths of 3–8 km bsl is not robust, and the conductive feature that extends deep beneath Mt. Motoshirane in a westerly direction is not required to explain the data.

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4. Discussion

As a result of 3-D inversion, several structural features were obtained as described above (Figs. 4

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and 5). The conductive feature C1, which appears to be discontinuously distributed within a depth of

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~1 km from the surface, occurs over a broad area on the eastern flanks of Mt. Motoshirane and Mt.

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Shirane. Previous AMT study (Nurhasan et al., 2006) reported this conductor in a 2-D section across Mt. Shirane and suggested that the conductor beneath the eastern flank (C1 in the present study) is a

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smectite-rich layer of Pliocene–Pleistocene volcanic rocks, based on evidence from 1500-m-deep

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boreholes on the southern flank of Mt. Motoshirane (Gunma Prefecture, 1989). A N–S section

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through the final 3-D model was compared with the borehole geology (Fig. 7). Although the spatial resolution of the resistivity structure is poor because of the sparsity of MT sites on the southern flank of Mt. Motoshirane, the modeled conductive layer intersects the boreholes at depths (~500 m) corresponding to that of the Takai lava (~5 Ma), which contains conductive clay minerals such as smectite. From the X-ray diffraction (XRD) analysis of the boring-core sample from 200-m-deep observation well located 600 m east of the Yugama crater rim, smectite was detected, which agreed with the vertical distribution of low resistivity in the electrical logging (Yokoyama et al., 2010). In addition, smectite was contained in the ejecta of recent phreatic eruptions occurred at the Yugama and nearby craters (Kurosaki et al., 1990; Ossaka, 2003). Therefore, it is reasonable to assume that

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the layer containing conductive smectite occurs over a broad area of the volcano and that the majority of C1 is a clay-rich layer, consistent with the conclusions of the AMT study (Nurhasan et al., 2006).

In the area south to southeast of the Sessho fumarolic area, in the vicinity of the source of highly acidic Bandaiko hot spring, the conductive area is slightly elevated in the final model (labeled as C1a

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in Fig. 4). Since the acidic hydrothermal fluid (pH < 2) shows electrically conductive nature because of high mobility of the hydronium ion (H3O+) (Byrdina et al., 2018), the conductor Cla is considered

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to reflect the presence of acidic water. This interpretation is supported by the airborne survey

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targeting on shallow (< 200 m) structures and by the sulfur isotope study. An airborne

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electromagnetic survey conducted over the Kusatsu-Shirane volcano revealed the existence of a 100-m-thick surface conductive (< 50 Ωm) layer, extending from the east of the Mt. Motoshirane

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summit to the Sessho fumarolic area (Asia Air Survey Co., Ltd. and Nippon Engineering Consultant

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Co., Ltd, 2014). A sulfide alteration zone was detected around this conductor, which suggests the

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emission of deeply sourced volcanic gases. The sulfur isotope ratio (δ34S) of H2S in the fumarolic gases collected at Furikozawa located in this sulfide alteration zone, about 0.8 km northwest of Sessho fumarolic area, showed low values of –2~–6 ‰, while δ34S of SO42- contained in the highly acidic (pH ~1.6) hot spring waters at Bandaiko showed a high value of ~+18 ‰ (Yamamoto et al., 1997). This difference in isotope ratio was explained by the sulfur isotope effect based on the disproportionation reaction of magma-derived SO2 from which H2S and H2SO4 are produced. Therefore, the conductor C1a is likely to be caused mainly by highly acidic fluids, although the resistivity values are similar to those of C1 interpreted as a clay-dominated layer.

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The C2 conductor extends beneath the summit areas of Mt. Motoshirane and Mt. Shirane at depths that range from 1.5 km below sea level (bsl) to 1 km above sea level (asl). Previous AMT study (Nurhasan et al., 2006) also recognized this conductor in a 2-D section across Mt. Shirane and interpreted the conductor as a fluid reservoir where magmatic gases condense (Ohba et al., 2000), based on the distribution of the hypocenters of volcanic earthquakes, which extend from the top of the conductor to the summit area. The hypocenter distribution documented by Mori et al. (2006) was

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superimposed on the N–S section of the final 3-D model (Fig. 7). There are two clusters of hypocenters: one beneath Mt. Shirane and the other to the north of Mt. Motoshirane. Most

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seismometers in the area are around the Yugama crater lake, so the number of earthquakes recorded

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beneath Yugama is higher than the number below northern Mt. Motoshirane. However, there is no

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noticeable difference in seismicity between the two clusters for earthquakes of magnitude greater than −1.2 (Mori et al., 2006). This hypocenter distribution suggests that the proposed fluid reservoir

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extends southwards from Mt. Shirane. The results of our study support the interpretation that the C2

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conductor beneath Mt. Motoshirane records the presence of fluids of magmatic origin within this

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region. The sub-vertical alignment of the hypocenter distribution is consistent with the ascent of volcanic fluids in the vicinity of these clusters (e.g., Tsukamoto et al., 2018). A heat source is necessary for the ascent of fluid, so it is likely that hot volcanic rocks and/or a magma chamber are present beneath the C2 conductor that extends from Mt. Motoshirane to Mt. Shirane. Given the consistent results of these previous studies, it is reasonable to assume that volcanic fluids heated by these thermal sources would ascend to the summit area and to the flanks of the volcano, where they can mix with groundwater flow and discharge through the Bandaiko and Kusatsu hot springs.

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A resistive body (R1) was detected in the vicinity of the Sessho fumarolic area at a similar depth to C2 (Figs. 4 and 5). The sensitivity tests suggest that a resistive feature in this location is necessary to explain the data (Supplementary Information; Fig. S1); this interpretation is supported by a gravity high (~2 mgal higher than the surroundings) at the position of R1 (Fig. 8a). Resistive features can occur where there are insufficient fractures for hydrothermal fluid flow, with a consequent lack of conductive clay minerals. A similar tendency of correlation with gravity anomalies can be seen in the

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resistivity section across the Yugama crater (Fig. 8b). Therefore, we propose that this dense resistive

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structure is the product of a paucity of fluids and clay minerals.

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We developed a conceptual model for the hydrothermal system beneath the Kusatsu-Shirane volcano

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(Fig. 9). Previous studies have described the shallow hydrothermal system on the eastern flank of the Kusatsu-Shirane volcano. Gunma Prefecture (1989) suggested that the rate of discharge from the

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Kusatsu hot spring was affected by precipitation, based on a compilation of previous work. That

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author proposed a hydrothermal flow model involving mixing between groundwater and

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high-temperature fluids ascending through the fault system. Aerial photographs and field surveys provided supporting evidence for this proposal (Ohta and Matsuno, 1970; Fig. 1). The groundwaters are thought to flow down to the Kusatsu hot spring through porous Quaternary lava. Ohba et al. (2000) concluded that the hot springs at the foot of the Kusatsu-Shirane volcano (Kusatsu, Bandaiko, and Manza) emit a mixture of meteoric water and primitive volcanic fluid of magmatic origin unaffected by fractionation, based on geochemical analysis of the hot spring waters and fumarolic gases. Conductor C2 containing volcanic fluids of magmatic origin is heated by a thermal source, not detected by our MT survey, which lies beneath the hydrothermal reservoir. Thermally driven ascent of volcanic fluids causes volcanic earthquakes beneath the summit area. The volcanic fluids are also

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supplied to the eastern flank of Mt. Motoshirane through a fracture system, which transfers heat and mass to the shallow subsurface and forms a region of acidic volcanic fluid (C1a: < 0.5 km deep from the surface) and hydrothermal alteration zones on the surface such as those of the Sessho fumarolic area. A mixture of groundwater and volcanic fluids flows down the eastern flank of Mt. Motoshirane close to the surface and discharges at the Bandaiko and Kusatsu hot springs.

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Hydrothermal fluids from the Manza hot spring on the western flank of the volcano are probably also supplied by conductor C2, based on the observation that the conductor extends westwards

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horizontally towards the hot spring. The origin of the Manza hot spring was discussed by Gunma

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Prefecture (1989), who noted that discharge rates were highest at the hot springs closest to the

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Okumanza fault, which extends from Mt. Ainomine to the Manza area (Fig. 9), and that the proportion of the vapor phase within the discharged fluid was high. The results of our model suggest

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that C2 supplies fluids directly to these hot springs through the fault system beneath the western

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flank of Mt. Motoshirane. Due to the similarity of the chemical composition, Kikawada et al (2002)

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suggested that the hot spring waters in the Manza area are derived from the single primitive hydrothermal fluids. Those authors also showed that temporal changes in the chemical composition of the major fumarolic gas in this area are affected by the volcanic activity of Kusatsu-Shirane Volcano. These geochemical results support our interpretation.

The R2 resistive body lies to the south of the Manza hot spring. The results of the sensitivity tests suggest that a highly resistive (> 1000 Ωm) feature in this location is required by the data (Supplementary Information; Fig. S2). According to borehole data, a layer of Miocene Ojo andesitic lava containing a hydrothermally altered zone occurs at the same depth as R2 (500−1500 m asl) (Fig.

Journal Pre-proof 7: Gunma Prefecture, 1989). Resistivities of 10−100 Ωm are indicated for this unit by electrical logging. The high resistivity of R2 is inconsistent with the measured resistivity of the Miocene lavas. However, the presence of low-porosity volcanic rocks, such as intrusive rocks associated with older magmatic activity, would explain the resistive feature. Previous MT studies conducted on the Asama and Kirishima volcanoes have interpreted the resistive body as old solidified magma (Aizawa et al., 2008; Aizawa et al., 2014). At Kusatsu-Shirane, geological and petrological studies suggest that the

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volcanic center of Matsuwozawa (~0.6 Ma) was located to the east of R2, on the western flank of Mt. Motoshirane (Hayakawa and Yui, 1989; Kaneko et al., 1991; Fig. 1). Therefore, the resistive body

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found beneath the western flank of Mt. Motoshirane might have been produced by the previous

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magmatic activity that produced Matsuwozawa. However, the distribution of MT sites for our survey

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was sparse, so it is difficult to constrain the extent of C2 and R2 precisely. A higher density of measurement sites is required to clarify the complex subsurface structure beneath the western flank

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of the volcano, which includes the Manza area hydrothermal system (Uto et al., 1983). This work

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should be performed in the future.

Similarly, our resistivity structure model may provide new information on the 2018 eruption at Mt. Motoshirane. The small scale of the phreatic eruption suggests that explosions occurred at shallow depths, but the sparse distribution of our MT sites and the absence of data at frequencies of >320 Hz mean that the spatial resolution of the model in the shallow subsurface is insufficient to explore the cause of the eruption in detail. One possible approach for constraining the eruption mechanism is to conduct a denser AMT survey. Since few geothermal manifestation is observed around the summit of Mt. Motoshirane, the hydrothermal fluid is expected to be confined by a less permeable structure such as the conductor C1 that is interpreted as a clay-rich layer. A characteristic resistivity structure,

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in which relatively high-resistive zone interpreted as a hydrothermal reservoir is surrounded by conductors, has been actually revealed by the AMT survey in other volcanoes (e.g. Yamaya et al., 2013; Seki et al., 2016; Tsukamoto et al., 2018).

Sensitivity tests indicate that the average resistivity of the deep structures is of the order of several tens of Ωm, or greater, and that large conductors (the order of several km) do not occur within the

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deep subsurface (Figs. S3−S4). Previous workers have suggested that a sub-vertical conductive body, interpreted as a magma conduit, exists beneath the volcanic center in other volcanoes (e.g., Aizawa

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et al., 2014; Comeau et al., 2015; Diaz et al., 2015; Hill et al., 2015; Peacock et al., 2015; Bedrosian

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et al., 2018). This feature does not occur in the Kusatsu-Shirane volcano; however, relatively small

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conductors (< ~1 km in extent) are difficult to detect if they are contained within a deeper structure,

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5. Conclusion

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so we cannot exclude the possibility that a magma chamber exists beneath Mt. Motoshirane.

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We investigated the subsurface resistivity structure of the Kusatsu-Shirane volcano, with a focus on Mt. Motoshirane. Mt. Motoshirane produced repeated magmatic eruptions until 1500 years ago and also erupted in 2018. A 3-D inversion of broadband MT data measured in 2015 and 2016 detected several conductive features (< 10 Ωm). Most part of conductive layer that occurs discontinuously over a broad area of shallow depths (< 1 km) on the eastern flank of Mt. Motoshirane is considered to be a conductive clay-rich layer, which is consistent with the conclusions of a previous AMT study. A part of this conductive feature around the Sessho fumarolic area could be interpreted as the zone containing acidic volcanic fluids rather than clays. A highly conductive layer at depths of 1–3 km, which extends from beneath the Yugama crater to Mt. Motoshirane, is reported for the first time in

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this study. Based on our data and the results of previous geochemical and geophysical studies, we interpret this layer as a hydrothermal fluid reservoir that supplies fluids to the Yugama crater lake and hot springs on the eastern and western flanks of the volcano. A heat source is necessary to drive hydrothermal fluid migration beneath this conductor, but the final 3-D resistivity structure model did not reveal a structure that could be interpreted as a heat source in the deeper parts of the model. However, the absence of such a feature may be an artifact of the low spatial resolution of the model.

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Two resistive bodies (> 100 Ωm) were found, but the interpretation of these bodies is tentative and incomplete; additional surveys over longer periods and over a wider frequency range are required to

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constrain the nature of these bodies.

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The results provide only limited information on the small phreatic eruption that occurred at Mt. Motoshirane in 2018. However, the C2 conductor that is interpreted as a hydrothermal fluid reservoir

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occurs over an area that extends from the Yugama crater to Mt. Motoshirane. This implies that a

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large-scale hydrothermal system has developed beneath the volcano, so the 2014 activity around the

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Yugama crater may have directly or indirectly triggered the 2018 eruption of Mt. Motoshirane. The data from Mt. Motoshirane are insufficient to confirm a link between volcanic activity at the Yugama crater and at Mt. Motoshirane, and further work is necessary to clarify the relationship between these two areas.

Funding This study was supported by the Joint Usage/Research Program of the Earthquake Research Institute, the University of Tokyo (ERI JURP 2015-G-10 and 2015-G-11), and by the MEXT Integrated Program for Next Generation Volcano Research and Human Resource Development (Y. Morita).

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Acknowledgments We are grateful to Nittetsu Mining Consultants for providing us with reference data from the Sawauchi MT station, Iwate Prefecture. The Kusatsu town office, the Agatsuma District Forest Office, the Manza Park Rangers Office, and the Kanto Regional Environment Office granted approvals for the field surveys documented in this study. Special thanks are offered to Weerachai

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Siripunvaraporn for provision of the 3-D inversion codes. The original manuscript was greatly improved by constructive comments from two anonymous reviewers. We also thank T. G. Caldwell

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for his valuable comments on the manuscript. Numerical computations were performed on the

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TSUBAME 2.5 supercomputer at Tokyo Institute of Technology. Most of the figures in this study

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Figure captions Fig. 1 Topographic map of the Kusatsu-Shirane volcano. Lavas and pyroclastic cones formed by the third eruptive stage, which started about 14,000 years ago and continues to the present (Hayakawa and Yui, 1989), are after Uto et al. (1983). The locations of major fumaroles and hot springs are shown.

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The two solid lines are the projected profiles across the final resistivity model (i: Figs 6 and 8a; ii: Fig. 7, iii: Fig. 8b). Green circles: MT sites used in this study. Black star: location of the

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Matsuwozawa volcano proposed by Hayakawa and Yui (1989). Red star: location of the 2018

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eruption vent. Boreholes used in previous geothermal investigations (55GT-1, 55GT-2, 53Isz-1,

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53Isz-2, and 50SN-1; Gunma Prefecture, 1989) are shown on the southern flank of Mt. Motoshirane.

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Fig. 2

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km northwest of Tokyo.

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The topographic contour interval is 50 m. Inset: Location of the Kusatsu-Shirane volcano, about 150

Comparison between the observed (left) and calculated responses (right) for the phase tensors and the real induction vectors (Parkinson’s convection) at five frequencies (97, 11, 1.4, 0.17, and 0.02 Hz). Each ellipse is normalized by its major axis (max) and filled with a color indicating the magnitude of the skew angle . The star indicates the location of the 2018 eruption vent. Y: Yugama crater, A: Mt. Ainomine, M: Mt. Motoshirane, B: Bandaiko hot spring. The topographic contour interval is 50 m.

Fig. 3

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Maps of the observed apparent resistivity (a) at five frequencies (97, 11, 1.4, 0.17, and 0.02 Hz). The apparent resistivity was calculated using a rotational invariant, det(Re(Z)) (Szarka and Menvielle, 1997). Symbols and labels are as for Figure 2. The topographic contour interval is 100 m.

Fig. 4 Horizontal sections through the final resistivity model. Depths below sea level are shown in the

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top-right corner of each plot. White dots indicate the locations of the MT sites. Symbols and labels

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are as for Figure 2. The topographic contour interval is 100 m.

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Fig. 5

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Vertical E–W sections through the final resistivity model. The six blue lines in the inset map indicate the locations of the sections. Star: location of the Matsuwozawa volcano proposed by Hayakawa and

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Yui (1989).

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Fig. 6

Regions used for the sensitivity tests. The upper diagram (a) shows a vertical section constructed along the solid black line shown in the map of the lower diagram (b). Resistivities were modified in the regions enclosed by the white solid lines on the vertical section and in those enclosed by the solid blue lines on the lower map. Green-filled circles in (b) and triangles with numbers in (a) indicate the locations of the MT sites.

Fig. 7

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North–south-oriented vertical section through the final 3-D resistivity model along the solid line (ii) shown in Figure 1. Triangles indicate the locations of the MT sites. The labeled vertical lines show borehole locations. The inferred geological units are superimposed on the resistivity model. The projection of the earthquake hypocenter distribution onto the resistivity section is also shown (black dots; Mori et al., 2006).

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Fig. 8

(a)Top: Bouguer gravity anomaly along line (i) calculated from the gravity database of the

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Geological Survey of Japan, AIST (Miyakawa et al., 2015), with a reduction density of 2.67 g/cm3.

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Bottom: Resistivity cross-section through the final resistivity model along line (i) in Figure 1.

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Triangles indicate the locations of the MT sites. (b)Top: Bouguer gravity anomaly along line (iii) in

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Fig. 9

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Figure 1. Bottom: Resistivity cross-section along line (iii) in Figure 1.

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Schematic illustration of the hydrothermal system that supplies the hot springs on both flanks of the volcano. (a): plan view, (b): E–W section. The peach fill color indicates the horizontal extent of C2 at around sea level. The blue arrows indicate the flow of meteoric water infiltrated into the ground.

Table 1 Temperature, pH, and total discharge rate of hot springs around Kusatsu-Shirane Volcano. Data are from Hirabayashi and Mizuhashi (2003).

Journal Pre-proof Yasuo Matsunaga: Conceptualization, Investigation, Formal analysis, Writing - Original Draft. Wataru Kanda: Conceptualization, Investigation, Writing - Review & Editing, Funding acquisition. Shinichi Takakura: Investigation, Resources. Takao Koyama: Investigation, Resources. Zenshiro Saito: Investigation. Kaori Seki: Investigation. Atsushi Suzuki: Investigation. Takahiro

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Kishita: Investigation. Yusuke Kinoshita: Investigation. Yasuo Ogawa: Investigation.

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be

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considered as potential competing interests:

Journal Pre-proof Table 1 Temprature, pH, and total discharge rate of each hot springs (Hirabayashi and Mizuhashi, 2003) Temprature

pH

Total discharge rate

Location

Date (m2/s)

(ºC) 38.9 - 52.9

1.95 - 2.15

9.30×10-2

2001/7/25

Bandaiko

94.8

1.51

1.55×10-1

2001/8/11

Manza

32.3 - 85.8

1.46 - 4.45

2.16×10-2

2001/7/27

Kagusa

55.7 - 63.6

1.12 - 1.35

9.5×10-4

2001/8/9

Yugama (lake water)

27.7

1.29

-

2001/8/20

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Kusatsu

Journal Pre-proof Highlights We conducted broadband magnetotelluric (MT) measurements in Kusatsu-Shirane volcano

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A 3-D electrical resistivity structure was inferred from the MT data

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We found an extensive low-resistivity layer at depths of 1–3 km in the summit area

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Conductor was interpreted as the source of crater-lake water and hot springs

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