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Title

Imaging the hydrothermal system beneath the Jigokudani valley,
Tateyama volcano, Japan: Implications for structures controlling
repeated phreatic eruptions from an audio-frequency magnetotelluric
surveyAuthors

Seki, K., Kanda, W., Ogawa, Y., Tanbo, T., Kobayashi, T., Hino, Y.,
Hase, H.

Citation

Earth, Planets and Space, Vol. 67, ,Pub. date2015, 1Creative Commons

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Seki et al. Earth, Planets and Space (2015) 67:6
DOI 10.1186/s40623-014-0169-8

LETTER

Open Access

Imaging the hydrothermal system beneath the
Jigokudani valley, Tateyama volcano, Japan:
implications for structures controlling
repeated phreatic eruptions from anaudio-frequency magnetotelluric survey
Kaori Seki1* , Wataru Kanda2 , Yasuo Ogawa2 , Toshiya Tanbo3 , Tomokazu Kobayashi4 , Yuta Hino1,5
and Hideaki Hase2,6
Abstract
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 andgeothermal activity and to elucidate the geologic controls on both its phreatic eruption history and recent increase inphreatic activity. The AMT data were collected at eight locations across the Jigokudani valley in September 2013, withhigh quality data obtained from most sites, enabling the identification of an underground 2D resistivity structure fromthe 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 tobe 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 theexistence 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 thesegranitic 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.
Keywords: Resistivity structure; AMT; Tateyama volcano; Phreatic eruption; Hydrothermal system; Clay cap

Findings
Introduction

Phreatic eruptions, or hydrothermal eruptions, occur
through the intense heating of groundwater by a hightemperature 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).
*Correspondence: seki.k.ad@m.titech.ac.jp
1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
Full list of author information is available at the end of the article

Phreatic eruptions are intrinsically connected to thehydrothermal systems present within volcanic edifices,
given that these eruptions often contain ash consisting ofhydrothermally altered minerals (e.g., Ohba and Kitade2005). In addition, volcanic hydrothermal systems areoften associated with low permeable clayey layers thatconsist 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© 2015 Seki et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly credited.

Seki et al. Earth, Planets and Space (2015) 67:6

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 beimaged 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 bymagnetotelluric surveys, including Kusatsu–Shirane volcano in Japan (Nurhasan et al. 2006) and Taal volcano in
the Philippines (Yamaya et al. 2013). A significant numberof these volcanoes are associated with near-surface conductive anomalies that have been interpreted to be low
permeable clayey layers.
Tateyama volcano (also called Midagahara volcano)is a composite volcano located in the Hida Mountains(Figure 1). The most significant volcanic activity associated with this volcano occurred during the Late Pleistocene (after 0.22 Ma; Harayama et al. 2000). Volcanic
edifices in this region are dominated by gneiss and granite units (Yamasaki et al. 1966). The Jigokudani valley is

Page 2 of 9an active solfatara field located in the northeast end of theMidagahara Plateau and is dominated by generally welded
pyroclastic flow deposits that were erupted at about 0.1
Ma. The Jigokudani valley formed as a result of repeatedphreatic eruptions some 40,000 years ago, during the laststage of volcanic activity in this area (Yamasaki et al. 1966;
Harayama et al. 2000). Figure 1 shows the geology of the
Jigokudani valley region (simplified from Harayama et al.
2000). The explosion craters around the Jigokudani valley define a NW–SE trend, as shown by dotted lines in
Figure 1, which show the traces of these craters. Although
the volcano has not erupted in historical times, four individual phreatic eruptions over the past 10,000 years havebeen identified from air-fall tephras (Kobayashi 1980).
Nakano and Itoh (1998) investigated the historic documents and inferred that a phreatic eruption occurred in
the Jigokudani valley in 1836 but was too small to produce
tephra deposits. In addition, the location of the magma
reservoir associated with the Tateyama volcano has been
seismically inferred to be a few kilometers southeast of
the Jigokudani valley at a depth of 2 to 4 km (Matsubara
et al. 2000). A low resistivity area was also detected by

Figure 1 A simplified geological map of the area around the Jigokudani valley, adapted from Harayama et al. (2000). The hatched area
represents the Jigokudani valley, and the dotted lines indicate the traces of hydrothermal eruption craters, some of which are connected one
another by repeated eruptions and have ill-defined borders. The magmatic eruptions that formed lavas and pyroclastic flow deposits in this area
occurred in the southeast of this map. Inset: Map showing central Japan, with red triangles indicating the location of active volcanoes, a gray shaded
ellipse representing the location of the Hida Mountains, and a dark blue triangle showing the location of the Tateyama volcano in the northern partof the Hida Mountains.

Seki et al. Earth, Planets and Space (2015) 67:6

data acquired from a wide-band magnetotelluric survey
at depths corresponding to this seismic anomaly (Ogawa
et al. 2002).
Several recent anomalous events have been repeatedly
observed in the study area, including a sulfur outflow in2010 (Japan Meteorological Agency 2013a) and fumaroles
with temperatures that exceed the boiling point of water,
reaching a maximum of about 140°C (Japan Meteorological Agency 2013b). These fumarole vent gases havehigh HCl/SO2 ratios, indicating that these gases were
derived from high-temperature sources (Tokyo Instituteof 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 resultsof 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.
Data

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 104 Hz) of theelectromagnetic 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 theelectric 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 thatdeparts from the underground Murodo terminal (at thesouthern edge of Figure 1), and commercial 60 Hz frequency power supplied to lodges in this area. AC power isalso supplied to the Jigokudani valley to monitor volcanic
gases in this area.
Apparent resistivity and phase sounding curves for
all the sites used during this study are shown inPage 3 of 9

Figure 2. High-quality data were obtained at most sites
for frequencies higher than 10 Hz, although the lower
frequency data obtained at several sites have a tendency
to have XY phase components that increase rapidly as
frequencies decrease (e.g., sites 01 and 08), providing evidence of the 3D nature of the underground structure
and/or the presence of local noise effects that cause breaks
in plane wave approximations. The dead band (about
1,000 to 3,000 Hz) sometimes contains low quality data,primarily as a result of the weakness of the natural-source
signal, a factor that in turn influences the quality of thedata obtained, particularly at site 03. The apparent resistivity at sites 04 to 06 is further impaired by the 60-Hz
power supply in this area, although the apparent resistivity
at sites in the Jigokudani valley (sites 04 to 06) is relativelylow.
2D analysis
Dimensionality of structures in the study area

Phase tensor analysis (Caldwell et al. 2004) was applied tothe entirety of the dataset obtained during this study todetermine the subsurface resistivity structure within the
study area. The phase-tensor skew angle (β) represents a
measure of the dimensionality of a structure and is zerofor 1D or 2D structures but increases in value as data
are influenced by the presence of 3D structures (Caldwell
et al. 2004). Figure 3 shows color-filled phase-tensor
ellipses that indicate the skew angle for three representative frequencies. The majority of the skew angle magnitudes for all of these sites are less than 5°, with the
exception of data obtained at low frequencies (<10 Hz),
indicating that the Jigokudani valley has a 2D subsurface
structure that is identifiable from the AMT data.
The 2D nature of the structures in the study area isalso constrained by the shapes of phase-tensor ellipses
and induction arrows (Figure 3). The major axes of thephase-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, theseellipses become more circular at higher frequencies (see1,100 Hz in Figure 3c). In addition, real induction arrows(Parkinson 1962; Figure 3), which intrinsically tend topoint toward lower resistivity areas, do not appear to haveany 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 thelow frequency data obtained during this study.
Strike estimation

The above discussion suggests that the study area has
a 2D resistivity structure, and we have estimated theSeki et al. Earth, Planets and Space (2015) 67:6

Page 4 of 9

Figure 2 Apparent resistivity and impedance phase. Apparent resistivity (upper panel: app. res.) and impedance phase (lower panel: phase)observed at all the sites in the study area. Impedance tensor (ZXY and ZYX ) off-diagonal components are shown in red and blue, respectively.

strike direction of this 2D structure using phase-tensor
parameters. One of the principal axis directions of thephase-tensor ellipse coincides with the 2D strike in ideal
2D cases (Caldwell et al. 2004). Figure 4 shows polar
histograms of the principal axis directions for three frequency bands obtained during this study; the lower frequency band (1 to 100 Hz; Figure 4b) is dominated by
N30°W and N60°E directions, whereas the higher frequency band has N and E dominated directions (100 to
10,000 Hz; Figure 4c). A histogram for data from all frequency bands (1 to 10,000 Hz; Figure 4a) also indicatesa similar directional property to that seen in Figure 4b.
However, this analysis is inherently ambiguous by 90°,
meaning that the geology of this area (Figure 1) and the
distribution of induction arrows (Figure 3) must also be
considered. A simplified geological map for the study areashows that the Tamadono lavas have a preferential SE
to NW orientation and cross-cut the Jigokudani valley(Figure 1), with a similar tendency present in the distribution of pyroclastic flow deposits below these lavas. In

addition, as stated above, the low frequency induction
arrows are focused toward a central area along an orientation that nearly parallels the survey line. This evidence
strongly suggests that the 2D structure in this area has a
2D strike of N30°W.
2D modeling

The impedance tensor was rotated into the assumed strikedirection (N30°W) before being decomposed into transverse magnetic (TM) and transverse electric (TE) modes.
Observation sites were projected onto a S60°W-N60°E
line, orthogonal to the strike of the structure in this area.
The data with large phase-tensor skew angles are inconsistent with the 2D structure assumed for the area; therefore,
only the data with small skew angles data (< 5°) were usedin the 2D inversion undertaken during this study. This 2D
inversion model was constructed using the inversion codedeveloped by Ogawa and Uchida (1996), using TM-mode
data with a 3% error floor and did not include any topographical influence. The best-fit model shown in Figure 5

Seki et al. Earth, Planets and Space (2015) 67:6

Page 5 of 9

Three characteristic features are present in the final
model (Figure 5); a conductive body (less than 10 m)
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 thenortheast 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 phasepseudo-section.
We carried out sensitivity tests against three featuresin the resistivity distribution and determined that thesefeatures were sufficiently sensitive. One example (a test
against C3) is shown in Figure 7. The sounding curves
for both sites obtained from the modified model (green
lines in Figure 7b) show significant deviations from theobserved responses (red dots) at frequencies lower than
100 Hz. This indicates that a relatively conductive zone(C3) is certainly present.
Discussion
Origin of the shallow conductor

Figure 3 Spatial distribution of phase-tensor ellipses and realinduction arrows. Spatial distribution of phase-tensor ellipses andreal induction arrows in the same area as is shown in Figure 1.
Phase-tensors and induction arrows at frequencies of (a) 9.4 Hz, (b)97 Hz, and (c) 1,100 Hz are shown. The magnitude of thephase-tensor skew angle (|β|) increases as the color of the ellipses
changes from warm to cold colors.

has a root mean square (RMS) misfit of 0.85. Pseudosections for the measured and calculated responses arecompared in Figure 6, with both figures agreeing well,
except in areas without data.

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 m extends beneath a significant part of
the Jigokudani valley (C1 and C2 in Figure 5); this nearsurface conductor is likely separated into upper and lower
sections, with the former having a convex shape and ismore 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 brecciasthat are exposed on the hill side of the valley and has avalley side where these breccias are overlain by laminatedlacustrine sediments (Kouno 1988). The distribution ofsolfataras 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)

Seki et al. Earth, Planets and Space (2015) 67:6

Page 6 of 9

Figure 4 Polar histograms for phase-tensor ellipse principal axis directions at all sites. Histograms for (a) all frequency bands, (b) lower
frequency bands (1 to 100 Hz), and (c) higher frequency bands (100 to 10,000 Hz) are shown. Sector width is 5°, and the radial scale is normalized by
the maximum value. Note that the directions have an intrinsic ambiguity of 90° as discussed in the main text.

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 theseupper 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 thisarea (Japan Meteorological Agency 2013b), which indicates the existence of the superheated vapor beneath the
Jigokudani valley. The apparent equilibrium temperatureof fumarolic gases estimated from the sulfur reaction suggests that the volcanic gases at shallow levels under the

Figure 5 Final 2D resistivity model along the S60°W-N60°E line.
Inverted triangles denote the AMT sites used for inversion, with threemajor fumarolic zones denoted by the cartoons of gray smoke.

Jigokudani valley are present at temperatures of 200°C
to 340°C (Mizutani et al. 2000). The capping of thesegases 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 thatwould either support or disprove this hypothesis. However, it should be noted that the active fumaroles discharging superheated steam in the Jigokudani valley arelocated 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 widespreadthroughout 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 theshallowing of the R2 resistive layer in this area.
This interpretation is supported from geochemical
studies. The fumarolic gases within the Jigokudani valley
have high 3 He/4 He 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 toseparate the resistive granitic basement into two zones.
Kobayashi and Hatano (1989) discussed the relationshipbetween 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Seki et al. Earth, Planets and Space (2015) 67:6

Page 7 of 9

Figure 6 Comparison of pseudo-sections between the final 2D model responses (Figure 5) and the observed data. Left and right columns
represent observed and calculated responses, respectively, with the upper chart showing TM-mode phase data and the lower chart showing
TM-mode apparent resistivity data. Inverted triangles denote the AMT sites used for inversion (see Figure 1).

the Midagahara Plateau to the Jigokudani valley and has
a length of approximately 7 km. The other is a 9-km-longfault extending nearly south-southeastward from around
the Jigokudani valley. These two faults are orthogonal toeach 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 andtheir associated eruptive activities were located along a
NW-SE oriented principal compression axis in this area
(Kobayashi and Hatano 1989). This model suggests thatthe area around the Jigokudani valley underwent NW-SEdirected compression that fractured the rocks in this area
as a result of strain accumulation between faulting events.
These fractures then led to interaction between nearsurface 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 magmaticgases and most likely formed as a result of the fracturing outlined above. The heat source, and hence the sourceof 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 thispossible magma chamber.

Conclusions

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 ofan 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 ofrather resistive volcanic gases. The less permeable natureof the clay in this area means that the upper clayey sediments act as a cap that stores the magmatic gases in thelower part of this zone.
(2) A deep resistive layer that is present at depths below
500 m and is interpreted to represent granitic rocks thatare widespread throughout the Jigokudani valley.
(3) A relatively conductive region that extends subvertically toward the shallow conductor and separates thedeep 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.

Seki et al. Earth, Planets and Space (2015) 67:6

Page 8 of 9

Figure 7 Results of the sensitivity test against a relatively conductive body (C3). (a) The resistivity of a region enclosed by the white dashedline was changed from the values of the best-fit model shown in the background to 200 m. (b) Sounding curves obtained from the modified
model (green line) are shown together with observed responses (red dots) and the best-fit curves (blue line). TM-mode apparent resistivity (upper)
and phase (lower) for sites 02 (right) and 03 (left) are shown.

The three features of the resistivity structure across thestudy area define a model where magmatic gases are supplied from a deep source to the Jigokudani valley, an areathat has undergone numerous phreatic eruptions. Further
research, including 3D modeling with denser AMT sites,
is required to confirm this preliminary model.
Competing interests
The authors declare that they have no competing interests.

Authors’ contributions
All authors participated in the acquisition of the data used during this study.
KS analyzed the data, constructed the 2D resistivity model, and drafted themanuscript. WK conceived the study and revised the manuscript. All authors
read and approved the final manuscript.
Acknowledgements
We are deeply grateful to the Tateyama Caldera Sabo Museum and the
Tateyama Nature Conservation Center for their cooperation during ourresearch. The Chubu Region Environmental Office, the Toyama District Forest
Office, and Toyama Prefecture granted permission for the AMT survey

Seki et al. Earth, Planets and Space (2015) 67:6

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 twoanonymous referees and the Editor-in-Charge (H. Toh) were highly helpful toimprove the manuscript. All of the figures used in this paper were preparedusing Generic Mapping Tools (Wessel and Smith 1998). This study was
supported by JSPS KAKENHI Grant Number 25350494.
Author details
1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan. 2 Volcanic Fluid ResearchCenter, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo
152-8551, Japan. 3 Tateyama Caldera Sabo Museum, Ashikuraji, Tateyama,
Toyama 930-1406, Japan. 4 Geospatial Information Authority of Japan,
Kitasato-1, Tsukuba, Ibaraki 305-0811, Japan. 5 Current address: NS Solutions
Tokyo Corp., Shinkawa-2, Chuo, Tokyo 104-8280, Japan. 6 Current address:
Geothermal Energy Research & Development Co., Ltd., Shinkawa-1, Chuo,
Tokyo 104-0033, Japan.
Received: 5 September 2014 Accepted: 9 December 2014

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