論文 / 著書情報
Article / Book Information
Title

Three-dimensional resistivity structure and magma plumbing system of
the Kirishima volcanoes as inferred from broad-band magnetotelluric
data

AuthorAizawa, K., Koyama, T., Hase, H., Uyeshima, M., Kanda, W., Utsugi,
M., Yoshimura, R., Yamaya, Y., Hashimoto, T., Yamazaki, K., Komatsu,
S., Watanabe, A., Miyakawa, K., Ogawa, Y.

Journal/Book name

Journal of Geophysical Research: Solid Earth, 119, ,Issue date2014, 1

DOI

http://dx.doi.org/10.1002/2013JB010682

権利情報 / Copyright

Copyright (c) 2014 American Geophysical Union (AGU)

Powered by T2R2 (Science Tokyo Research Repository)

198‒215

PUBLICATIONSJournal of Geophysical Research: Solid EarthRESEARCH ARTICLE
10.1002/2013JB010682
Key Points:• First 3-D inversion of the anomalous
magnetotelluric data
• Comparison of the structure with deformation source and seismicity• Magma plumbing system deduced
from above comparison

Three-dimensional resistivity structure and magma
plumbing system of the Kirishima Volcanoes
as inferred from broadband
magnetotelluric data
Koki Aizawa1,2, Takao Koyama1, Hideaki Hase1,3, Makoto Uyeshima1, Wataru Kanda3, Mitsuru Utsugi4,
Ryokei Yoshimura5, Yusuke Yamaya1,6, Takeshi Hashimoto7, Ken’ichi Yamazaki8, Shintaro Komatsu8,
Atsushi Watanabe1, Koji Miyakawa1, and Yasuo Ogawa3
1

Correspondence to:K. Aizawa,
aizawa@sevo.kyushu-u.ac.jp

Citation:
Aizawa, K., et al. (2014), Three-dimensional
resistivity structure and magma plumbing
system of the Kirishima Volcanoes as inferred from broadband magnetotelluric
data, J. Geophys. Res. Solid Earth, 119,doi:10.1002/2013JB010682.
Received 12 SEP 2013
Accepted 7 DEC 2013
Accepted article online 18 DEC 2013

Earthquake Research Institute, University of Tokyo, Tokyo, Japan, 2Institute of Seismology and Volcanology, Faculty of
Sciences, Kyushu University, Shimabara, Japan, 3Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo,
Japan, 4Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University,
Kumamoto, Japan, 5Disaster Prevention Research Institute, Kyoto University, Uji, Japan, 6Geological Survey of Japan,
National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, 7Institute of Seismology andVolcanology, Faculty of Science, Hokkaido University, Sapporo, Japan, 8Miyazaki Observatory, Disaster Prevention ResearchInstitute, Kyoto University, Miyazaki, Japan

Abstract Broadband magnetotelluric (MT) measurements were conducted in 2010 and 2011 in the vicinityof Shinmoe-dake Volcano in the Kirishima volcano group, Japan, where sub-Plinian eruptions took place 3 times
during 26–27 January 2011. By combining the new observations with previous MT data, it is found that an
anomalous phase in excess of 90° is commonly observed in the northern sector of the Kirishima volcano group.
Because the anomalous phase is not explained by 1-D or 2-D structure with isotropic resistivity media, 3-D
inversions were performed. By applying small errors to the anomalous phase, we successfully estimated a 3-D
resistivity structure that explains not only the normal data but also the anomalous phase data. The ﬁnal modelshows a vertical conductor that is located between a deep-seated conductive body (at a depth greater than
10 km) and a shallow conductive layer. By applying the ﬁndings of geophysical and petrological studies of the2011 sub-Plinian eruptions, we infer that the subvertical conductor represents a zone of hydrothermal aqueous
ﬂuids at temperatures over 400°C, in which a magma pathway (interconnected melt) is partially andoccasionally formed before magmatic eruptions. To the north of the deep conductor, earthquake swarms
occurred from 1968 to 1969, suggesting that these earthquakes were caused by volcanic ﬂuids.

1. Introduction
In the southern part of Kyushu Island, Japan, the Philippine Sea Plate subducts to the WNW at a rate of
approximately 5 cm/yr [Seno et al., 1993]. In this area, active volcanoes with large calderas are distributed
along a north–south trending volcanic chain (Figure 1). The Kirishima volcano group (the Kirishima
Volcanoes) is situated at the northernmost end of the southern Kyushu volcanic chain and consists of morethan 20 Quaternary volcanic cones within an area of ~20 × 30 km. Two calderas (Kakuto and Kobayashi) arelocated in the northern sector of the Kirishima Volcanoes, where some 100 km3 of magma was extruded
between ~300 and ~500 kyr ago [Tajima and Aramaki, 1980; Imura, 1994].
The Kirishima volcano group started to increase in extent after the formation of the calderas. Although thevolcanic cones are distributed roughly from NW to SE, a systematic shift of the eruption center is not recognized in its 300 kyr history (i.e., volcanic cones seem to be randomly located); therefore, the locations ofmagma chambers are not indicated from geological studies. Beneath the Kakuto caldera, ﬁve earthquake
swarms have been observed since 1913 [Miyazaki et al., 1976]. The 1968–1969 events were characterized bymore than 5000 earthquakes at a depth of 3–9 km, with a largest magnitude of 6.1 [Minakami et al., 1969].
To investigate the relationship between earthquakes and volcanic ﬂuid (presumably magma beneath Kakuto
caldera), Goto et al. [1997] conducted a series of broadband magnetotelluric (MT) surveys. However, the authors
discovered that the hypocentral area is in fact highly resistive and considered it unlikely that a large mass of
ﬂuid is located in the hypocentral area.
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130°

140°

40°

500 km30°

5 cm/year

Figure 1. Map showing the locations of major calderas and active volcanoes in southern Kyusyu, Japan (taken from Tajima and Aramaki[1980]). Inset shows the location of the ﬁgure within Japan. Solid circles indicate inferred positions of the vents of pyroclastic ﬂows
related to caldera formation. Hatched areas are active volcanoes. The solid rectangle indicates the area of Figure 2. The arrow shows the
subduction direction of the Philippine Sea Plate.

The most recent eruptions of the Kirishima Volcanoes occurred in the central and southeastern sectors of thegroup. Since AD 742, documented eruptions have taken place at three volcanoes: Iwo-Yama (altitude 1310 m),
Shinmoe-dake (1421 m), and Ohachi (1408 m) (Figure 2). Since the 1923 eruption of Ohachi Volcano, all other
eruptions (i.e., the 1959, 1962, 1991, 2010, and 2011 eruptions) have occurred at Shinmoe-dake Volcano [Imuraand Kobayashi, 1991; Tsutsui et al., 2005; Nakada et al., 2013], with the 2011 eruptions being the largest on
record in the group since a similar-scale eruption at Shinmoe-dake in 1717. The 2011 event started with a seriesof small eruptions between 19 and 22 January, followed by three sub-Plinian eruptions during 26–27 January
and the subsequent emergence of lava within the crater [Nakada et al., 2013]. Synchronous with the sub-Plinian
eruptions, tiltmeters, and global positioning system instruments detected ground subsidence in an area locatedapproximately 7 km WNW of Shinmoe-dake (Figure 2), at an estimated source depth of ~10 km [Kozono et al.,2013; Nakao et al., 2013; Ueda et al., 2013]. The estimated volume of the deﬂation was about 0.016 km3 [Kozono
et al., 2013; Ueda et al., 2013], which is within 1 order of magnitude of the amount of erupted material observed(0.021–0.027 km3 dense rock equivalent [Nakada et al., 2013]). At approximately the same location as the syneruptive deﬂation, inﬂation was observed from February 2010 to January 2011 and from February 2011 to
October 2011, which suggests that magma had accumulated at the source of the deformation both before andafter the late January 2011 eruptions [Nakao et al., 2013].
The 2011 syn-eruptive ground subsidence is regarded as clear evidence of subsurface magma migration, and its
characteristics have been studied in detail. Signiﬁcantly, a time lag was not detected between the subsidence
and the development of the eruption plumes [Kozono et al., 2013]. This suggests that a magma pathway existed
prior to the 2011 sub-Plinian eruptions. The existence of such a pathway is also supported by the fact that theprecursory increase in seismicity was not observed in advance of the 2011 sub-Plinian eruptions.
Although the magma pathway has not been imaged, some studies have investigated various aspects of themagma chamber beneath the Kirishima Volcanoes. Based on hypocenter distributions, Ida et al. [1986]

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F

A

A’

B

B’

C

C’

D

D’

E

E’

32°00'

31°54'

10130˚48'km
130°42'

F’130°54'

131°00'Figure 2. Broadband magnetotelluric (MT) observation sites at the Kirishima volcano group. The topographic contour interval is 200 m.
The hypocentral area of the 1968–1969 Ebino earthquake swarm, whose epicenter depth is 3–9 km [Minakami et al., 1969], is represented by the shaded grey area. The dashed blue circle indicates the possible location of the syn-eruptive deformation source (Ueda
et al., 2013; Kozono et al., 2013). The blue lines indicate the locations of transects along which vertical slices (Figure 12) of the ﬁnal 3-D
resistivity structure were taken.

speculated that a magma chamber is located NW of the Kirishima Volcanoes. On the basis of magnetotelluric(MT) surveys using the ultralow frequency band (over 10 s) and 1-D inversion, Utada et al. [1994] andKagiyama et al. [1997] suggested that magma does not exist beneath the SE sector of the Kirishima Volcanoes
but does exist beneath the NW sector. Yamamoto and Ida [1997] studied the P wave seismic structure andfound a high-attenuation zone located NW of the Kirishima Volcanoes at a depth of 5 km. From the frequency
dependence of the attenuation, the authors suggested the existence of an 80 m thick sill-like magma bodythere. However, the spatial resolutions of these previous studies were coarse, as low as 4 km; consequently,
the volume and shape of the magma chamber remain unknown. The possible magmatic pathway, whichshould connect the Shinmoe-dake crater and the source of deformation of the syn-eruptive ground subsidence, was not suggested by these previous studies.
In this study, we examine the 3-D resistivity structure of the Kirishima Volcanoes estimated using a broadband (0.003–2048 s) magnetotelluric (MT) data set. Compared with the previous MT survey [Utada et al.,1994], the MT data set covers a wider area and greater depth range, and we investigate from the deep
magma chamber level up to the shallow hydrothermal system at high spatial resolution. Because melt
[Tyburczy and Waff, 1983; Gaillard and Marziano, 2005] and saline water [Nesbitt, 1993] are both highly
conductive, a broadband MT survey and 3-D data modeling are appropriate for investigating themagma and/or magmatic ﬂuid upwelling under active volcanoes [Hill et al., 2009; Ingham et al., 2009;
Heise et al., 2010; Bertrand et al., 2012; Kelbert et al., 2012].

2. MT Observations
From July to August 2010 and March to April 2011, we recorded naturally occurring geomagnetic ﬁelds(Bx, By, and Bz) and induced Earth electric ﬁelds (Ex and Ey) at 28 broadband MT observation sites (Figure 2) using
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a Metronix ADU07 system. The sampling frequencies were set at 32 Hz (00:00–23:50 UT) and 1024 Hz (17:00–
18:00 UT). The typical record duration was 2 weeks for one site. In addition to our surveys, previous broadband
MT data [Goto et al., 1997; Uchida and Sasaki, 2006], which were recorded by Phoenix V5 and MTU5 systems,
were used in this study. Taking into account that the erupted volume of magma is on the order of 0.01 km3, we
assume that the resistivity structure has not signiﬁcantly changed since those observations were made.
Although the measurement sites in 2010 and 2011 do not overlap with the previous sites, the continuous MT
observation at Iwo-Yama since May 2011 suggested that a resistivity change occurred only at a shallow level(less than several hundred meters) and its amplitude was subtle less than ±10% [Aizawa et al., 2013].
The survey of Goto et al. [1997] was conducted to examine the existence of ﬂuid in the hypocentral area of the
1968–1969 Ebino earthquake swarms. By 2-D inversion, the authors concluded that the hypocentral area
corresponded to a zone of high resistivity. The extremely dense broadband MT data of Uchida and Sasaki[2006] were obtained for exploration around the Ogiri geothermal power plant by the New Energy andIndustrial Technology Development Organization, Nittetsu Kagoshima Geothermal Co., Ltd., and the
Geological Survey of Japan during the period 1996–2000. By using the off-diagonal components of the
impedance tensor in the period range 0.01–10 s, the authors imaged the 3-D resistivity structure to a depth of
4 km and suggested that high-temperature (above 200°C) hot water is trapped beneath the conductive caprock located at 1 km depth.

3. Time Series Analysis
Although there are no electrical power lines near the MT sites, artiﬁcial 60 Hz noise (and associated odd-order
overtones at 180, 300, and 420 Hz) originating from distant commercial power lines is still observed in therecorded power spectra of all ﬁve measured components (Ex, Ey, Bx, By, and Bz). Because ADU07 is not
equipped with a notch ﬁlter, the commercial noise presents a serious problem to the estimation of MT
response functions. To reduce this noise, we applied a digital notch ﬁlter. A raw time series of data was ﬁrst
transformed to the frequency domain by applying a Fourier transform with a time window of 213 s. In thisstep, the data were multiplied by the frequency responses of the induction coil and logger. Then, theamplitudes of the 60 Hz odd-order overtones were reduced to 1/100 of their values, with a notch width of
0.1 Hz. Finally, the time series data with physical units of mV/km and nT were recovered using an inverse
Fourier transform. By applying this comblike notch ﬁlter, the quality of MT response functions was significantly improved, especially at frequencies near 60 Hz and at odd-order overtones. We conﬁrmed that thecalculated MT response functions are robust against the slight change of ﬁlter parameters (notch width,
reduce factor, and time window).
The MT response functions were calculated using a robust estimation code [Chave and Thomson, 2004]. At aperiod less than 10 s, we also employed remote reference processing [Gamble et al., 1979] using MT data
recorded at other MT sites. At a period over 10 s, the 1 Hz-sampled geomagnetic data recorded at the Kakioka
Magnetic Observatory (located 1000 km ENE of Kirishima) were used for remote reference processing. Using
these procedures, we obtained the MT response functions across a broad period range of 0.003–2048 s.
The frequencies of the MT response functions in 2010–2011 differ slightly from the previous MT data due tothe difference in sampling frequency. This discrepancy created problems for further analysis, including
inversion. Because MT response functions are inherently smooth with frequencies, we interpolated the MT
response functions and error bars of the previous data in the frequency domain by using a cubic spline
function. The MT response functions were then deﬁned at the frequencies for 2010–2011.

4. Characteristics of the Data (Anomalous Phase in Excess of 90°)
Before inversion was performed, the characteristics of the MT data were examined. Because the data aroundthe Ogiri geothermal power plant [Uchida and Sasaki, 2006] are too dense to analyze (Figure 2), we pickednine sites from these data. Figure 3a shows a map of the phase at a period of 512 s. The phase for Zxy showshigher values than Zyx and exceeds 90° in the northern sector of the Kirishima Volcanoes. Note that Zyx phases
are quite normal (0–90°) at all periods. Figure 3b shows an example of the sounding curve. The phase over90°, which is usually present in one of the off-diagonal components and at a lower frequency band, is sometimes referred to as an “anomalous” phase, because the data cannot be explained by 1-D or 2-D structure withisotropic resistivity media. The Kirishima case is especially unusual in that approximately half of the data show

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Journal of Geophysical Research: Solid Earth(a)

Zxy (512 s)

10.1002/2013JB010682

Zyx (512 s)

Phase (°)
180

32°00'

5

5

1

1

135

4

4

2

2
3

31°54'

90

3

45

0
130°42'Phase (°)

Apparent resistivity(ohm−m)(b)

130°48'Site 1130°54'131°00'Site 2Site 3Site 4Site 5

1000
100
10
1
0.1
0.01

Zxx
Zxy
Zyx
Zyy

180
90
0
−90
−180
−3 −2 −1 0

1

2

Log period (s)3

4 −3 −2 −1 0

1

2

Log period (s)3

4 −3 −2 −1 0

1

2

Log period (s)34−3 −2 −1 0

1

2

Log period (s)3

4 −3 −2 −1 0

1

2

3

4

Log period (s)Figure 3. (a) Phase map of the Kirishima Volcanoes for the period of 512 s. To enable comparison between Zxy and Zyx, 180° is added to the phase of Zyx. Open squares indicate themeasurement sites, with sites numbered 1–5 indicating the locations of the sounding curves shown in Figures 3b, 9, and 13. The three solid triangles indicate, from northwest to southeast,
the volcanoes of Iwo-yama, Shinmoe-dake, and Ohachi. (b) Examples of sounding curves for ﬁve sites. The anomalous phase that exceeds 90° is marked by solid ellipse.

an anomalous phase. Note that MT data recorded at the ocean ﬂoor sometimes shows phases less than 0°,
which can be modeled by 2-D structure including high-conductivity seawater above measurement sites[Constable et al., 2009; Worzewski et al., 2011].
There are two general explanations for an anomalous phase. One explanation assumes resistivity anisotropy.
Numerical calculations show that an anomalous phase occurs in the case where one anisotropic layer isoverlain by another local anisotropic body and the anisotropic strikes differ signiﬁcantly [Pek and Verner,
1997; Heise and Pous, 2003]. An alternative explanation assumes electric current channeling by particular 3-Dfeatures of the resistivity structure [Egbert, 1990]. For example, an L-shaped conductor generates current
channeling and produces an anomalous phase around the corner of the “L” [Ichihara and Mogi, 2009; Ichihara
et al., 2013]. The model also requires 3 ~ 4 orders of resistivity contrast between conductor and surroundings.
In this study, we adopt this latter idea to explain the anomalous phase found here, because there is nogeological evidence that suggests the presence of two overlain anisotropic layers beneath the northern
sector of the Kirishima Volcanoes.

5. 3-D Modeling5.1. 3-D Inversion
It is usually difﬁcult to explain an anomalous phase in real data, and previous studies have applied theforward approach rather than inversion [Pous et al., 2002; Lezaeta and Haak, 2003]. However, here we try
to explain the anomalous phase using an inversion because volcanic structures are usually too complex
to be modeled using the forward approach.

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100

0

0

(km)

(km)

10

−10
−100
−10

0

10

−100

0

(km)
−2000

100

(km)
−1000

0

1000

2000

Altitude (m)Figure 4. Mesh used in the inversion. The solid circles show the 53 MT measurement sites used in the inversion.

We inverted the MT response functions by using a 3-D code (WSINV3DMT) with an algorithm that transforms
the model renewal equation from model space to data space [Siripunvaraporn and Egbert, 2000;
Siripunvaraporn et al., 2005]. We used the latest version of WSINV3DMT [Siripunvaraporn and Egbert, 2009] inwhich the full impedance tensor (four complex components: Zxx, Zxy, Zyx, and Zyy) and geomagnetic transfer
functions (two complex components: Tx and Ty) provided the inputs for the 3-D inversion. Fifty-three MT sites
with good data quality were chosen, and the MT response functions in 16 periods (0.004, 0.008, 0.015, 0.05,
0.125, 0.4, 1, 3.2, 8, 25.6, 64, 128, 256, 512, 1024, and 2048 s) were used in the inversion. Therefore, basicallythere are 12 impedance/induction vector elements at 16 periods at each of the 53 sites.
The horizontal mesh size was set to 500 m in the area around the observation sites and was logarithmicallyincreased with increasing distance from the Kirishima Volcanoes. The model also takes into account topography (Figure 4). The vertical mesh size was set at 100 m from the highest point to 1 km beneath sea level(bsl), and the air was approximated by 108 Ωm blocks. Between 1 and 10 km bsl, we set the vertical mesh size
at 200–500 m. For depths below 10 km bsl, the mesh size was increased with increasing depth. The total meshconsisted of 66 × 71 × 58 units (with seven vertical air layers as the default setting) in the x, y, and z directions,
respectively. The calculation area was 234 × 202 × 208 km, in which the ocean and bathymetry were represented
by 0.25 Ωm blocks. The initial resistivity of the land was set as 100 Ωm. In each iteration, we replaced theprior model [Siripunvaraporn and Egbert, 2000] by the smallest-misﬁt model of the previous iteration.
In the chosen mesh, the MT response functions in the higher-frequencies cannot be correctly calculated. For
example, diagonal components of impedance tensor and geomagnetic transfer functions (Zxx, Zyy, Tx, and Ty)
approach zero as the frequency increases, because the code calculated the MT response functions at thecenter of each mesh. In order to practically avoid such kind of numerical problems, we omitted the part of the
diagonal components and geomagnetic transfer functions at periods of less than 1 s.
In an initial inversion procedure, we ﬁrst removed the noisy data by visual inspection and then assumed
constant 10% error bars for the impedance tensor and constant 20% error bars for the geomagnetic transfer
functions. By using the smoothing parameter (τ = 10 with δx = δy = δz = 0.1) in the model covariance matrix,
the root-mean-square (RMS) misﬁt had reduced from 29.7 to 2.74 after 13 iterations, and an apparently
feasible structure was produced. However, importantly, the calculated phase was distributed over 0–90° andfailed to explain the anomalous phase at all sites. This result did not change when we applied othersmoothing parameters τ, which is initially set in each inversion, in the model covariance matrix[Siripunvaraporn and Egbert, 2000]. Because the anomalous phase is observed at approximately half the sites,
the resistivity structure is regarded as unrealistic.
To construct a resistivity structure that explains the anomalous phase, we conducted another inversion using
the following two-step inversion procedure. First, the error bars of Zxy for the periods over 0.4 s, in which an
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Figure 5. Apparent resistivity maps of the Kirishima Volcanoes for comparison of observed data (“obs.”) with calculated response (“calc.”). The off-diagonal components (Zxy and Zyx) of the
impedance tensor are shown for representative periods of 1, 8, 64, and 512 s. The solid circles show the 53 MT measurement sites used in the inversion. Note that the noisy data areremoved in the inversion. The three triangles indicate, from northwest to southeast, the volcanoes of Iwo-yama, Shinmoe-dake, and Ohachi (Figure 2).

anomalous phase is observed, were set to 2%. The other error bars were set at 10% for Zxx, Zyx, Zyy, and 20%
for the geomagnetic transfer functions. Using the uniform values of 100 Ωm for land and 0.25 Ωm for ocean,
the inversion was conducted until the RMS misﬁt reached a stable level. Then, the error bars were recovered
to the normal level (10% for Z and 20% for T), and the inversion was restarted by taking the ﬁnal model of theﬁrst step as the prior and initial model for the next inversion. This two-step inversion was performed withthree smoothing parameters (τ = 5, 10, and 20 with δx = δy = δz = 0.1) in the model covariance matrix[Siripunvaraporn and Egbert, 2000]. The model with τ = 10 was selected as the ﬁnal model that produced asmallest RMS. Although the initial model (100 Ωm for land) produced a relatively large RMS value of 38.6 due tothe small error bars on Zxy, the two-step inversion produced a ﬁnal RMS of 1.95 with 10% errors for Z and
20% errors for T, which is smaller than for the usual inversion (RMS = 2.74 with the same error bars).

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Journal of Geophysical Research: Solid Earth
Zxx

10.1002/2013JB010682

Zyy(Log ohm-m)
Figure 6. Apparent resistivity map for comparison of observed data (obs.) with calculated response (calc.). The diagonal components (Zxx and Zyy) of the impedance tensor are shown.
Other notations are the same as in Figure 5.

Figures 5–8 compare the observed data with the response calculated from the ﬁnal resistivity structure. For
simplicity, the apparent resistivity and phase of the impedance tensor (Zxx, Zxy, Zyx, and Zyy) are shown for
periods of 1, 8, 64, and 512 s. Note that the parts of the diagonal components (Zxx and Zyy) less than 1 s are
not used in the inversion. Overall, the model explains the observed data. Figure 9 presents an example ofthe data ﬁt shown by sounding curves. Although the ﬁt is not perfect, the anomalous phase is successfully
explained by the structure. This work (both of steps 1 and 2) is the ﬁrst inversion study that explains theanomalous phase in real ﬁelds. The anomalous phase is relatively well explained in the ﬁrst step of theinversion, whereas the better misﬁt is obtained in the second step and overall tendency of the data is well
reproduced (Figure 9).
Figure 10 shows the data ﬁt for the induction vectors [Parkinson, 1962] and phase tensor ellipses [Caldwell
et al., 2004; Moorkamp, 2007], which are robust to the galvanic distortion near the surface. It should be notedthat we did not invert the phase tensors but did invert the impedance tensor. The calculated response provides a reasonable explanation of the overall data, but there is a notable difference in the phase tensor for
512 s. In the data, the determinant of the phase tensor (Φ2) became negative at the northern sites where ananomalous phase is observed. In contrast, Φ2 of the calculated phase tensor are positive except two sites.
Note that negative Φ2 are reproduced in nine sites in step 1. It should be noted here that the main features of
the resistivity structure discussed in the following sections do not signiﬁcantly differ between step 1 and step
2. Therefore, we conclude that the polarity of Φ2 is not essential for the structure.
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Zyx

Zxy

phase (°)

phase (°)

Figure 7. Phase map of the Kirishima Volcanoes for comparison of observed data (obs.) with calculated response (calc.). The off-diagonal components (Zxy and Zyx) of the impedance tensor
are shown. Other notations are the same as in Figure 5.

5.2. 3-D Resistivity Structure
Figures 11 and 12 show the horizontal and vertical slices of the ﬁnal resistivity structure, respectively.
Horizontal slices (Figure 11) show that a widespread conductor in the shallower part gradually localizes withincreasing depth. In particular, the conductor is localized NE of Iwo-yama at 6 km bsl. The subverticalconductor is clearly seen in the vertical slices (labeled C1 in Figures 11, 12). At shallow depths, the verticalconductor becomes obscure and is replaced by the pervasive conductive layers. Figures 11, 12 also show alarge conductive body (labeled C2) exists northwest of the Shinmoe-dake. The vertical slices (Figure 12) show

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Zyy

Zxx

phase (°)
Figure 8. Phase map of the Kirishima Volcanoes for comparison of observed data (obs.) with calculated response (calc.). The diagonal components (Zxx and Zyy) of the impedance tensor are
shown. Other notations are the same as in Figure 5.

that the top of the deep conductor corresponds approximately to 10 km bsl. Although the entire shape of the
deep conductor is not clear due to its extension to the west, away from the site coverage, its volume isestimated to be in excess of 100 km3. The subvertical conductor (C1) is located between shallow conductive
layer and the deep conductive body (C2).
The resistivity structure shows a high contrast in the resistivity value. In particular, at 3 km bsl, this contrast
reaches up to 5 orders of magnitude. The Kakuto caldera corresponds to the resistive zone (R1 in Figures 11,12). At 3 km bsl, there is another resistive zone that lies just above the 2011 deformation source (R2 in
Figures 11, 12). At present, it is unclear whether this resistive zone (R2) is related to the Kakuto caldera, because the activity of the Kirishima Volcanoes destroyed and covered the southern rim of the caldera.
It should be noted that the features of the 3-D structure (high resistivity contrast, subvertical conductor, anddeep conductive body) are not present in the models that fail to explain the anomalous phase. The relationship between the anomalous phase and 3-D subsurface structure is discussed in section 5.3.
5.3. Veriﬁcation of the Main Features of the Structure
The ﬁt between the data and the calculated response (Figures 5–10) is not perfect, and therefore, there is apossibility that the structural features are not unique for the same level of ﬁt to the observations. Therefore,
the robustness of the structural features identiﬁed needs to be carefully checked.

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Journal of Geophysical Research: Solid Earth

Step1
RMS: 2.16(3.49)

10.1002/2013JB010682

Zxx
Zxy
Zyx
Zyy

Step2
RMS: 1.95

Figure 9. Representative sounding curves for comparison of observed data (symbols) with calculated response (lines). Error bars given in the inversion are smaller than each symbol. The
locations of the sites are shown in Figure 4. The data show the 16 periods used in the inversion. Note that the parts of the diagonal components (Zxx and Zyy) less than 1 s were not used in
the inversion. The inversion was conducted in two steps as detailed in section 5.1. The RMS misﬁts of 3.49 and 2.16 shown in step 1 were calculated by using the error bars used in step 1 andstep 2, respectively (both RMS are calculated from the ﬁnal model of step 1).

To check the presence of the conductors, two sensitivity tests were performed. First, the resistivities less than
100 Ωm in the zone of 1000 m < x < 5000 m, 3000 m < y < 5000 m, and at 2–10 km bsl were all changed
to 100 Ωm. This elimination of the subvertical conductor (C1) resulted in an increase of the RMS misﬁt from1.95 to 2.19, and the corresponding changes in the sounding curves are presented in Figure 13b. The sensitivity test was also applied to the zone around the deep conductor, which is deﬁned as the conductive zone
(less than 100 Ωm) at 7000 m < x < 9000 m, 15,000 < y < 0, and at 10–20 km bsl. This elimination of the
deep conductor (C2) resulted in an increase of the RMS misﬁt from 1.95 to 2.15. The corresponding changes
in the sounding curves are shown in Figure 13c. The sensitivity tests validated that both the subverticalconductors and deep conductors are necessary for the data. In particular, Figure 13 shows that the anomalous phase is essential for the structure.

6. Discussion
A subvertical conductor at a depth of 2–10 km is the most prominent feature of the resistivity structure.
Figures 11 and 12 show that the conductor corresponds to the zone of low seismicity. Assuming a low strain
rate (10 14/s), the lower bound of earthquakes (brittle–plastic transition) corresponds approximately to the370–400°C isotherm [Fournier, 1999]. A higher strain rate would make it possible for earthquakes to occur in ahigher-temperature regime. Given that the higher strain rate is plausible in a volcanic area, we suggest thatthe temperature in the vertical conductor is over 400°C. A subvertical conductor at upper crustal levels has
recently been imaged at other volcanoes: Ruapehu, New Zealand [Ingham et al., 2009]; St. Helens, U.S. [Hillet al., 2009]; and the southeastern part of the Taupo volcanic zone, New Zealand [Bertrand et al., 2012].
However, the interpretations in those studies are quite different from each other. For example, Hill et al.
[2009] suggested partial melt, whereas Bertrand et al. [2012] suggested hydrothermal ﬂuid as a cause ofhigh conductivity. Because both melt and hydrothermal aqueous ﬂuids can be conductive at 0.1 Ωm[Nesbitt, 1993; Gaillard et al., 2008; Ni et al., 2011], it is impossible to interpret the nature of the conductor
solely from the resistivity value.
Figure 11 shows that the zone between the deformation source and the Shinmoe crater is resistive, whereas
the northern part of the Shinmoe crater is signiﬁcantly conductive. In this study, we assume that magma
tends to travel through a conductor rather than a zone of high resistivity, because low resistivity suggests a

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Journal of Geophysical Research: Solid Earth
Induction vector (real)

10.1002/2013JB010682

Phase tensor

Figure 10. (left) Induction (Parkinson criteria) vectors and (right) phase tensor ellipse for the comparison of observed data with calculated response. In this study, axes of phase tensor
ellipse (Φmax and Φmin) are calculated according to the slightly modiﬁed expression [Moorkamp, 2007]. At a site where Φmin is negative, phase tensor ellipses are represented by using|Φmin| and are ﬁlled with grey color. Note that the noisy data were not used in the inversion.

good network of pore ﬂuids. Considering that precursory earthquakes were not observed in 2011 between
the deformation source and the Shinmoe crater, it is unlikely that the magma traveled straight to the crater
through the resistive zone. On the basis of the spatial relationship between the conductor and the source of
syn-eruptive subsidence (Figures 11 and 12), we propose that the subvertical conductor acted as a magma
pathway for the 2011 eruptions, which extends from the deformation source to the Shinmoe crater via thenorthern part of the Shinmoe Volcano. Two observations support the hypothesis that the magma wasderived from the northern part of the Shinmoe crater. The ﬁrst is the seismicity beneath the Shinmoe
Volcano. Line FF′ in Figure 12 shows that the hypocenter of the seismicity became shallower from northto the Shinmoe crater. The second observation is that shallow seismic tremors occurred during the 2011
eruptions. Based on continuous long-term seismic array observations, it is suggested that the volcanic tremors
occurred at 1 km depth and 1 km from the northern part of the Shinmoe crater [Nakamichi et al., 2013].
Although the subvertical conductor is interpreted as a magma pathway, we consider that the volume ofmagma is far too small compared with the volume of the aqueous hydrothermal ﬂuids. Taking into account thatthe subvertical conductor is signiﬁcantly larger than the volume of magma erupted in 2011 (0.021–0.027 km3), it
is unreasonable to assume that the interconnected melt is present everywhere in the subvertical conductor. A
previous study also rejected the presence of a large amount of melt at the depth of the subvertical conductor[Yamamoto and Ida, 1997]. In this study, we interpret the vertical conductor as a volcanic conduit through which
hot volcanic ﬂuids and, potentially, magma are transported to craters [Ingham et al., 2009]. The vertical
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Journal of Geophysical Research: Solid Earth

R1

10.1002/2013JB010682

C1

C1
R2

F
A

A’

B

B’

C1
C

C2

C’

D

D’

E

E’

F’

(Log ohm-m)
8

Figure 11. Horizontal slices of the ﬁnal resistivity structure at various depths. The air above sea level was approximated by 10 Ωm resistivity blocks. The seismicity between March 2008 and June 2012
(J. Oikawa, personal communication, 2013) is shown by small circles. Lines A–A′ to F–F′ indicate transects along which vertical slices were taken as displayed in Figure 12. C1 and C2 indicate thesubvertical conductor and the deep conductor, respectively. R1 and R2 indicate the resistive zone beneath the Kakuto caldera and the resistive zone above the 2011 deformation source, respectively.
Other notations are the same as in Figure 5. The region enclosed by the white dashed line shows the hypocentral location of the 1968–1969 Ebino earthquake swarm [Minakami et al., 1969].

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Journal of Geophysical Research: Solid EarthA

A’

Kakuto caldera

10.1002/2013JB010682(Log ohm-m)

R1

B

B’

C1

C2
Iwo-yama

C

C’

R2
C1
C2
D

Shinmoe-dake

D’F

Shinmoe-dake

F’

C1
C2
E

OhachiE’

Figure 12. Vertical slices of the ﬁnal resistivity structure W–E (transects A–A′ to E–E′ in Figure 11) and N–S (transect F–F′ in Figure 11)
across the study area. The seismicity between March 2008 and June 2012 (J. Oikawa, personal communication, 2013) is shown as a 2 km
wide swath. The white dashed region along A–A′ shows the hypocentral location of the 1968–1969 Ebino earthquake swarm [Minakami
et al., 1969]. The black dashed circle indicates the possible location of the source of deformation [Ueda et al., 2013; Nakao et al., 2013;
Kozono et al., 2013].

conductor extends upward to the volcanic cones (Iwo-yama and Shinmoe-dake) in the central sector of the
Kirishima volcano group, whereas it is directed away from the SW volcanic cone area (Ohachi). This result is
broadly consistent with the ﬁndings from 1-D inversion of previous ULF–MT data [Utada et al., 1994;
Kagiyama et al., 1997].

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Journal of Geophysical Research: Solid Earth

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Zxx
Zxy
Zyx
Zyy

(a)
Final model
RMS: 1.95(b)
Test 1
RMS: 2.19(c)
Test 2
RMS: 2.15

Figure 13. Results of sensitivity tests. Notations are the same as in Figure 9. (a) Sounding curve calculated from the ﬁnal resistivity structure. (b) Result of sensitivity test that eliminates thesubvertical conductor. (c) Result of sensitivity test that eliminates the deep conductor.

Hydrothermally altered minerals (especially smectite) show a signiﬁcantly low resistivity, and this has been
considered an important cause of the low resistivity within active volcanoes [e.g., Jones and Dumas, 1993;
Ogawa et al., 1998; Nurhasan et al., 2006; Coppo et al., 2008; Aizawa et al., 2009a; Yamaya et al., 2009; Garcia
and Jones, 2010; Kanda et al., 2010; Siniscalchi et al., 2012]. By taking into account bore data at Ogiri
geothermal ﬁeld [Uchida and Sasaki, 2006], the widespread, shallow (a few kilometers deep) conductors areinterpreted as a zone of hydrothermal alteration whose base corresponds to the 200°C isotherm. Conversely,
the alteration in the subvertical conductor is difﬁcult to evaluate because smectite breaks down attemperatures above 200°C. Little is known about hydrothermal alteration and its electrical resistivity attemperatures over 400°C.
Petrological data give important insights into the magma plumbing system of the Shinmoe-dake Volcano.
Suzuki et al. [2013] showed that the erupted material of the 2011 eruptions comprises a mixture of basaltic
andesite (BA) and silicic andesite (SA) and suggested that magma mixing occurred in two stages (precursor
mixing followed by ﬁnal mixing) before the sub-Plinian eruptions. The timescale from the ﬁnal mixing to theeruption was estimated at 2–50 h. According to melt inclusion analysis and phase equilibria experiments[Suzuki et al., 2013], the reservoir of SA magma was located at a depth of 5 km, whereas BA magma was
supplied from a deeper level (>10 km). Because the subvertical conductor exists on its own at around 5 km
depth, the SA magma is interpreted to be contained in the subvertical conductor. In turn, the BA magma isinterpreted as occupying the deep conductor (C2) below 10 km. The location of the deep conductor isroughly consistent with the locations of the deep low-velocity zone [Yamamoto and Ida, 1994] and the deep
low-attenuation zone [Yamamoto and Ida, 1997], and a large BA magma chamber may exist there. Here it
should be noted that the 2011 deformation source is located above the eastern and upper rim of the deep

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Journal of Geophysical Research: Solid Earth(a)10.1002/2013JB010682(b)
Depth (km)Iwo-yama

Shinmoe-dake

Ohachi0
Basaltic-andesite
magma chamber?

Resistive
body

5

Silicic andesite
magma ?

Pressuresource10Basaltic andesite
magma chamber?

10 km

Pressuresource15
7 km

Figure 14. Schematic illustration of the magma plumbing system of Shinmoe-dake Volcano. The model is inferred from the 3-D resistivity
structure (Figures 11 and 12) and studies of the 2011 sub-Plinian eruptions at Shinmoe-dake. (a) Horizontal view of the magma movement.
The dark shaded area indicates the chamber of the basaltic andesite (BA) magma. The dashed circle indicates the magma intrusion zone that
corresponds to the location of the syn-eruptive deformation source [Ueda et al., 2013; Nakao et al., 2013; Kozono et al., 2013]. The arrow
indicates the inferred magma movement of the 2011 Shinmoe-dake eruptions. (b) Vertical view of the magma movement in the 2011
Shinmoe-dake eruptions (corresponding to the arrow in Figure 14a). The black area represents the zone of interconnected melt, whereas thegrey area indicates the zone of aqueous ﬂuid. Both the black and grey areas are imaged as low-resistivity regions.

conductor (Figures 11 and 12). Therefore, the preeruptive and posteruptive inﬂations are interpreted as
the intrusion of BA magma from the deep chamber. In turn, the syn-eruption deﬂation is interpreted ascorresponding to the evacuation of magma through the identiﬁed magma pathway.
The spatial relationship between the syn-eruptive subsidence and the Shinmoe-dake crater (Figures 2 and 11)
indicates that the magma traveled obliquely to the surface. The highly resistive body is located just above thedeformation source (Figures 11 and 12) and possible BA magma chamber. We speculate that this highly
resistive body impeded the ascent of magma during the 2011 eruptions. Similar structural controls on
subsurface magmatic ﬂuid movement have been suggested for Asama and Iwate Volcanoes in Japan [Aizawa
et al., 2008, 2009b]. Figure 14 shows a schematic image of the magma plumbing system of the Kirishima
Volcanoes. The deep conductive body is the magma chamber from which BA magma was supplied to thevolcanoes (e.g., Iwo-yama and Shinmoe-dake Volcanoes) in the central sector of the Kirishima volcano group.
From February 2010 to January 2011, BA magma continuously intruded into the zone above the deep
chamber. Prior to the 2011 eruptions, the interconnected melt pathway was formed somewhere in thesubvertical conductor. From the shape of the subvertical conductor, it is inferred that the erupted magma in2011 moved from the NW via the northern part of Shinmoe-dake Volcano (Figure 14a).
The Kakuto caldera (Figures 1 and 2) was imaged as being of high resistivity, especially to a depth of 5 km bsl.
This result is consistent with a previous 2-D inversion [Goto et al., 1997]. Because the high resistivity(3000–10,000 Ωm) is inconsistent with the presence of an interconnected melt, there is unlikely to be an
accumulation of a large mass of magma beneath the Kakuto caldera. Alternatively, the resistive body may
represent a fossilized magma chamber, which has cooled and solidiﬁed. The hypocentral area of the 1968–1969
earthquakes corresponds to the deeper part of the resistive body (Figures 11 and 12), which is consistent withthe conclusion of Goto et al. [1997] that a large mass of ﬂuid is unlikely to exist in the hypocentral area. However,
because the conductor is located at the southeastern rim and bottom of the hypocentral area (Figures 11 and12), we further suggest that earthquakes beneath Kakuto caldera are related to volcanic ﬂuids. The intermittent
earthquake swarms may be caused by pressure increase around the conductor or the partial migration of
volcanic ﬂuid.

7. Conclusions
The 3-D resistivity structure in the vicinity of Shinmoe-dake Volcano reveals a subvertical conductor that is
located between the deep-seated conductive body (at a depth greater than 10 km) and shallow conductive
layers. By applying the ﬁndings of geophysical and petrological studies of the 2011 sub-Plinian eruptions at
Shinmoe-dake Volcano, and assuming that magma travels through a conductor rather than through a zone ofhigh resistivity, we propose that the subvertical conductor represents the hydrothermal zone at temperatures

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Journal of Geophysical Research: Solid Earth

10.1002/2013JB010682

over 400°C, in which a magma pathway (interconnected melt) is partially and occasionally formed before
magmatic eruptions. Subvertical conductors in the upper crust have recently been found under other active
volcanoes. We consider that the precise imaging of the subvertical conductor and comparing it with otherobservations (seismicity, deformation, and erupted material) is the key to investigating magma plumbing
systems under active volcanoes.

Acknowledgments
Existing MT data were kindly provided
by T. Goto (Kyoto University), T. Uchida(Geological Survey of Japan, AIST), New
Energy and Industrial Technology
Development Organization, andNittetsu Kagoshima Geothermal Co.,
Ltd. The geomagnetic data used for the
remote reference processing were provided by the Kakioka Geomagnetic
Observatory, Japan Meteorological
Agency. We thank landowners and regional forest ofﬁces (Kagoshima andMiyazaki prefectures) for permitting theobservations to be conducted. F.
Masutani, M. Yamaguchi, T. Kagiyama, Y.
Sasai, and H. Utada contributed to acquiring the previous MT data in the region of the Ebino earthquake swarms. Y.
Maehara and T. Sugano are thanked fortheir participation in the ﬁeld campaign
of 2010. The MT observations in 2011were conducted under the supervisionof the Japan Meteorological Agency.
Hypocenter data were kindly provided
by J. Oikawa (ERI: Earthquake ResearchInstitute, University of Tokyo). For thisstudy, We have used the computer
systems of the Earthquake Information
Center of the Earthquake ResearchInstitute, the University of Tokyo. We
thank W. Siripunvaraporn for supplying his
3-D inversion code. We thank A. Chave forproviding his magnetotelluric analysis
program. A discussion with H. Ichihara(JAMSTEC) about the anomalous phase
was helpful for the 3-D modeling. We
thank ERI for permitting the use of apublished ﬁgure (Figure 1). The commentsfrom Malcolm Ingham and an anonymous
reviewer and the associated editor(Max Moorkamp) greatly improved themanuscript. This work was supported
by a grant-in-aid from MEXT (KAKENHI
22900001 and the HIZUMI project).

AIZAWA ET AL.

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