Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 251-267

ArticleShallow Resistivity Structure of Sakurajima Volcano
Revealed by Audio-frequency Magnetotellurics
Wataru KANDA1）, Tomoya YAMAZAKI2）, Yasuo OGAWA1）, Takeshi HASHIMOTO3）,
Shinʼya SAKANAKA4）, Koki AIZAWA5, 10）, Shinichi TAKAKURA6）,
Takao KOYAMA5）, Kenta YAMADA4）, Tsukasa KOBAYASHI7）
and Shogo KOMORI8, 9）(Received October 5, 2011 ; Accepted April 27, 2012)
An audio-frequency magnetotelluric (AMT) survey was conducted at the foot of Sakurajima volcano in November
2007. This survey was carried out within the framework of the 7th National Project for Prediction of Volcano
Eruptions. The main objective was to clarify the shallow layers of Sakurajima volcano for better understanding of the
volcanic activity. We measured electromagnetic fields at frequencies from 1 to 10400 Hz at 27 locations along only
three lines set on the northern, western, and southeastern flanks of the volcano. The three profiles enclosed the summit
area where access is prohibited because of frequent explosive vulcanian eruptions. The data quality was generally
considered to be good at frequencies higher than 2 Hz. Two-dimensional and three-dimensional (3-D) modelingapproaches were applied to the data.
As a result of 3-D modeling, we obtained the following features in the resistivity model. The surface layer showeda high resistivity from several hundreds to thousands of ohm-meters and corresponded to the lava. This surface layer
covered a highly conductive layer with a resistivity of less than tens of ohm-meters, interpreted as containing seawateror groundwater. However, the basement structure was not clearly detected, except for in an area of northern
Sakurajima. The first lava layer was distributed thickly in northern and western parts of Sakurajima, and the boundary
between the first and second layers was typically located at around sea level.
The conductive second layer was found at deeper levels beneath areas such as the Haruta-yama lava dome and theNabe-yama pumice cone. This depression of the low resistivity layer is probably caused by the fracture zone associatedwith previous eruptions, which is formed above the conductive layer. In the proximity of Hikino-hira lava dome andthe Taisho crater, the second conductive layer was found at a depth shallower than sea level, suggesting that ahydrothermal system has developed underneath this area. In contrast, elevation of the second layer was not seen on theside of Taisho crater located nearest to the edge of Nabe-yama, implying that a heat source is not present beneath theeastern foot of Sakurajima.
Key words : Sakurajima volcano, electrical resistivity structure, hydrothermal system, flank eruption, prediction of
volcanic eruptions

Kusatsu-Shirane Volcano Observatory, Volcanic Fluid
Research Center, Tokyo Institute of Technology, 641-36
Kusatsu, Agatsuma, Gunma 377-1711, Japan.
2）
Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto 611-0011, Japan.
3）
Faculty of Science, Hokkaido University, N10W8, Kitaku, Sapporo 060-0810, Japan.
4）
Graduate School of Engineering and Resource Science,
Akita University, 1-1 Tegata gakuen-machi, Akita 0108502, Japan.
5）
Earthquake Research Institute, the University of Tokyo,
1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
6）
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba West, 16-1 Onogawa, Tsukuba,
1）

Ibaraki 305-8569, Japan.
Maizuru Marine Observatory, Japan Meteorological Agency,
901 Shimofukui, Maizuru, Kyoto 624-0946, Japan.
8）
Aso Volcanological Laboratory, Graduate School of Science,
Kyoto University, Minami-Aso, Kumamoto 869-1404, Japan.
9）
Now: Institute of Earth Sciences, Academia Sinica, 128
Academia Road, Section 2, Nankang, Taipei 115, Taiwan.
10）
Now: Institute of Seismology and Volcanology, Facultyof Sciences, Kyushu University, 2-5643-29 Shinʼyama,
Shimabara, Nagasaki 855-0843, Japan.
7）

Corresponding author: Wataru Kanda
e-mail: kanda@ksvo.titech.ac.jp

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Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI

Fig. 1. Map of AMT site. Three solid lines indicate measurement lines (AK, NS, and GK) used in the analysis. Note thatboth AK and GK lines include site 09. Open diamonds denote the position of boreholes (HARB and KURB) referred toin the interpretation. The main craters and lava distributions from different historic ages (Fukuyama and Ono, 1981) arerepresented by gray outlines and hachure patterns, respectively. A dashed line indicates the 2 km off-limits boundaryfrom Minami-dake (M) and the Showa (S) craters. Lava domes are indicated by open triangles (Hik: Hikino-hira; Har:
Haruta-yama; Yun: Yuno-hira). Other geographical features (Nab: Nabe-yama pumice cone; T: Taisho crater) and the
local name of places (Shirahama, Hakamagoshi, Miyamoto, and Kannon-zaki) are also labeled.

1．Introduction
This paper describes an outline and various results fromaudio-frequency magnetotelluric (AMT) surveys conducted
as a part of the 10th Joint Observation of Sakurajima Volcano in 2007. At Sakurajima, continuous volcanic activity
at the summit of Minami-dake has occurred since 1955
(Japan Meteorological Agency, 2005, p. 546). Repeatedleveling surveys around Sakurajima have revealed an
inflating tendency for the ground deformation since the
Taisho eruption in 1914 (Eto, 1967; Kamo and Ishihara,1980); however, in the late 1980ʼs this tendency swapped
to a deflating one, because eruption activities were accompanied by a large amount of ash ejection at thesummit. In the early 1990ʼs the eruption activity declined,
and the ground resumed a process of upheaval (Eto et al.,1997) that continues to the present. This upheaval implies
that magma has accumulated in the reservoir that isthought to exist beneath the Aira caldera (Iguchi, 2006).
From around 2001, volcanic activity in the region has been
at a low level. Subsequently, after 58 years of dormancy,
an eruption occurred at the Showa crater (S in Fig. 1) thatis located a few hundred meters east of the Minami-dakecrater (M in Fig. 1) in June 2006, and explosive eruption
activities in the Showa crater have been continued since(Iguchi et al., 2010).
Although the primary volcanic activity for the past 50
years has been explosive eruptions from the summit craterof Minami-dake, in historic ages (Tenpyo-Hoji, in 764;
Bunmei, in 1471-1476; Anʼ ei, in 1779; and Taisho, in
1914-1915) large amounts of lava were effused several
times at the flanks of the volcano (Fukuyama and Ono,1981). Each of these flank eruptions produced clusters ofcraterlets that are distributed mostly along a line passing
through the Minami-dake summit; however the direction

Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotelluricsof each line is different. This difference suggests that local
stress fields at shallow depths act in a different mannerfrom the regional stress field around Sakurajima. Fromthe vertical discontinuities in the occurrence frequency ofA-type earthquakes and in the source mechanism of thoseearthquakes, Ishihara (1988) proposed that a structural
boundary is present at a depth of about 2 km beneath
Minami-dake. He considered that the magma is temporarily stored around the structural boundary and that thepressure changes at this position control the local stressfield at depths shallower than 2 km. To investigate thishypothesis further, information on the shallow structure isessential.
Furthermore, the shallow layer of a volcano can induce adiversity of volcanic activity (e.g., Kagiyama, 2008). In
particular, the existence of groundwater is related directly
to the occurrence of dangerous phreatomagmatic explosions (e. g., Kagiyama et al., 1999), and this can cause
increasing flank instability through hydrothermal activity(e.g., Reid, 2004). Surveying the distribution of the groundwater layer around the crater of a flank eruption that has
effused large amounts of lavas and pumices can contributeto the evaluation of such volcanic hazards. Measurementof underground resistivity is a promising method ofinvestigating the groundwater layer or the hydrothermallyaltered layer resulting from circulation of volcanic fluids(e.g., Kagiyama et al., 1999; Finizola et al., 2006; Aizawa
et al., 2009).
However, sufficient information has not been acquired
in previous works. The electrical resistivity distribution at
Sakurajima has been investigated from the 3rd Joint
Observation Campaign to the 9th Campaign (Yukutake etal., 1980; Tanaka et al., 1982; 1986; Nishimura et al.,
1988; 1989; Karohji et al., 1989; Kubota et al., 1995;
Yamazaki et al., 1998). In addition, Kakioka Magnetic
Observatory belonging to the Japan Meteorological Agency
carried out resistivity surveys as part of routine monitoringof the volcanic activity (Tokumoto et al., 1988; Fukushima
et al., 1988; 1989). The main objective of these observations was to detect resistivity changes associated withvolcanic activity. Therefore, electrical methods were predominantly used in these studies such as dipole-dipole
methods or magnetotelluric (MT) methods in which themeasuring frequency was limited to 8-30 Hz. Recently,
Aizawa et al. (2011) investigated a temporal change in theresistivity structure of Sakurajima, found from the continuous MT observation using a broadband frequency for aperiod of one year, and discussed the relation between thischange and corresponding volcanic activity. However,since they used only two sites, sufficient information on
the structure of Sakurajima has yet to be acquired.
In this study, we set three measurement lines on theflanks of the volcano and carried out AMT measurements(Fig. 1). Since the frequency range used in the present study
was from 1 to 10400 Hz, the resistivity structure from the253

Fig. 2. Typical sounding curves of measured AMT dataobtained at sites 08 and 12. The apparent resistivity(app. res.; upper plots) and the phase (lower plots) computed from the ZXY-component (black dots) and fromthe ZYX-component (white dots) are shown. Note that
the ZYX -component of phase is expressed through arotation of 180°.

surface up to depths of 1-2 km, which have not been clarified in previous studies, can be estimated. The principal
objective of the present study was to elucidate the shallow
layers of Sakurajimaʼs electrical resistivity structure, and toobtain structural information about the conditions underwhich flank eruptions took place.
2．AMT observation and data
2-1 Outline of the observation
We carried out an AMT survey from 30 October throughto 4 November 2007. The off-limits area at Sakurajima
volcano is defined as being less than 2 km from the Minamidake summit crater or from the Showa crater, both ofwhich repeatedly have explosive eruptions. As a result of
this restriction, designing a measurement line across thesummit area is difficult, because there is no route thatpasses through the central part of the volcano. As a result,
we conducted electromagnetic field measurements at 27
locations on the flanks of the volcano along three lines(AK, GK, and NS in Fig. 1) enclosing the volcanic edifice.
To take the measurements, a total of seven MTU-5A
systems (Phoenix Geophysics Ltd., Canada) were used anddata were recorded during the nighttime (20: 00 to 06: 00
local time). Since data synchronized by a GPS clock wereobtained at five or six sites in a night, mutual local
reference processing was performed.
2-2 Data
In Fig. 2, examples of typical sounding curves for theapparent resistivity and phase are shown for two sites. The

254

Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI

data quality was considered to be good in the frequency
range from 2 Hz to values in the thousands of hertz, except
for at a small number of sites. At almost all sites, we see atendency for the apparent resistivities to decrease monotonically from hundreds of ohm-meters to less than tens of
ohm-meters as the frequency decreases. In phase curves,
an increasing trend from phases of 30°-50° to phases exceeding 70° is commonly seen as the frequency reduces.
However, differences are found in the low frequencies ofthe phase curves site-by-site. For example, although thephase decreases at low frequencies for sites along the AKline on the southeastern flank of the volcano, this tendency
is not as marked along the NS line on the western flank andhigh phases are maintained (Fig. 2).
At certain sites, we see that the phase rapidly increases
at the highest frequencies (7000-10400 Hz) and that, conversely, the apparent resistivity decreases. This inverse
relation is caused not by a conductor of the uppermost
layer but by a low-pass filter built into the MTU-5A system.
When the ground resistance reaches levels of tens of
thousands of ohms, such data are often obtained. Since
fitting a resistivity model to these areas is unfeasible, theassociated data were removed in the modeling processdescribed later.
2-3 Distribution of apparent resistivity and induction arrows
To obtain a rough understanding of the entire dataset,
we calculated the apparent resistivity based on a rotational
invariant of the impedance tensor. Fig. 3 shows the spatial
distribution of the apparent resistivity for six characteristicfrequencies calculated from the rotational invariant det(Re(Z)), which is thought to be a quantity that reflects thethree-dimensional (3-D) distribution of the conductors
appropriately (Szarka and Menvielle, 1997).
The trend in Fig. 3 matches that already shown in thesounding curves (Fig. 2); namely, the apparent resistivity
decreases as the frequency becomes low. At these lower
frequencies (<100 Hz), areas of comparatively high resistivity are found along each survey line; an area over whichAnʼei lava is distributed on the GK line (sites 22, 23, and24), a surrounding area of Nabe-yama on the AK line (sites
05, 06, and 07), and an area north of the Taisho crater (T in
Fig. 1) on the NS line (sites 12 and 13). In contrast, an
area from the Taisho crater to Hikino-hira, a lava dome on
the western flank, is relatively conductive (sites 14 and
15).
In Fig. 3, the distribution of the real induction arrows(Parkinson, 1962) is additionally shown. The induction
vector has properties that indicate contrasts in lateralresistivity and the vector will point in the direction of aconductive region. However, vectors tend to point toward
a topographically high region at frequencies above about
100 Hz, even if there is no good conductor in the area
(Müller and Haak, 2004). At 1100 and 530 Hz, the induction vectors along the AK and GK lines primarily point

toward the central part of the volcano (Fig. 3), and weassume this to be as a consequence of this topographic
effect. This tendency becomes weaker at a frequency of
about 100 Hz, and the arrows instead reflect the resistivity
contrast of the underground structure. At 49 Hz, thevectors at a number of sites point toward the sea, which is agood conductor, and this trait is even more pronounced atlower frequencies. However, at frequencies of less than
10 Hz, the induction vectors at some sites along the NS line
(sites 11, 13, 15, 16, and 17) point in the direction of
Minami-dake, suggesting the existence of another good
conductor beneath Minami-dake.
3．Dimensionality and strike
Calculating the phase-sensitive skew (a measure ofdimensionality), we found that this was less than 0.3 foralmost all usable data at all sites. Thus, we can presume
that the subsurface structure in this region is one-dimensional (1-D) or two-dimensional (2-D) and can, accordingly perform a 2-D analysis (Bahr, 1988). To this end,
first, the strike directions for this 2-D analysis were
estimated from individual impedance data by using a tensor decomposition technique (hereinafter referred to as
GBD; Groom and Bailey, 1989). Polar histograms of strikeestimates for frequency-dependent and site-dependent decompositions are shown in Fig. 4 for each line.
Strike estimates from data along the AK line (Fig. 4(a))
are principally in the N0° W-N10° W direction (or N80°
E-N90° E direction). The histograms of strikes estimatedfor data along the NS and GK lines (Figs. 4(b) and (c)) also
show a dominant direction of nearly N-S (or E-W). Since
Sakurajima volcano has grown in a regional stress fieldwith an approximately E-W extension (Hayasaka, 1987),
the underground structure of Sakurajima is considered tohave a directional property with an approximately N-S
strike.
We recognize that the second dominant direction isaround N30° E (or N60° W) in the NS and GK lines. In
Figs. 4(d)-(f), histograms of strike estimates obtained fromdata at frequencies lower than 10 Hz are shown for eachline. In the GK line (Fig. 4(f)), the estimated strikes areconcentrated between N15° E and N30° E (or between
N60° W and N75° W). This feature is also seen in the AK
and NS lines (Figs. 4(d) and (e)), although the concentration is not remarkable. The induction arrows at frequencies lower than 10 Hz at many of sites located in theeastern region of Sakurajima tend to point in a southeast
direction (Fig. 3). The presence of a resistivity contrast issuggested by the direction of the arrows, and hence, thestrike direction is roughly presumed to be northeast.
These results indicate the possibility that the direction of
the 2-D strike or the dimensionality of the structure differswith depth, and suggest that a simple 2-D analysis isdifficult to perform.
As a preliminary analysis we applied a 2-D inversion

Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics

255

Fig. 3. Spatial distribution of apparent resistivity at frequencies (freq) of 1100, 530, 97, 49, 9.4, and 5.6 Hz. Apparent
resistivity is calculated from det(Re(Z)), a rotational invariant of the magnetotelluric impedance tensor. White arrows
denote the real parts of induction vectors (Parkinsonʼs convention).

(Ogawa and Uchida, 1996) to the higher quality TM-mode
data at ≥2 Hz along each measurement line. Topographic
and bathymetric features were accounted for in the model,
and the resistivity of seawater was fixed at 0. 33 Ωm.

Static shift was also included as an inversion parameter.
The error floors for both the apparent resistivity and thephase were set at 5 %. In Fig. 5 (a), a 2-D resistivity
section for the AK line is shown as an example inversion

256

Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI

Fig. 4. Polar histograms of strike directions estimated from GBD results. Histograms of all frequency bands are shown for(a) AK line, (b) NS line, and (c) GK line, and those of lower frequency bands (<10 Hz) are shown for (d) AK line, (e)NS line, and (f) GK line. Radial scale is normalized to enable comparative measurement between cases. Sector width is
5°. Note that strike directions have an intrinsic ambiguity about 90°.

result. The root mean square (RMS) error for the fits was
0. 80. Pseudo-sections of the observed data and of themodel responses are shown in Figs. 5 (b) and (c). A
significant number of the features in the observed data are
reproduced by the inverted model.
The following features are seen in the resistivity structure. The first few hundred meters in depth along each line
have high resistivities of between hundreds and thousandsof ohm-meters. A non-uniform, heterogeneous layer witha low resistivity of less than 10 Ωm is then widely distributed below the surface layer. A resistive portion of lessthan 100 Ωm is also seen at high depths around the central
zone of the AK line. Although the models fit the datawell, the data corresponding to this deep high resistivity
are not clear in the pseudo-sections. The resistive portions
could be virtual images produced by inappropriate modeling of a 3-D structure by a 2-D analysis.
4．3-D modeling
As we described in Section 3, 2-D interpretation of theentire dataset is inappropriate, because the correct strikedirections cannot be assumed. As a result, a 3-D analysis
was also conducted. To do this, we used a parallel implementation version of the WSINV3D code for 3-D
inversion (Siripunvaraporn et al., 2005; Siripunvaraporn
and Egbert, 2009). In the inversion, we used all four
components of the impedance tensor (ZXX, ZXY, ZYX, andZYY) as input data. The error floor was again set at 5 %.
Sixteen different frequencies were used in the calculation,chosen from observed frequencies between 2.03-3000 Hz
such that the values had roughly equal spacing on the log
scale.
To cover an area of 12 km×12 km centered at Minami-

dake (hereinafter called the core area), a mesh length of
300 m was chosen. The core area encompassed almost allof the Sakurajima environs. Outside of the core area, themesh size was enlarged by  2 times as the distance fromthe core area became large. Consequently, the number ofhorizontal mesh points was 62 in the E-W and N-S
directions. In the vertical (depth) direction, the top 100 m
was divided equally into 10 layers, and the thickness wasthen gradually enlarged with depth. The number of vertical mesh points was thus 41 from sea level to a depth of
53.3 km.
From consideration of the obtained 2-D models, theinitial two-layer model (Model-U) was used: the first layer
had a thickness of 100 m from the surface and a resistivityof 1000 Ωm, and the second layer was a uniform halfspace with a resistivity of 100 Ωm. The seafloor topography was also sampled according to the above mesh, andwas incorporated into the initial model. Resistivity blocks
corresponding to seawater were fixed at 0.33 Ωm during
the inversion process. Moreover, blocks containing bothseawater and land was also fixed at a conductivity calculated from the weighted mean of the conductivities ofland (10−3 or 10−2 S/m) and of seawater (3 S/m) according to the volume ratio. The topography on land was nottaken into consideration in the modeling process, since theversion of WSINV3DMT code used was unable to manage
such information.
We first conducted a sensitivity test. Since our observation sites are distributed along the three lines on theflanks of Sakurajima, sensitivity to the conductor locatedbeneath the central part of the volcanic edifice, which isaway from the measurement lines, and to the conductor
beneath one of the measurement lines was checked by

Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics

Fig. 5. (a) 2-D resistivity model profile for AK line.
Inverted triangles denote AMT sites used for inversion(cf. Fig. 1). Horizontal dashed line indicates sea level.
(b) Pseudo-sections for the observed (upper plot) andthe calculated (lower plot) TM-mode apparent resistivity for AK line. (c) Pseudo-sections for the observed(upper plot) and the calculated (lower plot) TM-mode
phase.

using three models shown in Fig. 6(a). Model-U is theinitial model, which has a uniform second layer of 100 Ω
m. Model-A and Model-B have conductive 1 Ωm regions
A and B, respectively, in addition to Model-U. Region A
corresponds to the conductor beneath the summit area andregion B to that beneath the measurement line. The
calculated curves of the apparent resistivity and the phasefrom the three models are shown for four sites in Fig. 6(b).
Differences between Model-A (white dots) and Model-U(solid line) are seen in the phase curves of the observation

257

sites nearest to the conductor (05 and 15) at around 10 Hz;
however, the differences are as small as the error bars
shown in the plots. The responses of sites 08 and 13 aremostly in agreement with those of Model-U. In contrast,
notable differences between Model-B (gray dots) andModel-U are recognized in both the apparent resistivity
and the phase curves at site 15 located right above theconductor and site 13 close to the conductor. On the basisof these results, we can conclude that little sensitivity isfound to a conductive body located beneath the central partof the volcano, and that sufficient sensitivity is expectedaround the measurement line.
Fig. 7 shows a horizontal slice through the core area ofthe best-fit 3-D resistivity model, obtained as a result of
inversion by using all the observation sites. After the
RMS reached a sufficiently small value, a model with asmaller norm was searched for under a similar level of
misfit (Siripunvaraporn et al., 2005). Consequently, theRMS error for this model is as low as 2.18; however, thematch between the 3-D model and the non-diagonal components was less accurate at three sites to the south of theNS line (Fig. 8). This inaccuracy is due to the apparent
resistivity values at site 18, which are about one order ofmagnitude smaller than those at surrounding sites. Since
the phase curve at site 18 still maintains a similar profile,
this discrepancy is regarded as a static shift (static shiftcorrection was not applied in the 3-D study). To explain
the low apparent resistivity data, a highly conductive bodywas placed in the model near to the surface at site 18 (Fig.
7), although the existence of this body is not practical for
explaining the data at sites 17 and 19 (Fig. 8). Thus, theresults indicate that static-shift correction is required evenin 3-D modeling of AMT data.
Therefore, the inversion was conducted once more by
removing the data for site 18 and using that for the other 26
observation sites. Four horizontal sections of the resultant
final 3-D model are shown in Fig. 9. Moreover, typicalexamples of data-fits are shown for four sites in Fig. 10,
and the RMS error of this model was 1.62. Although the
response curves now reproduce the non-diagonal components well, at certain sites, the fit of the diagonal components of derived model and the data is inexact.
Features of the final model are summarized as follows.
The resistivity close to the ground surface has high resistivity in general. Although nearly all areas in the vicinityof the measurement lines have high resistivity at a depth of
100 m (not shown), conductive portions are also seen bothsides of the AK line and at the center of the NS line. At
depths of 200-250 m, from the southern to the easternregion of Sakurajima the resistivity structure again becomeslow. Finally, almost the entirety of Sakurajima has aresistivity of less than tens of ohm-meters at depths of
500-600 m, and this tendency continues at a depth of 1000
m. However, in an even deeper portion (〜1500 m indepth) beneath the west half of the GK line, the resistivity

258

Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI

Fig. 6. (a) 3-D resistivity models used to perform sensitivity tests. Model-A has a prism-like conductor of 1Ωm in region A
in addition to the initial model (Model-U), and similarly for Model-B in region B. The size of both conductors is 1200 m
×1200 m (horizontal)×500 m (vertical). Model-U is the initial model used for inversion. (b) Comparison of response
curves of apparent resistivity (upper plot) and phase (lower plot), which are calculated from the non-diagonalcomponents of impedance tensor, at four sites (site 13, 15, 05, and 08) for the three models. Solid lines indicate theresponses of Model-U. White and gray dots denote the responses of Model-A and Model-B, respectively.

Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics

value increases and a high resistivity of hundreds of ohmmeters is found.
Since the topography was not incorporated into ourmodel, the effect of the elevated center of the volcano,which may be included in the observed data, might haveaffected the inference of the 3-D structure. To estimate
this effect, we calculated the responses of a simple modelthat incorporated the actual topography of Sakurajima byusing another 3-D forward code (Mackie et al., 1994). As
a result, we found that the responses of the models (withand without the elevated center of volcano) mostly
overlapped at the majority of the sites, although slightdifferences were seen at lower frequencies at a few sites

Fig. 7. Horizontal slice at depths of 80-90 m of best-fit 3D resistivity model, obtained as a result of inversion
using all the sites.

259

located near the center of the measurement lines. The
maximum deviation was 12 % (about 3 % on the log scale)in the apparent resistivity curve, and was 2. 5 % in thephase curve. Because this maximum deviation in the apparent resistivity appeared at the lowest frequency calculated (1 Hz), we consider that the influence of the elevatedcenter of the volcano on the final model (Fig. 9) is fairly
limited.
5．Discussion
5-1 Shallow structure near sea level
In Fig. 11, cross sections of the final model along thethree measurement lines are shown. Since the topography
was not incorporated into the model, the depth corresponding to sea level (simply the reverse of the 2-D topography along a measurement line) is drawn by a whitedashed line. To interpret this 3-D model, we referred toborehole data from the Kurokami (KURB) and Harutayama (HARB) observation wells (Uto et al., 1999; Miki etal., 2000; Aizawa et al., 2011).
KURB is a 381 m deep drilling well located 150 m eastof site 08 (Fig. 1). According to the borehole lithology(Uto et al., 1999), the top 100 m is andesite lava effusedfrom Nabe-yama in the 8th century, which is underlain byscoriaceous tuff breccia down to 240 m. Below this layer,
lapilli tuff with pumice and block was found down to thebottom of the borehole. When compared with the obtained
3-D resistivity model around site 08 on the AK line (Fig.
11(a)), the boundary of the andesite lava coincides with thebottom surface of the high-resistivity layer. Therefore,
this layer can be interpreted as being lava.
Next, the low resistivity of the second layer is considered. From temperature data logged for KURB, at thebottom of the lava layer a high temperature peak wasfound, and the temperature decreased as the depth increased into the underlain layers (Aizawa et al., 2011).
With the survey area close to the coastline before the
Taisho eruption, we consider that cold seawater invaded

Fig. 8. Comparison between best-fit 3-D model from Fig. 7 and observed data. From left to right, ZXY− and ZYX−
components are shown for three southern sites (17, 18, and 19) of the NS line. Open squares with error bars indicate theapparent resistivity (upper plots) and the phase (lower plots) used for 3-D inversion. Black dots with a solid line denotemodel responses.

260

Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI

Fig. 9. Horizontal slices taken through the core area of the final 3-D model. Depths are indicated in the top-right corner of
each plot. White dots denote AMT sites and the red triangle indicates the center of Minami-dake crater. Outlines ofhistorical lava and craters (cf. Fig. 1) are also drawn with red lines.

into the second layer. That is, the conductive second layer
can be interpreted as an aquifer including seawater orgroundwater. For an area from the south of Sakurajima to
the east, the obtained 3-D model can be explained primarily by this two-layer structure (Figs. 11(a) and (c)).
HARB is a 408 m deep borehole located 300 m southwest of site 12 and 500 m northwest of site 13 (Fig. 1). A
top layer about 20 m in depth from the surface is occupied
by recent ejecta such as volcanic ashes, and this is underlain by andesite lava of greater than 300 m in thickness(Miki et al., 2000). In the lower section of the borehole,tuff breccias appeared at about 50 m above sea level (asl),and groundwater is contained in this layer (Iguchi, 1987).
If these geological features are compared with the resistivity structure of the NS line (Fig. 11 (b)), similar to
KURB, the bottom of andesite lava correspond to theboundary between the first high-resistivity layer and thesecond conductive layer. Therefore, the first layer is againinterpreted as a lava layer. From a comparison with theresistivity structure in the south and east area of Sakurajima (Fig. 11(a)), we clearly see that the andesite lava is
distributed thickly.

Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics

261

Fig. 10. Comparison between final 3-D model from Fig. 9 and observed data. From left to right, ZXY−, ZYX−, ZXX−, andZYY− components are shown for four sites (05, 13, 17, and 25). Symbols are as in Fig. 8.

Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI−400 S41ºW
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Fig. 11. Resistivity cross sections along measurement lines extracted from the final 3-D model: (a) AK line, (b) NS line,and (c) GK line. High-resistivity anomalies are labeled R1 to R4, while a conductive one is labeled C1. Black solid andwhite dashed lines indicate the topography of each profile and sea level, respectively. The vertical gray bar shows thelocation and depth of the borehole KURB in (a) and of HARB in (b). Labels for the geographical features are as in Fig.
1. Note that horizontal mesh intervals in the figures are not constant, because measurement lines are not in agreementwith the direction of the horizontal mesh of 3-D model.

Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics

However, in the lower portion of this layer, the resistivity has a low value of around 100 Ωm. Along the GKline the surface resistive layer also shows a low value at
depth (Fig. 11(c)). Given that HARB is close to the centerof the volcano, while groundwater may be contained in thesecond layer, this layer can also be interpreted as a hydrothermally altered layer that prevents groundwater from
penetrating into the deeper regions.
5-2 Basement structure
Komazawa et al. (2008) inferred the elevated basement
structure situated from the west to south side of Sakurajima from an analysis of gravity data. Since the depth to
the gravity basement along the NS line was about 500 m
bsl, it was expected that the basement structure would befound by our AMT measurements. However, the depth of
this gravity basement is equivalent to the low-resistivity
second layer in the 3-D model (Fig. 11(b)).
Regarding the reason as to why the basement structure is
not electrically clear, we consider two possibilities. The
first is that the boundary between the second conductive
layer and the underlain basement cannot be identified
because the basement rock also has low resistivity. However, along the GK line, a high-resistivity zone of hundredsof ohm-meters is found at depths greater than 1000 m
beneath sites 20 to 24 (R4 in Fig. 11(c)). This resistivezone is in agreement with the depth of the gravity basement proposed by Komazawa et al. (2008). A similar
high-resistivity zone is detected by the 2-D model along
the AK line at a depth of 1000 m underneath sites 1 to 5
(Fig. 5(a)). However, even though this also agrees withthe gravity basement, a high-resistivity portion is not seenin the corresponding 3-D model (Fig. 11 (a)). If theseresults are taken into consideration, the basement may
have a resistivity value in the hundreds of ohm-meters, but
is likely to be located at the sounding depth or at an evendeeper level.
The second possibility is that the actual basement islocated at a position deeper than that proposed by
Komazawa et al. (2008) in western Sakurajima. Aramaki(1977) stated that the 800 m class borehole located near thewest coast of Sakurajima did not reach the basement rock(the Shimanto Group), and that the borehole was largely
occupied by tuff corresponding to the Kekura formation
found in the Satsuma Peninsula (Oki and Hayasaka, 1970).
Yokoyama and Ohkawa (1986) proposed a model with adeeper gravity basement in light of this evidence. In theshallow-seismic-reflection-structure model for northeastern
Sakurajima proposed by Tsutsui et al. (2011), the least
reflective zone was found between the reflection horizon,
identified as the basement, and sea level, and the zone wasinterpreted as being due to pyroclastic materials ejectedfrom underwater eruptions. This zone was located between
100 and 800 m bsl and thus matches up with the second
conductive layer. In addition, Oshima (2008) investigatedthe borehole data for Shirahama, on the northern coast, and263

for Hakamagoshi, near the western coast (Fig. 1), andreported that hot spring water was drawn from the Kekura
formation layer at depths of between 900 and 1000 m forboth wells.
These results are consistent with the hypothesis that the
low-resistivity layer in western and northern Sakurajima
corresponds to the Kekura formation layer. If sedimentaryrocks of marine origin are widely distributed within
Sakurajima, the second layer might have been affected by
the highly conductive seawater, even in the western andnorthern areas.
5-3 Relation to flank eruptions
As seen so far, the shallow resistivity structure of
Sakurajima consists of a high-resistivity surface layer thatcovers a thick conductive layer, and only part of thebasement structure is clearly detected. The boundary between the first and second layers is situated approximately
at sea level. However, in the areas labeled R1 on the AKline (Fig. 11(a)), and R2 and R3 on the NS line (Fig. 11(b)), the conductive second layer found at depths below sea
level. In contrast, near the Hikino-hira lava dome between
R2 and R3, the second conductive layer is identified at a
depth shallower than sea level (marked C1 in Fig. 11(b)).
R1 is located beneath the Nabe-yama pumice cone (Nabin Fig. 1) that was formed by eruptions in 764 (the TenpyoHoji eruption). The majority of the craters that effusedlava during the Taisho eruption in 1914 are located at theedge of Nabe-yama. Moreover, the Haruta-yama lava
dome (Har in Fig. 1) and a further row of craters resulting
from the Taisho eruption (T in Fig. 1) are situated near the
ground surface above R2. These resistive zones are likely
to be fracture zones related to the eruptive activity, inwhich groundwater is undersaturated. As regards R3,
there is neither a corresponding crater nor a cone on the
ground surface over which M4 lava from the prehistoricage (Fukuyama and Ono, 1981) has spread. With theBunmei lava from 1476 southwest of R3, and theMiyamoto (M1) and Kannon-zaki lavas (M2) from 30004000 years ago (Miki, 1999), whose craters are unknown,
to the east of the Bunmei lava (Fig. 1), R3 may be related
to these eruptions.
Finally, C1 is considered. Various inconsistent data
have been reported around C1 or to its north. High fluxesof soil CO2 and Rn gases, which are considered to indicate
the presence of hydrothermal fluids, were observed on
Haruta-yama and Yuno-hira lava domes, although the fluxof those gases was low on Hikino-hira and Taisho crater(Kusakabe and Hirabayashi, 1988; Hirabayashi et al., 2008).
In addition, a positive anomaly of self-potential (SP) was
reported around the Taisho crater (Hashimoto et al., 1998).
Although the positive SP anomaly has been conventionallyinterpreted as an indication of the upward flow associatedwith hydrothermal circulation, recent studies suggest that ashallow high-conductivity zone enhances SP values aroundthe summit area (Ishido, 2004; Aizawa et al., 2009). The

264

Wataru KANDA, Tomoya YAMAZAKI, Yasuo OGAWA, Takeshi HASHIMOTO, Shinʼya SAKANAKA, Koki AIZAWA,
Shinichi TAKAKURA, Takao KOYAMA, Kenta YAMADA, Tsukasa KOBAYASHI and Shogo KOMORI

low CO2 emissions and positive SP anomalies above ahigh-conductivity zone were also observed in the summit
area at Stromboli volcano (Finizola et al., 2006). Theseobservational evidences can be explained by the modelthat the upper part of the hydrothermal zone is self-sealed
by hydrologically impermeable and electrically conductive
altered rocks (Aizawa et al., 2009). Therefore, the upper
part of C1 is likely to consist of such hydrothermallyaltered rocks, and a hydrothermal system has presumably
developed underneath the altered zone. Conversely, theelevated second layer is not seen beneath craters located on
the edge of Nabe-yama that erupted lava at the time of
Taisho eruption. Despite the fact that 90 years or more
have passed after the Taisho eruption, the heat source has
still affected the shallow subsurface under the crater on thewestern flank, whereas evidence suggests that a heatsource is not present beneath the eastern foot of Sakurajima.
6．Conclusions
We conducted an audio-frequency magnetotelluric
survey at the foot of Sakurajima volcano in order to reveal
the shallow region of the electrical resistivity structure.
From the result of 3-D modeling, the heterogeneous
resistivity structure beneath the flanks of Sakurajima,including the regions where lava effusion took place inpast large-scale eruptions, was clarified for the first time.
The following findings have been obtained.
(1) The shallow resistivity structure can be expressed by
two layers: a high-resistivity layer corresponding to surface
lava, and beneath this, a low-resistivity layer containing
seawater/groundwater. The first lava layer is distributed
thickly to the north and west of Sakurajima.
(2) The basement structure is not clearly detected,
except for along a section of the GK line. The conductive
second layer is distributed thickly over the entirety of
Sakurajima, and is likely to correspond to the Kekura
formation, a marine deposit layer.
(3) Although the boundary between the first and second
layers is generally located at around sea level, beneathcertain areas, such as the Haruta-yama lava dome and theNabe-yama pumice cone, the conductive second layer isfound at a deeper level. The fracture zone associated withprevious eruptive activity is expected to be formed above
this conductive layer.
(4) The conductive second layer is identified at a depth
shallower than sea level nearby the Hikino-hira lava dome,
suggesting that a hydrothermal system has developed underneath the vicinity of a crater row from the Taisho eruption
and around Hikino-hira. Conversely, no elevation of thesecond layer is found close to the Taisho crater on the
opposite side.
Acknowledgements
For assistance in implementing the AMT survey at

Sakurajima, we express our gratitude to the Osumi Officeof the River and National Highway and its Sabo Branch
Office in Sakurajima, the Kagoshima District Forest
Office, the Crisis Management Bureau of Kagoshima
Prefecture, and the Safety Division of Kagoshima City.
The staff at the Sakurajima Volcano Research Center of
Kyoto University is also acknowledged for their cooperation and useful discussions. A number of discussions with
Tomoki Tsusui of Akita University were fruitful forinterpreting the electrical structure model of Sakurajima.
Instruction concerning the use of the WSINV3DMT code
(Siripunvaraporn and Egbert, 2009) from Songkhun
Boonchaisuk of Mahidole University was very helpful.
Our thanks also go to the two referees, T. Kagiyama and an
anonymous one, and the Editor-in-Charge (M. Iguchi) for
their constructive comments and suggestions to improvethe manuscript. Numerical calculations were carried out
on TSUBAME2.0 at the Global Scientific Information andComputing Center of Tokyo Institute of Technology.
Most figures were prepared with General Mapping Tools(Wessel and Smith, 1991). This study was supported by
the Japanese Ministry of Education, Culture, Sports,
Science and Technology (MEXT), under the 7th National
Project for Prediction of Volcanic Eruptions.
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(Editorial handling Masato Iguchi)Shallow Resistivity Structure of Sakurajima Volcano Revealed by Audio-frequency Magnetotellurics

AMT 観測から推定される桜島火山の浅部比抵抗構造
神田径・山崎友也・小川康雄・橋本武志・坂中伸也・相澤広記・
高倉伸一・小山崇夫・山田健太・小林宰・小森省吾2007 年 11 月，第 7 次火山噴火予知計画の一環として，桜島火山の山麓で AMT 法比抵抗構造調査を実施した．南岳山頂火口および昭和火口では，ブルカノ式噴火が繰り返し発生するため，山頂付近へ近づくことができない．そこで我々は，山体の北，西，南東山麓にそれぞれ測線を設定し，合計 27 観測点で周波数
1〜10400 Hz の電磁場データを取得した．データは，2 Hz 以上の周波数帯で概ね良好であった．まず，各測線で得られたデータに対して 2 次元解析を行った．しかしながら，インピーダンスから推定される構造走向は，全ての測線においてほぼ南北方向を示し，2 次元解析は不適当であることが判明したことから，全てのデータを矛盾なく説明するために，3 次元解析を行った．3 次元モデリングの結果明らかとなった桜島の浅部比抵抗構造は，以下のような特徴を持つ．表層は溶岩層に対応する高比抵抗層で数百から数千Ωm の比抵抗値を示す．その下には，海水や地下水を含む層であると解釈される数Ωm 以下の低比抵抗層が続く．基盤
構造は，桜島の北部の一部を除いて明瞭ではなく，2 層目の低比抵抗層が厚く分布している．表層の高比抵
抗層と 2 層目の低比抵抗層との境界は，概ね海水準付近にあるが，ハルタ山溶岩ドームや鍋山軽石丘などの一部地域の下では，海水準より下に位置する．それらの地域では，過去の噴火に関係した破砕帯が形成されていると考えられる．また，引ノ平溶岩ドームから大正火口にかけての地下では，逆に低比抵抗層が地表近くまで盛り上がっており，熱水系の存在が示唆された．一方で，同じ大正噴火の際に溶岩流を流出させた鍋山の縁に位置する火口付近では，そのような低比抵抗層の盛り上がりは見られず，既に熱源が存在しないことを暗示している．

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