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8/8/2019 MT Study of Mt Fugi Hydro Thermal System AizawaEPSL05
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Hydrothermal system beneath Mt. Fuji volcano inferred from
magnetotellurics and electric self-potential
K. Aizawaa,*, R. Yoshimuraa, N. Oshimana, K. Yamazakia, T. Utoa, Y. Ogawab,S.B. Tankb, W. Kandac, S. Sakanakad, Y. Furukawad, T. Hashimotoe, M. Uyeshimaf,
T. Ogawaf, I. Shiozakig, A.W. Hursth
aResearch Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanb
Volcanic Fluid Research Center, Tokyo Institute of Technology, Ookayam 2-12-2, Meguro-ku, Tokyo 152-8551, JapancSakurajima Volcano Research Center, Kyoto University, Yokoyama Tsurusaki 1722-19, Sakurajima, Kagoshima 891-1419, Japan
dInstitute of Applied Earth Sciences, Faculty of Engineering and Resource Science, Akita University, Tegata-gakuen 1-1, Akita 010-8502, JapaneInstitute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo Kita 10 Nishi 8 cho-me,
Sapporo 060-0810, JapanfEarthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan
gDepartment of Civil Engineering, Faculty of Engineering, Tottori University, Koyama-cho minami 4-101, Tottori 680-8552, JapanhInstitute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand
Received 28 May 2004; received in revised form 10 December 2004; accepted 8 March 2005
Available online 17 June 2005
Editor: V. Courtillot
Abstract
Wideband magnetotelluric (MT) soundings were carried out on Mt. Fuji volcano along a northeast to southwest axis. It was
found by two-dimensional inversion using the highest quality data (in the frequency range 1300 Hz) that a good conductor
(resistivity of approximately a few ohm m) was located beneath the summit with a lateral extent of approximately 4 km. It begins
approximately 1 km below the ground surface; however, its depth cannot be resolved. In our previous study, an intense positive
self-potential (SP) anomaly (approximately 2000 mV), was found around a summit crater having a diameter of approximately 3
km. We interpreted the presence of the good conductor and positive SP anomaly as a strong indication of an active hydrothermal
system. Subsequently, we searched for conduction current sources to explain the SP distribution on the surface by using the
resistivity structure determined by the MT inversion. The results obtained were that a positive conduction current source of the
order of 1000 A should be located at the top of the conductor. From these results, we deduced that the conductor represents ahydrothermal system in which single-phase (liquid) convection is taking place. Since the resistivity at a distance from the good
conductor can be explained by the effect of cold groundwater, the hydrothermal system does not seem to extend throughout the
entire body of the volcano, but seems to be confined to the area beneath the summit crater. Finally, an estimate of the order of
magnitude of the subsurface hydrothermal flow was performed using a relation between the fluid volume flux and electric current
0012-821X/$ - see front matterD 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.03.023
* Corresponding author. Volcanic Fluid Research Center, Tokyo Institute of Technology, Ookayam 2-12-2, Meguro-ku, Tokyo 152-8551,
Japan.
E-mail address: [email protected] (K. Aizawa).
Earth and Planetary Science Letters 235 (2005) 343355
www.elsevier.com/locate/epsl
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density in the capillary model. The result suggested that there exists fairly low permeability within the shallow part of Mt. Fuji.
We speculate that the low permeability in the volcano has a correlation with the confinement of the hydrothermal system and
quiescence of volcanic activities, such as low seismicity, no gas emanations, and no natural hot springs.
D 2005 Elsevier B.V. All rights reserved.
Keywords: self potential; magnetotellurics; hydrothermal system; resistivity structure; volcano; permeability
1. Introduction
In recent times, the importance of investigating the
upper portion of the structure of volcanoes has been
recognized. This is because the interaction between
the magma and groundwater within the top few kilo-
meters might be the cause of precursory phenomenaand can control the type of eruption [1]. Most active
volcanoes have a hydrothermal system near the sur-
face, which transports heat from its depths to the
surface by convection. Hydrothermal systems in the
case of active volcanoes have been studied by various
geophysical and geochemical methods [26]. On the
other hand, hydrothermal systems are less understood
in the case of apparently dormant volcanoes, which
have no significant geothermal manifestations and
low seismicity. For a comprehensive understanding
of volcanic systems, it is important to know whether
the shallow part of apparently dormant volcanoes
differs from that of more active volcanoes. Further,
since apparently dormant volcanoes can erupt abrupt-
ly [7], the study of these volcanoes might be useful forgreater accuracy in prediction of eruptions.
Mt. Fuji, which is the highest (3776 m above sea
level) and largest stratovolcano in Japan, might be an
ideal volcano for studying such problems because of
its large eruption potential and present quiescence.
Mt. Fuji has had many historic eruptions, erupting at
least 17 times since 781 AD, and the most recent
eruption occurred in 1707 when approximately 1.7
Fig. 1. Self-potential distribution of Mt. Fuji [10] with self-potential (SP) and magnetotelluric (MT) observation sites. Topographic contours are
also shown. The black dots and star show the survey points and reference point of the SP surveys, respectively. The white triangles with
characters show MT survey sites and the site names. The thick lines indicate the locations of the summit crater and Hoei crater.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355344
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km3 ash was emitted and the Hoei crater was formed
on the southeastern flank 3 km away from the summit.
It is reported that the last eruption from the summit
occurred more than 2200 yrs ago [8]. Throughout itsentire history of approximately 100,000 yrs, more
than 99% of the erupted products of Mt. Fuji was
found to be basaltic. From geological study, it is
known that an old Quaternary volcanic body (10ka), which is 2400 m high above sea level, with high
SiO2 content (5160%) is superposed by Fuji ejecta
[9]. At present, Mt. Fuji is dormant. In recent years,
neither fumaroles nor geothermal manifestation can be
seen anywhere on the surface. Natural hot springs do
not exist on the outer areas of the mountain. Shallow
seismicity under Mt. Fuji is also low. Low-frequencyearthquakes are regularly observed at depths of 15 km
beneath the summit; however, no other related volca-
nic activity has been observed.
Recently, self-potential (SP) surveys revealed an
intense positive anomaly around the summit and
suggested the presence of a hydrothermal system
within Mt. Fuji [10]. Fig. 1 shows the SP distribution
using a reference point that is to the west of the Hoei
crater. A positive SP anomaly of approximately 3 km
diameter is centered on the summit crater and has an
amplitude of approximately 2000 mV. A remarkable
btopographic effectQ [11], in which SP has a negative
correlation of approximately 0.5 to 1 (mV/m)with an elevation due to the electrokinetic effect of
downward groundwater flow, was also observed at
elevations lower than 2000 m [10].
In this study, we investigate the shallow resistivity
structure obtained by the magnetotelluric (MT) sound-
ing and discuss the hydrothermal system of Mt. Fuji
using the SP distribution. Since the resistivity structure
might affect the SP distribution, a combined analysis is
more realistic for studying the dynamics of the hydro-
thermal system within the volcano.
2. Shallow resistivity structure of Mt. Fuji
2.1. Data acquisition and processing
In order to image the hydrothermal system that has
been suggested by the previous SP surveys [10], we
performed wideband MT soundings on a survey line
trending northeast to southwest (Fig. 1). We also
acquired data of the Hoei crater (H0 in Fig. 1). The
data was acquired during a period from 9 September
to 19 September 2002. The typical recording duration
for one site was 5 to 10 days. Natural electric andmagnetic fields in the frequency range of 300 to
0.0005 Hz were measured at the surface using Phoe-
nix MTU5 and MTU2E systems. Since the leakage
electric current from railway lines seriously contam-
inates the data in the lower frequency range, we used
remote reference processing [12] to calculate the
noise-free impedance by using geomagnetic data
that was recorded in northern Hokkaido (1000 km
north of Mt. Fuji). This procedure modified the im-
pedance processed at a single site in the frequency
range below 10 Hz. In this study, we used the highest-quality impedances in the frequency range between 1
and 300 Hz. At higher elevation sites (over 2200 m),
we did not record geomagnetic data; therefore, we
used geomagnetic data from other sites to calculate
the impedances.
2.2. Dimensionality and distortion analysis
Fig. 2 shows the impedance skew [13] for all sites.
The skews are less than 0.1 except at the sites M0,
M1, and M2, which are located at a high elevation on
the mountain; this suggests a regional 2D or 1D
resistivity structure. However, in the summit area,
the presence of a local three-dimensional distortion
or 3D structure is suggested.
We investigated the dimensionality of the data
using the tensor decomposition technique of Groom
and Bailey [14]. In order to investigate whether the
Fig. 2. The impedance skews for all sites. The curves for the summit
sites, M0, M1, and M2 (as shown in Fig. 1) are individually labeled.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355 345
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structure is regionally 2 D with local shallow 3D
anomalies, distortion parameters were first determined
with no constraints (i.e., site-dependent and frequency-
dependent constraints). Fig. 3 shows the histograms ofthe estimated regional strike in two frequency ranges.
The cumulative estimated strikes slightly scatter at
northeastern sites; however, they basically converge
in one direction suggesting that there exists a NWSE
trending regional 2D resistivity structure within the
shallow part of Mt. Fuji. The distortion parameters
(twist and shear) were also determined to be frequency
independent (maximum fluctuation is less than 38) on
each site.
2.3. Two-dimensional analysis
Mt. Fuji has numerous parasitic cones trending a
NWSE direction [9], suggesting that the subsurface
structure is two dimensional with a NW SE trend.
The estimated regional strike directions (Fig. 3) are
in agreement with this geological trend. In this study,
we assumed the regional strike direction (parallel to
the 2D structure) to be N45W and conducted a 2D
inversion using the code of Ogawa and Uchida [15].
After fixing the N45W regional strike direction, all
the impedances were decomposed using site-depen-
dent and frequency-independent distortion para-
meters. Subsequently, we inverted the apparent
resistivity and the phase in the decomposed TM
mode (electric current flowing across the structure)
because the TM data is more robust in the presence
of three-dimensional anomalous structures [16]. The
starting model had a uniform resistivity of 100 V m
as well as 2D topography. The different locations of
electric and magnetic fields at certain observation
sites were taken into account in the inversion. Fig.
4 shows comparisons between the observed and
calculated data sets. The overall fit was good forall sites.
Fig. 5 shows the best-fit resistivity model. A good
conductor with a lateral extent of approximately 4 km
is located approximately 1 km beneath the summit.
We performed two forward calculations (sensitivity
tests) in order to assess the uniqueness of the conduc-
tor with modifications to the best-fit resistivity. First,
we changed the resistivity value of the conductor to a
uniform 30 V m value. Secondly, we placed the top of
the conductor at a depth of 700 m. Since the calcu-
lated sounding curves do not fit the observed sound-ing curves in the sites around the summit in either case
(Fig. 6), our model of a conductor with a resistivity of
approximately 10 V m located 1 km beneath the
summit is confirmed. Due to attenuation of the elec-
tromagnetic field penetrating into the body of the
conductor, the downward extent of this conductor is
uncertain.
Since the topography of Mt. Fuji is 3D and the SP
data suggest the presence of a 3D local structure
beneath the summit, the 2D modeling is not strictly
valid. However, when the 3D conductive structure is
located below a survey profile and has a horizontal
extent greater than one-half of a skin depth, it is found
that the effects of finite horizontal extent of the 3D
conductor in a 2D analysis are not significant [17].
The shallow resistivity structure of Mt. Fuji is inter-
preted as a regional 2D structure with an imbedded
3D conductor beneath the summit. The SP distribu-
tion, which suggests the presence of a hydrothermal
Fig. 3. Histograms for the regional strikes from Groom and Bailey [14] tensor decompositions in two frequency ranges. Distortion parameters
were set to be site-dependent and frequency-dependent. Note that the 908 ambiguity is included in each histogram. Site names are shown above
each histogram.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355346
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system beneath the summit, might support this inter-
pretation. Therefore, we interpret the best-fit 2D re-
sistivity structure as a first-order cross section of a 3D
structure for which some features might have a finite
horizontal extent. Although we modeled the 3D to-
pography as 2D, this will not enhance the resistivity
value of the good conductor because we used only the
TM mode in the 2D model and replaced air with finite
resistivity blocks. The 3D modeling will be the sub-
ject for future study.
2.4. Features of the resistivity structure
Besides the summit area, the shallow resistivity
structure of Mt. Fuji is essentially characterized by 3
layers. The first layer is a surface layer with a
thickness of several hundred meters and a resistivity
of approximately 1000 V m. The result of the sci-
entific drilling [18], which was carried out on the
northeast flank (1 km northwest of Y3) in 2003,
showed that the first layer is a porous and unsatu-
Fig. 4. Comparison between calculated and observed sounding curves. Only the decomposed TM mode data was used for 2D inversion. The
solid circles indicate observed responses with one standard deviation. The solid lines indicate calculated responses from the best-fit resistivity
model. Site names are shown above each panel.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355 347
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rated layer. Core samples collected at a depth of 650
m revealed that the composition was primarily mud-
flow deposits with three sheets of lava flows. The
well-logging data showed that the porosity was from
10% to 40% and the resistivity was from 500 to
1000 V m. Further, the results revealed that aquifers
did not exist in the first layer because the water
pressure did not rise significantly during the drilling.
The second layer is located at depths between
several hundred meters and 2 km and is relatively
conductive (resistivity of approximately 50 to 200 V
m). Since the resistivity of the spring water at 18 8C
is between 50 and 200 V m at any place on the outer
surface of Mt. Fuji [19], the second layer is inter-
preted as an aquifer where a large quantity of ground-
water saturates the host rock. The third layer is the
layer that is deeper than 2 km and it shows resistivity
from 500 to 1000 V m. This layer might represent a
water poor region in which the cracks were closed by
pressure.
The first and second layers are more conductive by
one order of magnitude within a radius of about 2 km
around the summit area than in other places at the
same depths. The positive SP anomaly, whose radius
is about 1.5 km, is located above this conductor ( Fig.
1). This relation between a conductive zone and pos-itive SP anomaly is similar to that for the Miyakejima
volcano [20,21], where changes of the hydrothermal
system are suggested to precede eruption activity [21].
Since anomalous crustal movements have not been
detected around Mt. Fuji and the seismicity is low,
there is no reason to believe that the good conductor is
a magma body. In this study, we interpret the presence
of a good conductor and a positive SP anomaly as an
indication of an active hydrothermal system. The
existence of active fumaroles at approximately 80
Fig. 5. Best-fit resistively model obtained by the 2D inversion of the MT data. The decomposed TM mode data were used for inversion. The 2D
regional strike direction (parallel to the 2D structure) is assumed to be the NWSE direction. The inverted triangles with characters indicate the
measurement sites. Note that the good conductor beneath the summit prevents the penetration of the electromagnetic field into the region beneath the summit.
Fig. 6. The results of sensitivity tests at site M0. The solid black dot
shows observed data, and the solid line represents a sounding curve
calculated from the best-fit resistivity structure (Fig. 5). The heavy
dashed line shows the sounding curve calculated from the structure
on which the resistivity of the good conductor, which is located at a
depth from 1 to 2.4 km and has a horizontal extent of 4 km, was
changed to a uniform 30 V m. The fine dashed line represents the
sounding curve calculated from the structure on which the conduc-
tors top was made 700 m deep with 5 V m resistivity blocks.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355348
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8C on the summit 70 yrs ago [9] might support the
theory of the presence of the hydrothermal system.
Although several SP-generating mechanisms (e.g.,
electrochemical, thermoelectric, electrokinetic, andrapid fluid disruption) have been proposed [22,23],
it is widely believed that only the electrokinetic (EK)
effect can be the source of large SP anomalies of the
order of volts [24]. Hereafter, we interpret the SP
anomaly on the basis of the EK effect.
3. The modeling of SP sources
3.1. Electrokinetic potential
The flow of a fluid through a porous medium gen-
erates an electrical potential gradient (referred to as
belectrokineticQ orbstreaming potentialQ). Sill [25] pre-
sented a method for the investigation of self-potential
based on induced current sources. The general relations
between the electric current density J (A m 2), the
fluid volume flow density (Darcy velocity) U (m3
m2 s1), the electric potential U (V), and the hy-
draulic potential n (Pa) are expressed by the coupled
equations [26] in the case of static conditions as
follows:
J L11jU L21jn; 1
U L21jU L22jn; 2
where r=L11, L21, and L22 represent the electrical
conductivity (S m1), the cross coupling coefficient
(m2 s1 V1), and the hydraulic conductivity (m2
Pa1 s1), respectively. When the effects of the sec-
ondary electric potentials on the fluid flow are small,
the primary flow equation is decoupled and the result-
ing equations are as follows:
U L22jn Darcy0s law ; 3
and
J rjU L21jn: 4
The first term on the right-hand side of (4)
(rjU) represents a conduction current (Jcond) driv-en by the electric potential, while the second term on
the right-hand side of (4) ( L21jn) represents aconvection current (Jdrag) driven by fluid flow. The
electric potential difference that is measured on the
surface is related only to the conduction current. Since
external current sources can be neglected in static
conditions,
jd J 0; 5
and by substituting this in Darcys law results, we
obtain
jd Jcond jdJdrag jL21
L22
dU
L21
L22jdU: 6
Thus, sources of conduction current exist wherever
there are gradients of the cross-coupling coefficient or
permeability parallel to the fluid flow or wherever
there are external or induced sources of the fluid
flow. This mechanism is schematically illustrated inFig. 7. Since it can be concluded that the self-potential
measured on the surface is generated from conduction
current sources, an analysis that focuses on the fea-
tures of conduction current sources would provide
information on the subsurface fluid flow.
3.2. Estimation of the SP sources
The fundamental purpose of the SP and MT sur-
veys is to elucidate the subsurface hydrothermal flow.
However, achieving this is difficult because of the
Fig. 7. Schematic illustration of the generation of a streaming
potential. A situation of uniform fluid flow perpendicular to plane
boundaries in a static condition is shown. In the porous media,
electric charge is conveyed by convection (convection current);
however, no charge is conveyed outside the porous media. There-
fore, the charge accumulation occurs on both sides of the porous
media, and the conduction current flows in the direction opposite
the convection current to cancel the charge separation. The voltage
difference is caused by the conduction current.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355 349
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large uncertainties in the physical parameters, such as
permeability, cross-coupling coefficient, resistivity,
and phase of fluids. However, In this study, the resis-
tivity structure is known, and relatively realistic mod-eling can be performed. We conducted an approximate
estimate of the conduction current sources by forward
modeling that included the resistivity structure. The
purpose is to deduce the spatial relationship between
the location of the SP source and resistivity structure
and to investigate the subsurface electric charge accu-
mulation process.
The forward code used in this study is based on the
finite-difference schemes proposed by Dey and Mor-
rison [27]. In forward modeling, we used the simple
structure, as shown in Fig. 8, representing the primaryfeatures of the best-fit resistivity model. The good
conductor was expressed as 8 V m blocks and was
located at depths greater than 1 km. We interpreted the
best-fit 2D resistivity structure (Fig. 5) as the first-
order cross section of the 3D structure that has a finite
horizontal extent and assumed that the 3D structure is
symmetric around the summit. Although theseassumptions are inexact, they might be sufficient for
an approximate estimation of the SP sources. The
mesh consisted of 73 73 horizontal points (onegrid point interval is 150 m) and 100 vertical points
(one grid point interval is 100 m) and air was defined
as a highly resistive zone (resistivity of 3 1013 V m).For simplicity, the conduction current sources are
represented by a pair of point sources of positive and
negative polarities that is located in a vertical line and
their locations, depths, and intensity were changed in
order to fit the observed SP data on the surface. Wealso incorporated the position of the SP reference
point and the btopographic effectQ (TE) coefficient
as model parameters. The TE coefficient, as noted in
Fig. 8. 3D electrical resistivity model used in the forward modeling for the SP source estimation. X, Y, and Z axes denote the east, north, and
vertical direction, respectively. The mesh consisted of 73 73 horizontal points (one grid point interval is 150 m) and 100 vertical points (onegrid point interval is 100 m). The color scale bar is identical to the one in Fig. 5.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355350
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the introduction, is assumed to be a linear function of
elevation, and the calculation was performed with four
TE coefficients (Cases 1 to 4) 0, 0.5, 1, and 1.5
(mV/m). We also performed the calculation withoutthe constraint of the TE coefficient (Case 5) in order
to estimate a plausible range for the TE coefficient. In
each case, the appropriate model parameters are de-
termined by a grid-search technique in order to min-
imize the RMS-misfit between the observed and
calculated SP on the surface. The searched space
was the range that extends 1500 m horizontally and
5300 m vertically from the center of the summit crater.
The modeling procedure is similar to the analytical
approach of Kanda and Mori [28], which assumed that
the resistivity structure is uniform and current sourcesare located just beneath the fumarolic vent in the
summit. However, this study considers the resistivity
structure, and the horizontal source locations are not
constrained.
Table 1 shows the determined parameter range and
RMS misfit for each case. We interpreted those para-
meters among the results that produced an equivalent
misfit level (RMS in the best-fit model + 0.002 for
each TE coefficient) as the reasonable parameter
range. The smallest RMS is obtained in Case 3, and
this suggests that a TE coefficient of 1.0 mV/m isreasonable in the case of Mt. Fuji. Since the deter-
mined horizontal source locations were within a circle
with a diameter of 300 m and a center located 400 m
northwest of the summit crater center in all cases, only
the depth of the source is shown in the table. Consid-
ering that the RMS in Case 1 is relatively large, the
depth of the positive source is determined to be
between 1.0 and 1.5 km. This is an important result
because this depth corresponds to the top of the good
conductor (Figs. 5 and 8). In addition, the strength of
the sources was determined to be of the order of 1000
A, and the negative source was determined to be
deeper. With regard to the depth of the positive source
and strength of the sources, the results did not varywhen we placed two sources obliquely instead of in a
vertical line and searched the current sources that
express an asymmetric positive SP anomaly on the
surface by trial and error.
4. Discussion and conclusion
When the presence of a hydrothermal system on
Mt Fuji was suggested, it seemed reasonable that it
would merely be a single-phase (liquid) hydrothermalsystem because there is no gas emanation on the
surface. The laboratory experiments show that the
zeta potential, which primarily contributes to convec-
tion currents, depends on the temperature [26,29]. By
numerical modeling, Ishido and Pritchett[30] showed
that positive charges accumulate due to this tempera-
ture dependence of zeta potential in the zone where
the upwelling fluid begins to move sideways and
cools. The positive charges produce a positive SP
anomaly on the surface in a single-phase (liquid)
case. The results of our forward modeling that the
conduction current sources are primarily accumulated
at the top of the conductor are consistent with these
results of Ishido and Pritchett [30]. By considering
that either a rock with a large quantity of hot water or
a hydrothermal altered rock is highly conductive
[31,32], we interpret that the good conductor beneath
the summit represents the hot-water saturated and
altered zone in which single-phase convection is
occurring. The schematic model for the hydrothermal
system beneath Mt. Fuji is shown in Fig. 9. At the top
Table 1Estimated source parameter range by forward calculation in which two conduction current sources in a vertical line are imposed on the
resistivity structure shown in Fig. 8
Increment Case 1 Case 2 Case 3 Case 4 Case 5
Topographic effect (mV/m) 0.1 0 (fix) 0.5 (fix) 1 (fix) 1.5 (fix) 0.8 to 1.2Source intensity (A) 10 70330 2901230 8701500 13401387 7801880
Depth of a positive source (km) 0.1 0.51.1 0.91.5 1.11.5 1.21.5 1.11.7
Depth of a negative source (km) 0.1 1.55.3 3.35.3 4.44.9 4.55.1 4.15.3
Best-fit RMS (V) 0.3092 0.3035 0.3002 0.3024 0.3002
The results that produced the equivalent misfit level (RMS bRMS in best-fit model+0.002) were shown in each case. The grid search was
performed in five cases that assume the topographic effect (TE) coefficients as 0, 0.5, 1, 1.5 (mV/m), and unconstrained (Case 5). In thecalculation without the constraint of TE coefficients, 1.0 mV/m was the best-fit TE coefficient.
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of the hot-water saturated zone, the upwelling flow
turns sideways and cools; hence, the positive current
sources accumulate and generate the positive SP
anomaly on the Earths surface. The hydrothermal
system probably does not extend within the whole
volcano because the resistivity value outside the good
conductor can be explained by cold (18 8C) ground-
water content. It should be noted that the current
discussion holds only when the upwelling zone does
not have a large lateral extent because we expressed
the charge accumulation process using point currentsources.
Under static conditions, we can conduct an order of
magnitude estimation of the fluid mass flux by making
some more assumptions. When fluids are incompress-
ible and dehydration from magma does not occur,
there is no divergence of fluids and the current source.
Hence, Eq. (6) reduces to the following equation:
jdJcond jL21
L22
d U: 7
When the capillary model [26] is considered, the
equation becomes the following:
jdJcond j gt2enk
dU; 8
where g, t, and k denote porosity, tortuosity, and
intrinsic permeability (m2) of the porous media, re-
spectively. e denotes the dielectric constant of the
fluids (F m1), and f denotes the zeta potential (V).
Ifg, t, k, e, and U are constant in the region of charge
accumulation, we obtain
Jscond gt2eDf
kU; 9
where Jscond and U denote source intensity (A) and
fluid volume flux (m3 s1). Df denotes the difference
in the zeta potential through the direction of fluid flow.
When we assume the order of magnitude of each
parameter as gt2=0.1, e= 1010, k= 1014 [28],
Df= 0.1 [26], and Jscond=1000 (Table 1), the fluidmass flux is calculated to be of the order of 106 (ton/
day). If we assume the area of the upwelling path to
be from 104 to 106 (m2), the speed of the fluid
upwelling is calculated to be approximately 1 to
100 (m/day). Although the large uncertainty of phys-
ical parameters, especially in permeability, makes
this estimation somewhat uncertain, the speed of
the fluid seems to be extremely high. In the case
of Mt. Fuji, a considerably lower permeability might
be more realistic.
Even in the apparently dormant Mt. Fuji, the resis-
tivity structure seems to require hydrothermal circu-
lation as is suggested in the case of certain active
volcanoes; however, there is a difference in its struc-
ture. The difference lies in the horizontal extent of the
conductor beneath the summit crater. In the case of
active volcanoes, there are examples in which theshallow conductor is not limited to only the region
beneath the main summit, but extends to the entire
region beneath the volcano [3136]. A possibility that
explains this feature of the resistivity structure of Mt.
Fuji is the permeability that was discussed previously.
It is speculated that the hydrothermal system cannot
develop and extend to the whole volume of Mt. Fuji
due to the rather low permeability in the volcano. The
quiescence of volcanic activities, such as low seismic-
ity, no gas emanations, and no natural hot springs,
Fig. 9. Schematic model of the hydrothermal system of Mt. Fuji.
The thick arrows show the movement of hot fluids. The thin arrows
show the movement of relatively cool fluids. The good conductor
imaged by the magnetotelluric sounding was interpreted as a hot-
water saturated zone. The circles with plus and minus signs repre-
sent positive and negative conduction current sources, respectively.
The positive conduction current sources are accumulated at the top
of the hydrothermal upwelling.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355352
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might be related to the structure of and permeability in
the volcano.
The SP distribution (Fig. 1) suggests certain fea-
tures of the hydrothermal system. A remarkable SP positive anomaly exists only around the summit
crater and not around the Hoei crater. The Hoei
eruption in 1707 was the latest eruption, whereas
the last eruption on the summit occurred more than
2200 yrs ago [8]. Since the sounding curve obtained
in the Hoei crater does not show any significant
difference from those of the other sites at the same
elevation, the good conductor might not exist direct-
ly beneath the Hoei crater (Fig. 10). Therefore, the
hydrothermal system does not seem to be active
directly beneath the Hoei crater at present; this sug-gests that the growth process of the hydrothermal
system differs between the summit and the flank.
The heat that drives the hydrothermal system might
have been supplied only beneath the summit crater
for a long duration.
In Mt. Fuji, it is found that the good conductor is
located beneath the positive SP anomaly. The goodconductor has a lateral extent of approximately 4 km
and extends downward from 1 km below the surface.
The positive SP anomaly has a diameter of approxi-
mately 3 km. Based on the SP modeling using a pair
of positive and negative conduction current sources in
a vertical line imposed on the axisymmetrical resis-
tivity structure, the positive source was determined to
be located at the top of the conductor. These results
suggested the presence of a single-phase (liquid) hy-
drothermal system even in the apparently dormant
volcano. However, there are some volcanoes wherea dominant positive SP anomaly is not observed
around the peak; instead, only a strong btopographic
effectQ is observed [24,37]. The knowledge regarding
Fig. 10. Comparison between the sounding curves that are obtained at the Hoei crater and other sites at the same elevation. Site names and
elevations are shown above each panel. Regional strikes are assumed to be N45W and un-decomposed data are shown. The impedance skew of
H0 (at the Hoei crater) is approximately 0.2 for all frequencies. Note that the good conductor is not seen directly beneath the sites S1 and M2.
The curves for H0 are not significantly different; therefore, probably, a good conductor does not exist directly beneath the Hoei crater.
K. Aizawa et al. / Earth and Planetary Science Letters 235 (2005) 343355 353
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the extent to which good conductors exist within
apparently dormant volcanoes is sparse. In order to
better understand the SP generating mechanism and
hydrothermal systems in volcanoes, it would be im- portant to study the relationship between SP anoma-
lies and subsurface conductors by conducting surveys
on both active and quiescent volcanoes.
Acknowledgments
We thank T. Kagiyama and N. Osada for providing
support for the arrangement of our field survey. We
also thank the Fujinomiya-Sengen Shrine and Yama-
nashi and Shizuoka Prefectures for permitting the fieldobservations. T. Mogi allowed us to use the MT data
sets of his own survey in Hokkaido for remote-refer-
ence processing in this study. Critical and useful
comments from Dr. Dominique Gilbert and an anon-
ymous reviewer greatly improved the manuscript. The
field operation was partly supported by the new Pro-
gram of the Study and Observation for Earthquake
Prediction. This research was partly supported by
special coordination funds for promoting science
and technology.
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