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Title

Three-dimensional magnetotelluric imaging of crustal fluids andseismicity around Naruko volcano, NE Japan

Authors

Yasuo Ogawa, Masahiro Ichiki, Wataru Kanda, Masaaki Mishina, Koichi
AsamoriCitation

Earth, Planets and Space, Vol. 66, ,Pub. date2014, 12Creative Commons

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Ogawa et al. Earth, Planets and Space 2014, 66:158
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LETTER

Open AccessThree-dimensional magnetotelluric imaging ofcrustal fluids and seismicity around Naruko
volcano, NE Japan
Yasuo Ogawa1*, Masahiro Ichiki2, Wataru Kanda1, Masaaki Mishina2 and Koichi Asamori3

Abstract
We analyzed the 3-D resistivity structure beneath Naruko volcano, northeastern Japan, with the aim of imaging
3-D distribution of fluids in the crust for its volcanic and seismogenic implications. The data were recorded at 77
sites in total: 30 sites are new and are arranged in an approximately 5 × 5 km grid whereas the remaining older sites
constitute two separate east-west profiles. We ran a 3-D inversion using full components of impedance tensors in
the period range between 0.13 and 400 s. The resulting model showed that a sub-vertical conductor exists a few
kilometers below Naruko volcano. The conductor extends from the surface of the volcano and dips towards thesouth, away from the volcano towards the backbone range. High levels of seismicity are observed in the upper
crust above and around the conductors. We suggest that the seismicity is fluid driven and that a fluid trap iscreated by the precipitation of quartz owing to a reduction in solubility at shallow depth. The Quaternary volcanic
front is characterized by a sharp resistivity contrast and a high-resistivity zone and extends 10 to 15 km towards
the east. A fore-arc conductor was observed at mid-crustal levels even farther towards the east. The sub-verticalconductors along the arc and the fore-arc conductor have resistivities of 1 to 10 Ωm. Assuming a Hashin-Shtrikman
model with saline fluids of 0.1-Ωm resistivity, a porosity of 1.5% to 15% is required to explain the observed
conductive anomalies.
Keywords: Magnetotellurics; 3-D modeling; Saline fluids; Northeastern Japan; Quaternary volcanoes; Seismicity

Findings
Introduction

The magnetotelluric (MT) method involves the measurement of time-varying natural electromagnetic fields
for the remote detection of small volumes of fluid or
melt in the crust and upper mantle (e.g., Hill et al. 2009;
Wannamaker et al. 2009; Ingham et al. 2009; Aizawa
et al. 2014). The bulk resistivity of the fluid- or meltbearing rock is not controlled by the resistivity of thehost rock, which is normally as resistive as 106 Ωm, butby the interconnectivity of the conductive fluid or meltin pore spaces or along grain boundaries, which can reduce the host resistivity to below 1 Ωm. Thus, the sensitivity of the electromagnetic method allows minor* Correspondence: oga@ksvo.titech.ac.jp
1
Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1
Ookayama, Meguro, Tokyo 152-8551, Japan
Full list of author information is available at the end of the article

amounts of fluid, and the interconnectivity of fluids ormelts in the crust and mantle, to be mapped. The transport of these minor amounts of fluid through the rockcan also be estimated since the required connectivity
must fit the low bulk resistivity of the rock.
Fluids play an important role in subduction systems(e.g., Iwamori 1998). Northeastern (NE) Japan can beclassified as a typical subduction zone and is therefore asuitable test site for the study of fluid distributions. The
dehydration of the subducting slab in the mantle wedge
results in the upward migration of fluids, causing peridotite to be converted to serpentine, which may be detected as conductive anomalies. Fluids can also lower
the melting point of peridotite and magmatic melts aswell as fluids will be created and transported upward towards the crust (Kawamoto et al. 2012). The presence of
fluid in the crust causes localized deformation and seis-© 2014 Ogawa et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly credited.

Ogawa et al. Earth, Planets and Space 2014, 66:158
http://www.earth-planets-space.com/66/1/158

Page 2 of 13

micity (Hasegawa et al. 2005; Ogawa and Honkura
2004).
The MT method has been used to detect the presenceof fluids or melts in the crust. Electromagnetic induction studies of NE Japan have been performed since the1950s and have used band-limited instruments. The 2-D
models presented in the 1980s were primitive andshowed lower crustal conductors in the back-arc(Ogawa et al. 1986; Ogawa 1987; Utada 1987). In the
2000s, wideband MT equipment and 2-D inversion
methodologies allowed detailed 2-D crustal resistivity sections in seismogenic zones to be determined (Mitsuhata
et al. 2001; Ogawa et al. 2001). These studies showed thatzones of high seismic activity were associated with resistive zones underlain by mid-crustal conductors. The observations are consistent with a fault-valve model (Sibson
et al. 1988; Sibson 2009). Mishina (2009) and Asamori
et al. (2010) performed 2-D inversions on MT profile dataand suggested the existence of sub-vertical conductors
under Naruko volcano and surrounding regions (Figure 1).
The present study considers the two previous profile
datasets together with recent data recorded by gridspaced MT sites. The objective of this research was togenerate a 3-D resistivity model that is correlated withthe distribution of volcanic and seismic activity around
Naruko volcano, NE Japan.

The present study area is located in the central region of
the NE Japanese arc (Figure 1). The area now known as
NE Japan was in the past located at the margin of theEurasian continent and rifted apart during the openingof the Sea of Japan in the Miocene (Sato 1994). In thelate Miocene (7 to 5 Ma), when the tectonic stress was
close to neutral, large collapsed calderas with diametersof approximately 10 km were formed in the study area
(Yoshida 2001; Yoshida et al. 2013). From the late Pleistocene (1 Ma to the present), the area has been subjected totectonic east-west compression, resulting in active volcanism such as Naruko and Mukaimachi volcanoes in the
study area, Onikobe caldera volcano located farther north,and Funagata-yama volcano located farther south.
This study includes 32 new MT sites and 45 sites fromprevious studies along profiles (Mishina 2009; Asamori
et al. 2010). All data, new and old data, were obtainedusing a Phoenix MTU5 system (Phoenix Geophysics,
Toronto, Canada) with periods ranging from 0.003 to
2,000 s. The new sites span the area to the south of
Naruko volcano using a grid arrangement with approximately 5-km spacing designed to image the crust around
Naruko volcano in three dimensions.
Figure 2 shows the distribution of apparent resistivity
and phase from the determinant of impedance tensors

OnikobeShinjoBasin
L1Data acquisition

Mukaimachi
Naruko

L2
L3
AkakuraL4

SendaiPlain

Yamagata
Basin

Funagata-yama

Figure 1 Study area in the northeastern Japan arc (inset map) and magnetotelluric sites around Naruko volcano. Purple dots denotenew magnetotelluric sites used in this study. Yellow and blue dots denote magnetotelluric sites from previous studies by Mishina (2009) andAsamori et al. (2010), respectively. Major Quaternary volcanoes are shown by triangles. Late Miocene collapse calderas are outlined by dottedlines following Geological Survey of Japan (2013).

Ogawa et al. Earth, Planets and Space 2014, 66:158
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Page 3 of 13

log10(det ρa)

5

3

N

0

2

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1

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0.4s (Cal)

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20Distance(km) N+

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N

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10

(deg)

M

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Distance(km) N+

0.4s (Obs)det Phase4log10( m)15

Distance(km) N+

(a)

5
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Distance(km) E+

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20

Figure 2 Apparent resistivity and phases from determinant of impedance tensors for four representative periods. (a) 0.4 s, (b) 4 s,(c) 40 s, and (d) 400 s. For each period, observed and calculated apparent resistivities and phases are shown by colored circles. The two triangles
represent Naruko volcano in the east and Mukaimachi caldera in the west.

Ogawa et al. Earth, Planets and Space 2014, 66:158
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for the four representative periods (0.4 to 400 s). They
are calculated as below:
ρinv
a ¼
φ¼

j detZ jωμ1
tan−1 ð detZ Þ
2These values do not depend on the coordinates, arerotation invariant, and are useful in understanding thegeneral feature of the dataset, although the apparent resistivity may still be affected by the near-surface inhomogeneities. Observed low resistivities in the shortperiod (yellow and orange circles in Figure 2a,b) correspond to the sedimentary layers of Sendai plain, Yamagata, and Shinjo basins (see Figure 1). As the period
increases, Naruko volcano (N in the figure) is surrounded by resistive apparent resistivities (Figure 2c,d).
However, the area around Mukaimachi caldera (M in the
figure) to the west of Naruko volcano is surrounded bymore conductive apparent resistivities. This basicallycorresponds to the existence of the major conductor between Naruko volcano (N) and Mukaimachi caldera(M), as we will show later. The determinant phases arealso consistent with this feature. In the right column of
Figure 2a, high determinant phases are evident at thesoutheastern corners of the map, corresponding to thesedimentary conductor of Sendai Plain. In the long periods (Figure 2b,c,d), high phases are getting evident to
the west of Naruko volcano, implying an existence of the
major conductor. All of the panels in Figure 2 also showrespective model responses of the final model, which will
be described later.
The phase tensor of the recorded dataset (Caldwell
et al. 2004) is shown in the left column of Figure 3 atfour representative periods. The phase tensor can graphically show the directionality and dimensionality of thedataset without being affected by local conductivity
anomalies or any assumptions about 2-D structures. The
upper and lower panels in Figure 3a show the phase tensor for the observed and calculated impedances at 0.4 s,
respectively. The ellipse sizes are normalized by the
maximum axis. The color bar represents the arithmetic
average of tan−1(φmin) and the tan−1(φmax) of the phasetensor where the warmer colors indicate that the structures are more conductive. To prevent misleading conclusion from the poor-quality data, only good data witherror of both off-diagonal impedance phases less than
10° were plotted. The shapes of the phase tensor ellipses
are more circular than those with longer periods.
Figure 3b shows the distribution of phase tensor ellipses
at a period of 4 s. The colors and shapes of the phasetensor show more consistency between neighboringsites. The southern central sites show E-W elongatedPage 4 of 13

elliptical shapes, implying a N-S elongated resistive block
surrounded by conductors (see R2 in Figure 4b,c,d). The
distribution of phase tensor ellipses for the period of 40
s is shown in Figure 4c. In this figure, the shapes are
more consistent and the thin ellipses align in a WNWESE orientation, suggesting the presence of a conductor
with a NNE-SSW orientation. The suggested conductor
follows the arc direction, as well as the coastline in thearea. The model calculation described later in this paper
also includes coastal effects from the Pacific Ocean and
the Sea of Japan, but such thin ellipses were not explained without crustal conductors elongated NNESSW. We also observed changes in elliptic eccentricity
at Naruko volcano where the ellipses are more circular
towards the west and more elongated towards the east.
Figure 3d shows the phase tensor ellipses for the periodof 400 s. At this period, the colors and shapes of the ellipses exhibit a similar distribution to those for theperiod of 40 s.
The β values in phase tensor analyses were plotted in
the right column in Figure 3, which represent threedimensionality. The sites around Naruko volcano havelarge β values (Figure 3a). This is due to the shallow
conductor located at Naruko volcano, which is surrounded by a more resistive upper crust. Towards longer
periods, there are more sites with large β. At the periodsof 40 and 400 s (Figure 3c,d), the high β values occupy
the eastern part of the study area. This feature seems
contradictory to the consistency of the distribution of
the ellipses in the left column of Figure 3c,d. The causeof this feature will be discussed later.
3-D inversion

We used the 3-D inversion code by Siripunvaraporn andEgbert (2009), which had been parallelized for multiple
frequencies. Full components of the impedance tensors
for 77 sites were used in the inversion to produce 3-D
resistivity structures. We selected eight periods of data,from 0.13 to 400 s in half-decade spacing. The horizontal mesh consisted of 1 × 1 km cells in the central coreregion (30 × 30 km), and the outer cells' sizes were gradually increased to cover 465 × 465 km. This mesh design
allowed the coastal effect from both sides of the oceans,
and from the Tsugaru Strait between Honshu and Hokkaido, to be incorporated. For vertical cells, the surface
cells were 100 m in thickness and the thickness was increased with depth. The base of the cells was at a depthof 139 km. In total, 60 × 60 × 37 cells were used, including seven layers above ground.
The initial model assumed a uniform earth having a
resistivity of 100 Ωm surrounded by ocean with a fixedresistivity of 0.25 Ωm. This initial model was also used
as a prior model, and a smooth model was obtainedusing the Occam algorithm. The error floor of theOgawa et al. Earth, Planets and Space 2014, 66:158
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Page 5 of 13

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Figure 3 (See legend on next page.)

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Ogawa et al. Earth, Planets and Space 2014, 66:158
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Page 6 of 13(See figure on previous page.)
Figure 3 Phase tensor ellipses (left column) and β (right column) for four representative periods. (a) 0.4 s, (b) 4 s, (c) 40 s, and (d) 400 s.
For each period, observed and calculated phase tensor ellipses and β are shown for comparison. The size of each ellipse is normalized by the
major axis. The color of each ellipse shows 12 ðatn ðΦ minÞ þ atn ðΦ maxÞÞ. The two triangles represent Naruko volcano in the east and Mukaimachi
caldera in the west.

(a)

c3

L1
L2

N
c2

L3
L4

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c4

2
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10(g)15 25km depth
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c3

distance(km) N+

distance(km) N+10

log10( m)(e) 15km depth
4

15

−15

0
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0
distance(km) E+

10

20

Figure 4 Resistivity depth sections for different depths. (a) 1 km, (b) 3 km, (c) 5 km, (d) 10 km, (e) 15 km, (f) 20 km, (g) 25 km, and(h) 30 km. The color denotes log10 (resistivity). The red triangles labeled N and M denote Naruko volcano and Mukaimachi caldera, respectively.
The dotted line shows the outlines of the calderas as shown in Figure 1. The black and white dots represent hypocenters of normal earthquakes
and long-period volcanic earthquakes at respective depths from the 1997 to 2011 JMA catalogue, respectively. The hypocenters were plotted with
2-km depth ranges for respective depths. Four east-west lines (L1 to L4) are profiles for the sections in Figure 5.

Ogawa et al. Earth, Planets and Space 2014, 66:158
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impedance was set at 10%. The RMS misfit was reduced
to 3.12 after the fifth iteration. Here, we call this modelas the first-generation model. This model was then used
as an initial model and also as a new prior model for another series of iterative inversions. The fifth iteration
gave a new model with minimum RMS of 1.87, and wetake this as the second-generation model. In the sameway, we had the third- and fourth-generation models
with RMS of 1.63 and 1.60, respectively. As RMS is almost converged, we used the fourth-generation model as
our final model. The near-surface telluric distortions
were not explicitly incorporated into the model. We suggest that the resistivity of the model surface blocks accounts for static shift by producing contrasts between
the adjacent sites.
The responses of the final model are shown as determinant apparent resistivity and phase (in Figure 2) andphase tensor ellipses and β (in Figure 3) for the representative periods. The observed and calculated responses
generally fit well.
Results and discussion

Figure 4 shows representative depth sections. Each panel
also shows the seismic hypocentral locations with adepth tolerance of 2 km from the 1997 to 2011 seismic
catalogue compiled by the Japan Meteorological Agency.
At depths of 1 and 3 km (Figure 4a,b, respectively), thedistribution of resistivity is consistent with the surface
geology. Conductors c1 and c2 are located at Naruko
and Mukaimachi volcanoes, respectively, and represent
caldera fill. Conductors c3 and c4 represent sedimentarylayers of the Shinjo and Yamagata basins, respectively(Figure 1). Towards the southeastern edge of the model,
conductor C2 is evident at a depth of 3 km but this area
is not covered by the MT data and requires careful investigation before interpretation. The existence of C2
was confirmed by the sensitivity test as described later inthe Appendix.
Structures at 5-km depth (Figure 4c) show more resistive features than those at shallower depths. At a depthof 10 km (Figure 4c), a conductor in 10-km diameter
appears to the west of Naruko volcano. The seismicity atthese depths is distributed in the resistive region, in particular R2, which is sandwiched between conductors C1
and C2.
Deeper structures (Figure 4e,f,g,h) show greater elongation of conductor C1 along the NNE-SSW direction,and the contrast with the eastern resistive block R2 isclearer. This direction is consistent with the 2-D strikedirection determined by Asamori et al. (2010). The
eastern conductor C2 disappears at depths below 25 km.
Long-period earthquakes at 20- to 30-km depth are
shown in Figure 4f,g,h in white dots, and imply injections of magmatic melt into the crust, although theirPage 7 of 13

locations are distant from conductor C1 and Naruko
volcano (approximately 10 km away). The long-period
earthquakes are located at the edge of the resistive block
R2 and are not associated with a significant conductive
anomaly.
Resistivity sections for L1 to L4 in Figure 4 are shownin Figure 5. Profile L1 (Figure 4a) almost overlaps with
one of the profiles shown by Mishina (2009). Althoughsection L1 is oriented east-west and the corresponding
profile (Mishina 2009) is rotated by 15° clockwise fromthe north, the geometry of the conductors is relatively
consistent. We observe clearly defined clusters of earthquakes above conductor C1. The brittle-ductile transition is defined by the cutoff where the seismicity
coincides with the top of conductor C1. Thus, we interpreted C1 not as a melt, but as a saline fluid reservoir.
The conductive feature extends smoothly to the west;
however, there is a sharp resistivity contrast between C1
and resistor R2 in the east. Farther to the east of resistor
R2, there are clusters of long-period volcanic earthquakes (shown as white dots) in the lower crust.
Section L2 (Figure 4b) can be compared with that of
Asamori et al. (2010). Figure 6a,b compares sections
from 3-D and 2-D inversions using the same color scale.
The sub-vertical conductor (C1) in the 3-D model has
lower resistivity than the conductor (C1’) in the 2-D
model (Figure 6b). There are two possible reasons. One
is the difference in treating the shallow galvanic effect.
The 2-D model is inverted together with static shift asparameters (Ogawa and Uchida 1996), whereas the 3-Dmodel assumes that the shallow blocks represent galvanic distortions including static shift. The other reason
is that the 3-D structure has a finite strike length ofapproximately 10 km (Figure 4d,e,f,g,h). Lower resistivity
is needed to produce strong inductive responses to account for a low apparent resistivity where the electric
current flows along strike. In this 3-D model, we cannot
expect enough charge buildup at the ends of the elongated C1 conductor, which may lower apparent resistivity (Ting and Hohmann 1981). We have also foundthat the crustal anomaly C’ in the 2-D model disappeared in the 3-D model. This C’ conductor in the 2-D
model may be erroneous projection of an off-line sedimentary conductor.
The leading edge of the C1 conductor approaches thesurface conductor c2, which is in the middle of Naruko
caldera. The level of shallow seismicity is high along the
profile, and the hypocenters are distributed in the resistive body surrounding the C1 conductor. The C1 conductor shows a pathway of saline fluid at depth to thesurface. A helium isotope anomaly provides direct support for such supply from the upper mantle to Naruko
volcano (Asamori et al. 2010; Horiguchi et al. 2010).
Similar vertical conductors were also observed below

Ogawa et al. Earth, Planets and Space 2014, 66:158
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Page 8 of 13

Figure 5 Resistivity sections for profiles L1 (a), L2 (b), L3 (c), and L4 (d) in Figure 4. Left and right columns show resistivity models withand without seismic hypocenters, respectively. The black and white dots indicate normal earthquakes and long-period volcanic earthquakes,
respectively. The triangles on the surface denote Quaternary volcanoes. M and N indicate the locations of Naruko volcano and Mukaimachi
caldera, respectively. Labels for the conductors and resistors correspond to those in Figure 4.

Onikobe caldera (Fukino 2011) to the north of the study
area.
Section L3, oriented east-west and located 5 km southof Naruko volcano, shows that the top of the subvertical conductor (C1) is deeper and its resistivity ishigher than those in Figure 3b. We also observed deeper
hypocenters above the C1 conductor. Section L4, located
5 km farther south, is characterized by a more blurredand deeper C1 conductor. Correspondingly, the hypocenters are also deeper, being close to 15-km depth.

The mid- to lower crustal conductors in this study
showed resistivities of 1 to 10 Ωm. As shown in Figure 5,
the top of the sub-vertical conductor is directly below
the hypocenters, i.e., just below the brittle-ductile transition. The existence of the conductor could imply thepresence of saline fluids rather than magmatic melt,
although these are not easily discriminated purely on theresistivity value alone. If we assume a fluid salinity,
we can infer the minimum porosity, based on a fluidbearing rock model such as Hashin-Shtrikman bounds

Ogawa et al. Earth, Planets and Space 2014, 66:158
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M

3.5

c1
R1

c3

−5

4

c2

3

R2

−10

2.5
2

−15

C1

1.5

−20

1

−25

0.5

−30

0
−30

−20

−10
0
distance(km) E+

20

N

M

(b)

10

4
3.5

0

depth(km)

−5

c3’

3

C’

2.5

−10

2
−15

C1’

1.5

−20

1

−25
−30
−10

log10( m)0N

log10( m)(a)depth(km)Page 9 of 13

0.5
0
0

10

20
distance(km)

30

40

50

Figure 6 Comparison between (a) the 3-D model and (b) the 2-D model. 3D model along line L2 is compared with 2-D model (Asamori
et al. 2010) with the same color scale. Major conductors are labeled.

(Hashin and Shtrikman 1962; Pommier and Le-Trong2011). According to Nesbitt (1993), typical crustal fluids
have resistivities of 0.01 to 1 Ωm. Recent mantle xenolith studies by Kawamoto et al. (2013) reported the salinity of fluids in the mantle wedge to be 5.1 wt%, which
is more conductive than that of seawater, but the difference is by a factor of <10. Here, we made a first-orderassumption that the fluid conductivity is 0.1 Ωm, whichcorresponds to four times as high salinity as that ofseawater. The Hashin-Shtrikman bounds for the fully
connected fluid case require a porosity of 1.5% to 15%
for a conductive anomaly of 1 to 10 Ωm (Pommier and
Le-Trong 2011). Recently, Shimojuku et al. (2014) also
showed that fluid of high salinity (>10 wt%) is required
to account for 10-Ωm bulk resistivity, from the resistivity measurement of fluid-bearing rocks under lower
crustal conditions.
The resistivity sections can be compared with theseismic tomography results presented in Nakajima andHasegawa (2003) and Okada et al. (2014). The sub-verticalconductor C1 below Naruko volcano corresponds to bothVp and Vs images that show low velocity anomalies (approximately 5%) from the lower crust to the surface. However, the lower part (>15-km depth) shows high Vp/Vs(>1.82), whereas the upper part (<15-km depth) shows
low Vp/Vs (approximately <1.75). The lower part, whichshows low Vp and Vs but high Vp/Vs, is interpreted as
representing a partial melt. The upper part, which shows
low Vp and Vs but low Vp/Vs, is interpreted as a zone ofhigh fluid content (Nakajima and Hasegawa 2003; Okada
et al. 2014). The lower solubility of quartz in the upper
crust (Newton and Manning 2000) will create a sealing
cap for the deeper fluid due to precipitated quartz. The reduced solubility of quartz in the upper crust may also contribute to the lower Vp/Vs ratio (Christensen 1996). Thus,
our electrical image, as well as the seismic image, is consistent with quartz solubility controls on the distributionof saline fluid in the crust.

Ogawa et al. Earth, Planets and Space 2014, 66:158
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Page 10 of 13

Conclusion

The 3-D resistivity structure was inverted from wideband MT data recorded by 77 sites around Naruko volcano, NE Japan. Full-component impedance tensors ateight representative periods were used in the inversion,using a 100-Ωm uniform earth with surrounding ocean
as the initial model. The model fit was quantified by
RMS misfit and by mapping the observed and calculated
phase tensor ellipses. The model showed a sub-verticalconductor C1 below Naruko volcano. The conductor
showed a sharp contrast at the eastern border comparedwith a blurred western border. The depth to the conductor was shallow in the vicinity of Naruko volcano
and increased towards the north and south. At a depthof ≥15 km, the conductor was elongated NNE-SSW. The

crustal seismicity distribution showed a cutoff depth,
consistent with the top of the conductor at depth. Near
Naruko volcano, levels of shallow seismicity (<5 km)
were high and were associated with the top of the subvertical conductor, which was close to the surface. Thisobservation suggests that the crustal seismicity is fluid
driven. The fluids are presumably supplied by a partial
melting zone at a greater depth, and the top of the fluid
reservoir is capped by a self-sealed precipitate of silicate
solution at temperatures of approximately 400°C.
At greater crustal depths, the backbone ranges areassociated with 2-D conductors elongated in the NNESSW direction. The Quaternary volcanic front is characterized by a sharp resistivity contrast and a highresistivity zone of 10- to 15-km width to the east. Fartherβ(atan(Φmin) + atan(Φmax))/2
15

10

75

10

5

60

0

45

K222

−5

214

−10
−15

−20

−10
0
Distance(km) E+

10

Distance(km) N+

0

−15

15

75

10

5

60

0

45

K222

−5

214
227
−30

−20

−10
0
Distance(km) E+

10

20

30
15

−15

15
10

5

60

214
227
−30

−20

−10
0
Distance(km) E+

10

20

30
15

−15

15
10

5

60

214

−10
−15

227
−30

−20

−10
0
Distance(km) E+

10

20

30
15
0

214
227
−30

−20

−10
0
Distance(km) E+

10

20

−5

−10

10

0

75

45

0

K222

0

K222

−5

90

K222

−10

10

214

−10

0

20

5

10

0

10

−5

5

15

−5

−10
0
Distance(km) E+

−10

0

45

−20

0

75

K222

227
−30

−5

90

0

214

5

10

−5

0

K222

5

15

−15

Distance(km) N+

−10

90

−10

400s (Cal)

0
−5

10

−15

400s (Obs)

5

15

−10

(b)

20

10

5

15

Distance(km) N+

Distance(km) N+

40s (Cal)

227
−30

30

Distance(km) N+

90

Distance(km) N+

Distance(km) N+

40s (Obs)

15

Distance(km) N+

(a)

227
−30

−20

−10
0
Distance(km) E+

10

20

−5

−10

10

5
5
0

0

K222

−5

214

−10
−15

227
−30

−20

−10
0
Distance(km) E+

10

20

−5

−10

Figure 7 Phase tensor ellipse and β for sensitivity test for two representative periods. (a) 40 s and (b) 400 s. The dotted squares show thehorizontal location of the area where the C2 conductor is replaced with 100 Ωm. The calculated β values of the sites west of the C2 conductor
are significantly underestimated. We do not see significant differences with and without the C2 as seen in phase tensor ellipses; however,
without the C2 conductor, the observed large β value to the west of the C2 is not explained.

Ogawa et al. Earth, Planets and Space 2014, 66:158
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Page 11 of 13

calculated phase tensor ellipses for the periods of 40
and 400 s. The dotted square at the lower right cornershows the location of the C2 conductor. From the phasetensor shapes, we do not see any clear differences. However, the β values are very sensitive to C2. The calculated β near the C2 anomaly now have low values.
Figure 8 shows typical MT sites in Figure 7. Final model
responses and test model responses are shown by solidlines and broken lines, and observed responses areshown by symbols. At site K222 (Figure 8a), the offdiagonal phase curves (green solid and broken lines)
differ significantly at periods longer than 40 s. The final
model successfully explains phase over quadrant responses. The difference between final and model responses is clear in diagonal elements. The off-diagonal
elements at site 227 (Figure 8b) are not very sensitive toC2, but the amplitudes of the diagonal elements are sensitive. The diagonal elements of site 214 (Figure 8c) are
sensitive to C2 at periods longer than 40 s. These differences in sounding curves are easily seen in the mapof β (Figure 7).

to the east, a fore-arc conductor was observed in the midcrust. This conductor did not have a deep root. The resistivity image is consistent with results from seismic
tomography. The crustal conductors along-arc and in thefore-arc have resistivities of 1 to 10 Ωm. If we assume a
Hashin-Shtrikman model using saline fluids of 0.1 Ωm, aporosity of 1.5% to 5% would be required to explain thebulk resistivity of 1 to 10 Ωm.
Further data acquisition and compilation of previous
datasets are both underway, and the distribution ofcrustal fluids will be imaged for a larger area (Ichihara
et al. 2014; Kanda and Ogawa 2014) together with seismic data (Okada et al. 2012; Okada et al. 2014).

AppendixSensitivity test of the C2 conductor
The existence of the C2 conductor in Figure 4 wastested by the forward modeling because the C2 conductor was outside the site coverage. The C2 anomaly
was replaced by a 100-Ωm resistivity. The left columnof Figure 7 shows the comparison of observed andsite K222

3

180
135
90
45
0
−45
−90
−135
−180
−1

phase(deg)

1
0
0

1
2
log10(period) (s)site 227
3
2
1
0
0

1
2
log10(period) (s)site 214
3
2
1
0
0

1
2
log10(period) (s)12
log10(period) (s)

3

0

1
2
log10(period) (s)3site 214

4

−1
−1

0

site 2274phase(deg)log10(app res) ( m)log10(app res) ( m)

3

180
135
90
45
0
−45
−90
−135
−180
−1

2

−1
−1

(c)

3

3

−1
−1

(b)

site K222
180
135
90
45
0
−45
−90
−135
−180
−1

4

phase(deg)log10(app res) ( m)

(a)

0

1
2
log10(period) (s)3

Figure 8 Sounding curves at three representative sites for sensitivity test with and without the C2 conductor. (a) Site K222. (b) Site 227.
(c) Site 214. See Figure 7 for their locations. Blue, red, green, and black symbols with error bars (only where error bars are larger than the symbols)
represent xx, xy, yx, and yy components, respectively. Those with solid lines and broken lines represent model responses with and without the C2
conductor, respectively.

Ogawa et al. Earth, Planets and Space 2014, 66:158
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
YO and MI designed the field survey for the new dataset. YO carried out thefield survey, processed the time series data, performed the 3-D inversion, anddrafted the manuscript. WK contributed to the inversion and compilation ofsome of the figures. MM and KA compiled the previous datasets. All authors
read and approved the final manuscript.
Acknowledgements
We had fruitful discussions with E. Takahashi, T. Matsuzawa, T. Okada, H.
Sakuma, M. Nakamura, H. Iwamori, H. Ichihara, and T.G. Caldwell. Commentsfrom the two anonymous reviewers improved the manuscript. We thankW. Siripunvaraporn of Mahidole University for the WSINVMT3D code
(Siripunvaraporn and Egbert 2009). Numerical calculations were carried out
on TSUBAME 2.0 at the Global Scientific Information and Computing Centerof the Tokyo Institute of Technology. Some figures were prepared using theGeneric Mapping Tools (Wessel and Smith 1991). This study was supported
by JSPS KAKENHI Grant Number 21109003. We used hypocenter dataprocessed by the Japan Meteorological Agency with the Ministry of
Education, Culture, Sports, Science and Technology. These data were
provided by the National Research Institute for Earth Science and Disaster
Prevention, Geographical Survey Institute, Hokkaido University, Hirosaki
University, Tohoku University, University of Tokyo, Nagoya University, Kyoto
University, Kochi University, Kyushu University, Kagoshima University, Japan
Marine Science and Technology Center, National Institute of Advanced
Industrial Science and Technology, Tokyo Metropolitan Government, Aomori
Prefectural Government, Shizuoka Prefectural Government, Hot Springs
Research Institute of Kanagawa Prefecture, and City of Yokohama.
Author details
1
Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1
Ookayama, Meguro, Tokyo 152-8551, Japan. 2Research Center for Predictionof Earthquakes and Volcanic Eruptions, Tohoku University, 6-6 Aoba, Aramaki,
Aoba-ku, Sendai 980-8578, Japan. 3Tono Geoscientific Research Unit,
Geological Isolation Research and Development Directorate, Japan Atomic
Energy Agency, 959-31, Jorinji, Izumi, Toki 509-5102, Japan.
Received: 4 April 2014 Accepted: 13 November 2014

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Cite this article as: Ogawa et al.: Three-dimensional magnetotelluric
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Japan. Earth, Planets and Space 2014 66:158.

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