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Title

Three-dimensional electromagnetic imaging of fluids and melts beneaththe NE Japan arc revisited by using geomagnetic transfer function data

Authors

Wataru Kanda, Yasuo Ogawa

Citation

Earth, Planets and Space, 66:39, ,Pub. date2014, 5Creative Commons

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Kanda and Ogawa Earth, Planets and Space 2014, 66:39
http://www.earth-planets-space.com/content/66/1/39

LETTER

Open AccessThree-dimensional electromagnetic imaging
of fluids and melts beneath the NE Japanarc revisited by using geomagnetic transfer
function data
Wataru Kanda* and Yasuo Ogawa

Abstract
The three-dimensional distribution of fluids and melts under the NE Japan arc was imaged using its resistivity
structure, modeled with geomagnetic transfer functions. The data were collected at 37 stations located on a 20-km
grid, at periods ranging from 16 to 256 s. In spite of the narrow period band nature, these periods turn out to besensitive to conductors in the deep crust and upper mantle. The geomagnetic transfer functions represent lateralresistivity variations, which yield inherently nonunique model results when using the geomagnetic transfer functions
alone. However, by fixing the resistivity structure of the surrounding seawater distribution, the intrinsic
nonuniqueness is alleviated. In this study, we show an inversion result using a 100-Ωm uniform Earth with fixedresistivity of surrounding oceans. As a result, it was found that the features of the short period transfer function
require shallow conductors in the upper crust, which is suggested to represent the northern Tohoku conducting
belt of a previous study. The final model is characterized by a highly conductive zone along the quaternary volcanicarc in the depth range of the lower crust to the upper mantle. The conductor, which is obtained mainly from thefeatures of longer-period data, is particularly clear beneath the Sengan geothermal area. The deep crustal conductor
implies the existence of partial melt and/or high-salinity fluids.
Keywords: NE Japan; Quaternary volcanoes; Arc magma; 3-D modeling; Electrical resistivity

Findings
Introduction

Geomagnetic transfer functions (TFs) represent the ratioof vertical to horizontal magnetic variations and are robust to the effects of near-surface local anomalies. Pioneering electromagnetic induction studies of the NE
Japan region in the 1970s were based on geomagnetictransfer functions (Kato et al. 1971; Honkura 1974;
Nishida 1976; Yamashita and Yokoyama 1976; Ogawa
et al. 1986). These studies used three-component fluxgate
magnetometer data in the period range of 10 min to 2 h
to map the undulations of upper mantle conductors.
Kato et al. (1971) modeled the upper mantle conductor in NE Japan, which shallows at the backbone

range, using TFs. However, later studies (Yamashita andYokoyama 1976; Honkura 1974) pointed out that major
features of the TFs are influenced by the effects of induced electric currents flowing in the surrounding
ocean, including in the Tsugaru Straight between Honshu
and Hokkaido islands. On the other hand, in an analysisof the lower crustal conductor to the west of the volcanic
front, Ogawa et al. (1986) pointed out, using a twodimensional (2-D) finite element approach, that the effectof the ocean alone cannot account for the distribution of
the TFs at the period of 15 min. A similar conductor was
also revealed on the back-arc side of central Japan byusing the TFs obtained over four periods each of whichranged from 15 to 120 min (Utada et al. 1986). However,in those studies, the periods of the data used were restricted to a narrow range and the sites were distributed* Correspondence: kanda@ksvo.titech.ac.jp
Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1
Ookayama, Meguro, Tokyo 152-8551, Japan© 2014 Kanda and Ogawa; licensee Springer. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly credited.

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Avdeev et al. (1995) used thin sheet forward modelingto explain the distribution of induction arrows at theperiod of 16 s and confirmed the existence of crustal
conductive anomalies (NTCBs), but they could not fully
explain the TFs at longer periods. In this study, we
attempted to interpret the TF data quantitatively by useof the three-dimensional (3-D) inversion scheme of Siripunvaraporn and Egbert (2009).

along the measurement lines, which led to qualitative interpretations using simplified 2-D forward modeling.
Ogawa (1987a) qualitatively interpreted geomagnetictransfer function data at 37 sites located on a 20 × 20-km
grid (as shown in Figure 1) using induction coils in the
period range of 16 to 256 s, which was shorter than the
periods used in the preceding studies because of the needto clarify the structure of deep crust but was narrow ascompared with present-day wideband magnetotelluric
measurements (typically 10−2 to 104 s). Nonetheless, those
periods turned out to be sensitive to the distributions ofdeep crustal and upper mantle conductive anomalies, as
shown below.
Although the sites were distributed on a grid, Ogawa’s(1987a) data were still interpreted qualitatively because
realistic three-dimensional modeling including the conductive ocean for TF data has long proved difficult.

139˚

Data

The TF is a complex quantity that is sensitive to thegradient of the conductivity distribution (Rikitake andYokoyama 1953; Schmucker 1970). When the verticalmagnetic field variation (Hz) is expressed as a linear
combination of the two horizontal components (Hx andHy) at a given period T, the TFs (A and B) are definedby the following relationship:

140˚

141˚

ru
suga

142˚

it

Stra

OSR

T
41˚

41˚

HKD

IWK

L0
L1
L2
L3
L4

TWD

40˚
YKY HCMIWT
KMG

Pacific Ocean

Range

L5

Ou Backbone

Japan Sea

39˚

CHK

40˚

KKM

NRG

39˚

HJO

139˚

140˚

−4000

−3000

142˚

141˚
−2000

−1000

0

bathymetry (m)Figure 1 Location map of the sites described in this study. Latitude and longitude are from Ogawa (1987a). The area outlined in pink shows
the Northern Tohoku Conducting Belt (NTCB), following Ogawa (1987a). A gray rectangle indicates the Sengan geothermal area. A thick blue line
shows the Ou backbone range. Thin horizontal lines (L0 to L5) indicate the locations of cross-sectional profiles shown in later figures. Dashed lines
denote prefectural boundaries. Red triangles indicate the locations of major quaternary volcanoes; triangles with three-letter codes denote active
volcanoes, as identified by the Japan Meteorological Agency (JMA). OSR, Mt. Osore; IWK, Mt. Iwaki; HKD, Mt. Hakkoda; TWD, Mt. Towada; YKY, Mt.
Yakeyama; HCM, Mt. Hachimantai; IWT, Mt. Iwate; KNG, Mt. Komagatake; CHK, Mt. Chokai; KKM, Mt. Kurikoma; NRG, Mt. Narugo; HJO, Mt. Hijiori.

Kanda and Ogawa Earth, Planets and Space 2014, 66:39
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H z ðT Þ ¼ AðT ÞH x ðT Þ þ BðT ÞH y ðT Þ

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ð1Þ

The TF data obtained at 37 sites at three periods (16,64, and 256 s) are listed in table two of Ogawa (1987a).
For the graphical representation of the TF data, weused the induction arrows of Parkinson’s convention(Parkinson 1962), which are shown in Figure 2. It can beconsidered within an x-y plane that the pair (A, B) ofcomplex coefficients is a vector and that this vector defines the induction arrows. The magnitude and azimuth
calculated from the real and imaginary parts of the vector
give the length and direction of those arrows, respectively, in which the arrow direction is taken reversely for
Parkinson’s convention. The real arrows intrinsically tend

Figure 2 Observed and modeled induction arrows. Induction arrows representing data and calculations from the initial model (Figure 3) andfrom the inverted model (Figure 4) are drawn at periods of (a) 16 s, (b) 64 s, and (c) 256 s. Real (left) and imaginary (right) arrows are shown ineach plot.

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to point towards a good conductor (e.g., Simpson and
Bahr 2005).
The main features of the distribution of the induction
arrows (see Figure 2) and the features of the subsurface
structure inferred from those arrows are described in
Ogawa (1987a) and are summarized as follows. At aperiod of 16 s, an upper crustal conductor is suggestedby the pairs of induction arrows (red arrows, Figure 2a)
at sites east of the volcanic front, which are pointing towards each other, suggesting the existence of a crustal
conductor (NTCB, Figure 1). At longer periods, the induction arrows generally show a spatially smoother distribution (Figure 2b,c). Most coastal sites show large real
arrows perpendicular to the coastline. The sites on thenorthernmost profiles (L0 and L1) show large northward
components (Figure 2c), but their lengths are shorternear the volcanic front. These data were qualitatively
interpreted in terms of conductors along the volcanic
front that were reducing the lengths of the northward
arrows (Ogawa 1987a).

Methods

We designed finite difference meshes for a 3-D inversion
as follows. In the horizontal direction, the core area, centered on the Sengan geothermal area, consisted of a 60 ×
60-mesh with a mesh unit length of 4 p
km.
ﬃﬃﬃ Outside thecore area, the mesh size was enlarged by 2 times the distance from the core area (Figure 3). In the vertical direction, the minimum mesh length was 10 m at the top layer,with the mesh size gradually increasing with depth. Consequently, the calculation domain of 830 × 830 × 349 km
was divided into 78 × 78 × 47 resistivity blocks (Figure 3).
We used TFs at three periods. The error of the TFs wasestimated by Ogawa (1987a) as the 95% confidence limit
under the assumption of F-distributed data. For the 3-Dmodeling, we approximated the standard deviation of the
TF as half of the TF errors, which are listed in Ogawa(1987a). An error floor of 5% was used for the inversion.
The inversion using the TF data alone has an inherent
ambiguity and a resistivity structure that cannot be recovered uniquely unless a proper starting model is given

45˚

44˚

43˚

42˚

41˚
40˚
39˚

38˚
37˚
36˚

km
0

100 200 300

3

4

35˚
135˚ 136˚ 137˚ 138˚ 139˚ 140˚ 141˚ 142˚ 143˚ 144˚ 145˚ 146˚ 147˚
−1

0

1

2

log10 resistivity (Ωm)

5

Figure 3 Initial model of the 3-D inversion. The resistivity structure of the Earth was a uniform-half space with a resistivity of 100 Ωm; resistivity
blocks corresponding to seawater were fixed at 0.33 Ωm. Resistivities of blocks containing the land-sea boundary were given as the weighted
mean of two values, according to the volume ratio.

Kanda and Ogawa Earth, Planets and Space 2014, 66:39
http://www.earth-planets-space.com/content/66/1/39(Siripunvaraporn and Egbert 2009). In this study, the initial model consisted of a 100-Ωm uniform Earth and surrounding seawater of 0.33 Ωm distributed over known
bathymetry. Blocks containing both seawater and land
were fixed at a resistivity calculated from the weighted
mean of the conductivities (reciprocals of the resistivities)of land and seawater, according to the volume ratio. The
resistivity of seawater was fixed during the inversions. The
initial model was also used as a prior model.
From the responses of the initial model, plotted in
Figure 2, we can compare modeled and observed induction arrows. As the initial model specifies a simple uniform Earth with surrounding oceans, the induction
arrows are simply pointing to nearby coastlines and theocean is affecting induction further inland at longer periods. As expected, the following two features are notexplained by the initial model: (1) the real arrows to theeast of the volcanic front are pointing towards eachother at 16 s and (2) the large northward arrows at
northeastern sites at 64 and 256 s.
The 3-D inversion uses an Occam’s style inversion
scheme (Constable et al. 1987). The model was searchedso as to minimize model roughness relative to the priormodel as well as data misfit, where the root mean square(RMS) is not less than the target RMS value (1.0 in thisstudy). The selection of model is performed on an RMS
basis. In this inversion, a first-generation best model was
found after several iterations, using the initial model described above. Then, the best model was used as the initial model, and as the prior model for the series of
iterations in the next generation. In this way, we obtained a final resistivity model from the inversion of thethird generation with an RMS of 1.4. The final model
explains major features of the observed responses, inparticular, the two features mentioned previously (directions of arrows to the east of the volcanic front and thelarge northward arrows at northeastern sites).
Results and discussion

Figure 4 shows representative depth sections of the final
model. Figure 4a shows the upper crustal feature of themodel. The short-period induction arrows require conductors C1 to C4, the geometry of which implies thatthey are sedimentary layers. In particular, C1, which runs
in a N-S direction to the east of the volcanic front, is responsible for the pairs of arrows pointing towards eachother at the period of 16 s. Conductors C2 and C3 likely
represent thick Miocene marine sedimentary layers (Sato
1994), and C4 presumably represents Paleozoic sedimentary layers which lie between two resistive regions(R2 and R3) of Cretaceous granitic intrusions (Finn
1994; Ogawa 1992). Conductor C5 is evident at the eastern coast at a depth of 10 km. This conductor is required to explain the large northward induction arrows

Page 5 of 8

at sites to the south of C5; however, the implications of
the anomaly remain unknown. The anomaly outside of
the observed array requires further investigation. A conductor is present below the volcanic zone centered in
the Sengan area, located at a depth of 20 km (Figure 1).
Figure 4d shows a depth section of the Moho in thisarea. Conductor C6 spreads in a N-S direction along the
volcanic area.
Figure 5 shows resistivity sections for the six profilesin Figure 1. The arrows at the surface represent the location of the backbone range. The Conrad and Moho discontinuities are drawn with broken lines.
The upper crustal conductors (C2 and C3), which areobserved on the Japan Sea side as deep as 10 km, can beinterpreted as Miocene marine sedimentary layers, dating
to the rifting of the Japan Sea (Sato 1994). Conductors C1
and C4 are shallow features (depths of ca. 2 km), probably representing Paleozoic sediments in the Kitakami
Mountains and presumably separated by resistive granitoids.
Conductor C5 is a notable feature, but its implications are
not clear, as it is outside of the coverage area of the study.
The distributions of lower crustal conductors areobserved from north to south in the depth sections.
Conductor C6 is evident in the southern four profiles.
The distribution of conductor C6 and its resistivity (1 to10 Ωm) are also consistent with the results of previous
studies (Ogawa et al. 1986; Ogawa 1987b). Recent wideband magnetotelluric profile measurements with site spacings of several kilometers have yielded more detailed imagesof the locations and depths of these conductors, althoughthose images were limited to two-dimensions (Mitsuhata
et al. 2001; Ogawa et al. 2001; Umeda et al. 2006; Mishina
2009; Asamori et al. 2010; Ichihara et al. 2011).
The distribution of the N-S trending regional conductor C6 is consistent with a zone of low P-wave velocity (Vp), low S-wave velocity (Vs), and a high Vp/Vs
ratio, as inferred from seismic tomography results(Nakajima et al. 2001a). This low-velocity and high-Vp/
Vs zone is located in the upper part of a sheet-like lowvelocity zone inclined and parallel with the slab in a
mantle wedge (Nakajima et al. 2001b). The inclined
sheet-like low-velocity zone is considered to represent
the hot upwelling flow of mantle material accompanying
subduction, which is partially molten because of theaddition of water emanating from the deeper parts ofthe subducting slab (Hasegawa and Nakajima 2004;
Hasegawa et al. 2005). Since this inclined low-velocity
zone reaches the Moho beneath the volcanic front, it has
been proposed that partially molten materials repeatedly
intruded the crust and formed volcanoes (Hasegawa andNakajima 2004).
In our 3-D resistivity model, an inclined low-resistivity
structure is not observed because the depth of our investigation is ca. 80 km, at most. The low resistivity is

Kanda and Ogawa Earth, Planets and Space 2014, 66:39
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100

2.0 – 2.5 km

(a)
80
C2

40

20

20

?
C3

R3

−20

−20

−40

−40

C4

R2

C5
3

0

R1

−80

5

4

60

40

0

10.0 – 12.0 km

(b)
80

C1

−60

2

1

−60

0

−80

−100
−100 −80 −60 −40 −20
100

0

20

40

(c)
80

−100
100 −100 −80 −60 −40 −20
10018.6 – 22.0 km
60

80

20

40

60

40

40

20

20

80

100
5

4

3

C6

C6

0

0

−20

−20

−40

−40

−60

−60

−80

−80

0

60

26.4 – 32.1 km

(d)
80

60

−100
−100 −80 −60 −40 −20

−1
0

20

distance (km)

40

60

80

2

1

log10 resistivity (Ωm)

distance (km)100

log10 resistivity (Ωm)

distance (km)60

Page 6 of 80−100
100 −100 −80 −60 −40 −20

−1
0

20

distance (km)

40

60

80

100

Figure 4 Depth sections of the final 3-D model. Sections at depths of (a) 2.0 to 2.5 km, (b) 10.0 to 12.0 km, (c) 18.6 to 22.0 km, and (d) 26.4
to 32.1 km are shown. Red triangles indicate the locations of major quaternary volcanoes (Figure 1); white dots denote observation sites.

continuously distributed from the uppermost mantle tothe lower crust along the backbone range, which is notcontradictory to the abovementioned seismological images of a volcano generation model. This suggests thatthe regional conductor inferred beneath the backbone
range can be interpreted as arc magma or high-salinity
fluids (Kawamoto et al. 2012).
Hasegawa and Nakajima (2004) also proposed a modelof volcano generation on the back-arc side. They suggested that upper mantle melt has an along-arc variation, with ‘hot fingers’ extending upwards beneath thevolcanoes. The conductor visible in our 3-D model
under the Sengan region (C6) corresponds to one suchfinger. However, no conductive region is found in the

back-arc side, except that the regional conductor C6 seems
to extend towards the Iwaki volcano (IWK in Figure 1).
We presume that a conductive region was not resolvedby the tipper inversion because of the presence of asmall-scale structure in either the deep crust or uppermost mantle. Future 3-D magnetotelluric studies will confirm and provide details of such melt/fluid distributions
under the volcanoes.

Conclusion

We applied a 3-D inversion to magnetic transfer function
data from 37 sites, as presented by Ogawa (1987a). Starting from a 100-Ωm uniform Earth with a fixed resistivity

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Figure 5 The six E-W cross sections of the final model (L0-L5 in Figure 1). The arrow shows the location of the backbone range and indicatesthe origin of the horizontal axis. The white dashed lines indicate the Conrad and Moho seismic discontinuities, following Zhao et al. (1994).

Kanda and Ogawa Earth, Planets and Space 2014, 66:39
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structure and a surrounding ocean, the inverted resistivity
model showed the following features.
1. Observed induction arrows cannot be accounted forsolely by the effect of a conductive ocean enclosing ahomogeneously resistive Earth. The ocean effects are
particularly small at a period of 16 s.
2. The NTCB, previously inferred from pairs of
induction arrows pointing towards each other at aperiod of 16 s, is attributed to a shallow conductor(C1) located east of the volcanic front. Upper crustal
conductors (C2, C3) are required to explain the
short period data. We also found a NW-SE trending
conductive belt (C4) in the Kitakami Mountains.
3. A N-S trending conductor is located along the
volcanic arc in the lower crust. The conductor
shallows under the Sengan geothermal area, which isconsistent with the results of seismological imaging.
This conductor implies a magmatic melt and/or ahigh-salinity fluid.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
WK constructed the 3-D conductivity models and drafted the manuscript.
YO conceived of the study and revised the manuscript. Both authors readand approved the final manuscript.
Acknowledgements
The authors thank Weerachai Siripunvaraporn of Mahidol University for theuse of the inversion program WSINVMT3D (Siripunvaraporn and Egbert,2009). Constructive comments and suggestions from two anonymousreferees were most helpful in improving the manuscript. Numericalcalculations were carried out on TSUBAME2.0 at the Global Scientific
Information and Computing Center of Tokyo Institute of Technology. Figures
were prepared using the Generic Mapping Tools (Wessel and Smith 1991).
This study was supported by JSPS KAKENHI Grant Number 21109003.
Received: 15 January 2014 Accepted: 12 May 2014
Published: 27 May 2014
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doi:10.1186/1880-5981-66-39
Cite this article as: Kanda and Ogawa: Three-dimensional electromagnetic
imaging of fluids and melts beneath the NE Japan arc revisited by using
geomagnetic transfer function data. Earth, Planets and Space 2014 66:39.