Full text of DOI:10.1186/BF03351661

Earth Planets Space, 52, 539–548, 2000
Resistivity structure of high-angle subduction zone in the southern Kyushu
district, southwestern Japan
Masahiro Ichiki1, Norihiko Sumitomo2, Tsuneomi Kagiyama1
1Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2Faculty of Humanities and Sciences, Kobe Gakuin University, 518 Arise, Ikawadani-cho, Nishi-ku, Kobe 651-2180, Japan
(Received January 19, 2000; Revised May 2, 2000; Accepted June 14, 2000)
Magnetotelluric observations were carried out in the southern Kyushu district of southwestern Japan to investigate
the characteristics of the electrical resistivity structure of a high-angle subduction zone. We constructed a 2-D
resistivity model parallel to the subducting plate motion by using the inversion technique with the Akaike Bayesian
Information Criterion (ABIC) smoothness constraint. The general features of the obtained resistivity structure are
as follows: (1) a conductive block (below 1 ꢀ·m) is found beneath the volcanic zone and is widespread bilaterally
below 40 km depth, (2) a resistive block (about 1000 ꢀ·m) distributes from 10 to 25 km depth in the forearc region
and (3) a conductor (1 ∼ 30 ꢀ·m) is embedded beneath the resistive block, which may correspond to the negative
Bouguer gravity anomaly observed in this region. We propose the following for the high-angle subduction zone: A
serpentinized block is generated in the lower crust of the forearc region and a partial melting and hydrothermal fluid
are well developed beneath the volcanic front.
1. Introduction
physical parameters on the basis of the model of Kanamori
Electrical resistivity structures of subduction zones have (1977). However, so far, we have never come across elec-
been investigated since the 1980’s and have contributed to tromagnetic investigations in high angle subduction zones
wards recognizing the role of the dehydration process of except for that of Toh (1993). Hence, more investigations
the subduction zone. Kurtz et al. (1986), Utada (1987), are required to establish the general features of the resistivity
Hyndman (1988) and Wannamaker et al. (1989) investigated structures of high angle subduction zones.
the resistivity structures of the subduction zones where in-
The southern Kyushu district is one of the typical subduc-
termediate and deep dip angles (Jarrard, 1986) are from 30◦ tion zones in Japan, where the Wadati-Benioff zone dips over
to 45◦ and revealed the following features: (1) a conduc- 60◦ beneath the volcanic front, as shown in Fig. 1 (Yamazaki
tive dipping layer lies on the subducting slab and (2) a lower and Ooida, 1985; Nagamune and Tashiro, 1989; Kakuta et
crust conductor is found beneath the volcanic fronts. On al., 1991; Ishihara and Yoshida, 1992). Moreover, the south-
the other hand, in a low-angle subduction zone, a different ern Kyushu district is remarkable for other characteristic tec-
type of resistivity structure has been detected by recent stud- tonic features. For example, the extreme negative Bouguer
ies. For example, Shiozaki (1993) investigated the resistivity gravity anomaly (Kobayashi et al., 1997) is observed in the
structure in Shikoku Island, southwestern Japan, where the forearc region (Fig. 2) and the Kirishima volcano group,
Philippine Sea plate subducts at a low angle smaller than Sakurajima volcano, and other volcanoes, which are the most
20◦, and showed that no dipping conductive layer lies on the active in Japan, are located along the volcanic front in the
subducting slab. Such a difference of resistivity structure southern Kyushu district (Fig. 3). In spite of such geophysi-
can be possibly explained by the subduction angle or evo- cally interesting features, little is known about the subsurface
lution of the subduction zone (Kanamori, 1977; Uyeda and structure except the uppermost crust in the southern Kyushu
Kanamori, 1979). The process of subduction starts with low- district. Therefore, the main purpose of this paper is to obtain
angle thrusting. Then the oceanic plate descends to a greater a resistivity structure from the southern Kyushu district as a
depth and inclines with a higher angle as the subduction pro- typical high-angle subduction zone that reflects the last evo-
ceeds. Lastly, the oceanic plate sinks and the subduction lution stage. We collected the magnetotelluric (MT) data, as
angle becomes very large. During this last stage, the heat shown in Fig. 1, and constructed a two-dimensional (2-D) re-
flow increases in the mantle wedge and the new subduction sistivity model parallel to the subducting plate motion. This
appears at the back arc region. Our final goal is to clarify the paper will also discuss the resistivity structure with special
evolution of the subduction zone from the viewpoint of the attention being paid to its possible relations to the Bouguer
dehydration process by compiling resistivity and other geo- gravity anomaly and the active volcanism in this area.
c
Copy rightꢀ The Society of Geomagnetism and Earth, Planetary and Space Sciences
(SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan;
The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.
2. Magnetotelluric Data Acquisition
The MT observation was carried out at the 10 sites shown
in Fig. 1. In order to avoid the effect of Kinko Bay, we
539

540
M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
Fig. 1. Site locations for the magnetotelluric surveys (triangles). The A–Aꢁ is the profile line of pseudosection (Fig. 8) and 2-D resistivity model (Fig. 9).
Dotted lines illustrate the isodepth of the Wadati-Benioff zone compiled by Yamazaki and Ooida (1985). The volcanic front approximately coincides
wit the location of the 80 km isodepth of the Wadati-Benioff zone. Child map: the Philippine sea plate subducts beneath the Eurasian plate in the study
area. The convergence rate was calculated as 5.2 cm/yr (Seno et al., 1993).
Fig. 2. Bouguer anomaly map of the southern Kyushu district (modified
from Kobayashi et al., 1997). The average density is assumed to be 2.67
g/cm3. Units are represented in mgal and the contour interval is 10 mgal.
Reverse triangles represent the observation sites.
Fig. 3. Surface geological facies around the study area. Closed triangles
represent active volcanoes. Gray lines are caldera rims. BTL represents
Butsuzo Tectonic Line.
made the traverse as far as possible. Four MT instruments
of the same type (Tierra Tecnica U43 system) were used
in the observation, the resolution of which is 1 pT in the
magnetic field and 1 μV in the telluric field, respectively.
The internal clock system is continuously synchronized by remote-reference processing (Gamble et al., 1979). Both
a GPS clock signal and the accuracy of the internal clock the magnetic and telluric fields were digitally recorded in a
is within 10−8. This clock system enabled us to carry out flush memory (10 MB) and the typical recording duration
the MT experiments with a high sampling rate and reliable was 3 weeks with a 1 Hz sampling rate and 1 hour with

M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
541
Fig. 4. Dimensionality criteria used in this study. Dimensionality is esti-
mated by Swift’s skew, phase difference skew and regional skew param-
eters. 3-D/1-D means a local 3-D and regional 1-D resistivity structure
and 3-D/1-D means a local 3-D and regional 1-D resistivity structure,
respectively. ‘Convent.’ and ‘Superimp.’ mean conventional and super-
imposed, respectively (cf. Bahr, 1991).
128 Hz, respectively. The total observation ran from 30
July to 22 September 1997. The data quality and quantity
were generally sufficient except those at Site 2 where battery
troubles were encountered. The data of Site 2 were not used
in the following analyses.
3. Data Processing
The MT impedance responses, Zi j (i, j = x, y), are de-
fined as follows:
Fig. 5. Dimensionality parameters at 341, 85, and 21 sec periods in the
study area. (a) Swift’s skew parameter. Gray histograms represent the
sites where the parameters are over 0.1. (b) Phase difference skew pa-
rameter. Gray histograms indicate the sites where the regional skewness
parameters are over 0.05. (c) Regional skewness parameter. Gray his-
tograms indicate the sites where the regional skewness parameters are
over 0.3.
ꢀ
ꢁ
ꢀ
ꢁ ꢀ
ꢁ
Ex
Ey
Zxx Zxy
Zyx Zyy
Hx
Hy
=
,
(1)
where Ei , Hi (i = x, y) are electric and magnetic field spec-
tra and x, y represent the N-S and E-W components, re-
spectively. The impedance responses are transformed into
apparent resistivity and phase responses, which are used in
the interpretations, modelings and inversions.
Calculation of the impedance responses from the time se- eters.) The three parameters at the periods of 341, 85, and
ries data was executed by using the robust processing tech- 21 sec in the study area are presented in Fig. 5. Although
niques (Chave et al., 1987). Impedance response data from Swift’s skew parameters are all over 0.1 and the phase differ-
a period range of between 0.1 and 3000 sec were obtained.
ence skew parameters are all over 0.05, most of the regional
Before inverting the apparent resistivity and phase re- skew parameters are less than 0.3. This result suggests that
sponses, we first estimated the dimensionality of the resis- 3-D/2-D (local 3-D over regional 2-D structure) processing
tivity structure by using the criteria of Bahr (1991) to carry is appropriate for the magnetotelluric data in this study area
out the processing. Although the detailed recipe of Bahr according to Bahr (1991).
(1991) classifies the behavior of the electromagnetic field
Figure 6 is a rose diagram of the principal axes direc-
into seven classes, we assessed the dimensionality according tions of all the sites for each decade of periods. Each rose
to the essentials of his criteria, as illustrated in Fig. 4. (Refer diagram is composed of five-degree divisions. In estimat-
to Bahr (1991) for details of the definitions of Swift’s (1967) ing the principal axes directions, we used the decomposi-
skew, the phase difference skew, and regional skew param- tion technique developed by Groom and Bailey (1989). We

542
M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
Fig. 6. Rose diagrams of the principal axes accumulated for all sites. The
diagrams are represented separately by four period bands. Since the
principal axes have uncertainty at 90◦, the diagrams are symmetrical at
every 90◦.
Fig. 8. Phase pseudosections of TM mode (top) and TE mode (bottom)
along the A–Aꢁ of Fig. 1. White dots represent sampling points.
Fig. 7. Plot of 2-D regional strike angles vs. global χ2 misfits calculated
from Toh and Uyeshima (1997).
perpendicular to the regional strike) and TM (magnetic field
parallel to the regional strike and electric field perpendicular
to the regional strike) modes along the A–Aꢁ profile of Fig. 1.
Note that the vertical scale is indicated by a log-period unit
and is a non-linear function of depth, because the penetration
depth depends on both resistivity and period. We do not
present apparent resistivity pseudosections, which are more
or less affected by ‘static shift’ (e.g. Jones, 1988).
Figure 8 implies following two features: (1) low TE-mode
and high TM-mode phase values are observed below the east-
ernmost sites between 0.1 and 10 sec periods, which suggests
the presence of a resistive block in the eastern shallow part
and (2) TE-mode phase values are high below Sites 4 to 6
over short periods, which suggests a possible intrusion of a
conductive block into the central part of the profile spreading
bilaterally to greater depths.
followed the procedure of determinating the distortion pa-
rameters (twist and shear) and principal axes directions as
recommended by Jones and Groom (1993). We first decom-
posed the impedance tensors by letting all the parameters be
variable with regard to periods and estimated the shear and
twist parameters at each site, respectively, and ultimately de-
termined the principal axes directions with the shear and twist
parameters fixed at each site. The principal axes directions
are almost consistent with each other over longer periods,
although they vary widely over shorter periods. The aver-
ages of all principal axes directions are N28◦E and N62◦W.
(More prudent calculation of regional strike is proposed by
Toh and Uyeshima (1997).) Then we attempted to calculate
the regional strike by using the Toh and Uyeshima (1997)
calculations in order to test these average values. Figure 7
shows that the sum of χ2 misfits against the regional strike. 4. 2-D Inversion
The sum of χ2 misfits is minimized at the regional strike of
The 2-D resistivity model along the A–Aꢁ profile was con-
N26◦E, which almost coincides with N28◦E. We, therefore, structed by inverting the apparent resistivity and phase re-
confidently adopted the regional strike as N28◦E, which is sponses of Sites 1 to 10 under the ABIC (Akaike Bayesian
approximately parallel to the trench axis.
Information Criterion) minimization with a smoothness con-
Figure 8 represents the phase pseudosections of the TE straint (Uchida, 1993; Uchida and Ogawa, 1993). The resis-
(electric field parallel to the regional strike and magnetic field tivity of the oceanic region was constrained to 0.3 ꢀ·m and

M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
543
Fig. 9. Final resistivity model along the A–Aꢁ profile deduced from 2-D ABIC inversion. K.V.G. and V.F. represent the Kirishima volcano group and
volcanic front, respectively.
Fig. 10. Observed responses and calculated sounding curves from the final model (Fig. 9). Solid circles: observed responses of the TM mode. Solid
triangles: observed responses of TE mode. Solid lines: calculated sounding curves of the TM mode. Broken lines: calculated sounding curves of the
TE mode. Some observed responses are reduced and were not used in the inversion process owing to missing the trends.

544
M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
Fig. 11. Modified models to carry out the sensitivity check. (a): The hatched area is replaced by one block with resistivity, ρ2, which we varied in the test.
(b): Resistivity of the hatched area, ρ2, is changed. (c): The upper part of the ρ3 block is fixed at 30 ꢀ·m and resistivity at a depth of between 40 and
60 km, ρ3 is changed. (d): Resistivity, ρ4 is changed. (e): The upper part of the ρ5 block is fixed at 5000 ꢀ·m and resistivity below 30 km depth, ρ5 is
changed.
the starting model was taken to be uniformly 100 ꢀ·m ex- is embedded in the middle crust of the eastern part of the
cept in the oceanic region. Sites are distributed at intervals profile and a conductor (1 ∼ 30 ꢀ·m) underlies the resistive
of between 10 and 15 km along the A–Aꢁ profile of about block at a depth of from 25 to 40 km. Beneath the Kirishima
100 km in length. The profile section of the Earth was di- volcano group, a high conductive block of about 1 ꢀ·m can
vided into 37×44 meshes. The mesh widths are larger than be found which extends bilaterally to below a depth of 40 km.
5 km and were determined by the intervals of the adjacent A middle to lower crust seems to be rather uniform to the west
sites. Then we carried out the inversion assuming that the ob- of the Kirishima volcano group.
served responses had small static shifts, because the apparent
5. Sensitivity Check
resistivity responses over the shortest period are almost con-
tinuous from site to site along the profile. Thus we inferred
that static shifts do not drastically affect the model. After
five iterations with 136 degrees of freedom, we obtained the
final 2-D resistivity model as shown in Fig. 9. The observed
and calculated response curves at each site are compared in
Fig. 10. The χ2 misfit, Eq. (2), was reduced from 8.34 to
0.883.
After obtaining the 2-D resistivity model by using the in-
version technique, we carried out the sensitivity tests for
some of the typical features.
We appraised the sensitivity by using the χ2 misfit weight-
ed by reciprocal standard errors. It is defined as follows:
χ2 = S2/D,
(2)
(3)
S2 = ꢁ2 + η2,
The model shows that a resistive block of about 3000 ꢀ·m

M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
545
Fig. 12. χ2 misfit calculated from the models of Fig. 11. (a): the χ2 misfit vs. resistivity, ρ1, in Fig. 11(a). (b): the χ2 misfit vs. resistivity, ρ2, in Fig. 11(b).
(c): the χ2 misfit vs. resistivity, ρ3, in Fig. 11(c). (d): the χ2 misfit vs. resistivity, ρ4, in Fig. 11(d). (e): the χ2 misfit vs. resistivity, ρ5, in Fig. 11(e).
Dotted lines denote the χ2 misfit calculated from the final model as shown in Fig. 9.
Nρ
ꢂ
misfit for the various resistivity values of ρ1 shown in
2
ꢁ2 =
Wi {log(ρaobs/ρacal)} ,
(4)
Fig. 11(a). The resistivity, ρ1, was changed as A × 10n
(A = 1, 2, · · · , 9; n = −2, −1, · · · , 2), 1000, 1500, 2000,
· · ·, 9000, 9500, and 10000.
i=1
Nφ
ꢂ
η2 =
wi (φobs − φcal)2,
1/(σiρ)2
(5)
(6)
Figure 12(a) presents the χ2 misfit versus resistivity, ρ1, in
Fig. 11(a). The misfit calculated from the model of Fig. 11(a)
for ρ1 > 1ꢀ·m is worse than that of the final model. This
result implies that the conductive block is well determined
beneath the Kirishima volcano group.
i=1
Wi =
,
ꢃN
ρ
ρ
1/Nρ j=1{1/(σj )2}
1/(σiφ)2
5.2 The spacial extent of the conductive block in the
western side of the Kirishima volcano group
wi =
,
(7)
ꢃN
φ
φ
1/Nφ j=1{1/(σj )2}
To confirm the lateral extent of the conductive block in the
western side of the Kirishima volcano group, we tested the
sensitivity of the conductive block below depth of 40 km.
We examined the changes in the χ2 misfit for changes in the
resistivities, ρ2 and ρ3, as shown in Fig. 11(b) and 11(c). The
way of changing the resistivity is the same as in the previous
case.
where D is the degree of freedom, Nρ is the apparent resis-
tivity data, Nφ is the phase data, ρa is the apparent resistivity,
φ is the phase, σρ is the standard error of the apparent resis-
tivity, and σφ is the standard phase error, respectively.
The result can be summarized as follows:
5.1 The conductive block beneath the Kirishima vol-
cano group
Figure 12(b) shows the χ2 misfit versus resistivity, ρ2, in
Fig. 11(b). The χ2 misfit values were found to be much larger
than those calculated from the final model. We carried out
The sensitivity of the conductive block beneath the
Kirishima volcano group was tested by calculating the χ2

546
M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
the F-test (Agarwal et al., 1993; Weaver and Agarwal, 1993; 6. Discussion
Agarwal and Weaver, 1997). The deterioration of fit, ꢂS2,
is defined as
A geological interpretation of the final 2-D resistivity
model is illustrated in Fig. 13.
Our final model suggested that a high conductive block
underlies the active volcanic zone of the Kirishima volcano
group. Because of the geological setting, this conductive
block may be interpreted as the presence of partial melt-
(St2est − Sfi2nal)/2
ꢂS2 =
,
(8)
χfi2nal
where S2 is provided by Eq. (3) and the subscripts test and ing and/or geothermal fluid that are directly related to the
final represent tested and final models, respectively. On the present magmatism. The bulk resistivity of rocks is mostly
F-test, the deterioration for the present case is 5.15, which controlled by the resistivity, connection, and volume fraction
is greater than 4.79 of a 99% confidence level. This result of the pore fluid or melt (Archie, 1942; Bahr, 1997), and the
deduces that the resistivity structure cannot be approximated resistivity of the melt is lower than that of brine (Presnall et
by a uniform resistivity block between a depth of 5 and 60 km al., 1972; Rai and Manghnani, 1978). Therefore, we may
in the western side of the Kirishima volcano group .
exclude the presence of free water as a possible cause of the
Figure 12(c) presents the χ2 misfit versus resistivity, ρ3, in low resistivity. Is such a very conductive structure below 1
Fig. 11(c). The misfit calculated from the model of Fig. 11(c) ꢀ·m really possible and able to be explained by partial melt-
for ρ3 < 10ꢀ·m approaches that calculated from the final ing and/or geothermal fluid? Recently, Schwarz and Kru¨ger
model. The ꢂS2 is 0.723 and is much smaller than 4.79 (1997) calculated the bulk resistivity of a medium which is
of a 99% confidence level. These results reveal that the composed of a matrix and isolated spherical melt fractions.
considered part should be represented by two blocks and the The resistivity of the matrix was constrained to 1000 ꢀ·m
lower block is more conductive than the upper block.
5.3 The resistive block in the forearc region
and that of the melt was constrained to 0.1 ꢀ·m. They found
that the bulk resistivity was not able to decrease below 500
The sensitivity test for the resistive block in the forearc ꢀ·m, even for a 14% melt fraction. They also simulated
region was carried out by calculating the χ2 misfit for the the case in which 1% connectivity is assumed and found
various resistivity values of ρ4 shown in Fig. 11(d). Fig- that the bulk resistivity easily reaches about 1 ꢀ·m. On the
ure 12(d) presents the χ2 misfit versus resistivity, ρ4. The other hand, the melt is thought to always exist as thin films or
misfit is obviously larger than that calculated from the final network channels under a mantle condition with partial melt-
model, as ρ4 decreases below 1000 ꢀ·m. This indicates that ing (Toramaru and Fujii, 1986; von Wagen and Waff, 1986;
the resistive block is surely more resistive than its surround- Harte et al., 1993). These studies imply that partial melting
ings.
can cause a resistivity of below 1 ꢀ·m. Furthermore, another
5.4 The conductive block in the forearc region
field exploration also revealed a low resistivity of the order
To check the resolution of the conductive block below a of 1 ꢀ·m beneath the Kirishima volcano group (Kagiyama et
30 km depth in the forearc region, we calculated the χ2 mis- al., 1996). The conductive area beneath that group extends
fit by changing the resistivity, ρ5 shown in Fig. 11(e). The bilaterally below 40 km depth. The geometry of the obtained
shallower resistive block was constrained at 5000 ꢀ·m. Fig- conductive structure seems to correspond well to the features
ure 12(e) presents the χ2 misfit versus resistivity, ρ5. Even if of the magmatic process in the subduction zones analyzed by
ρ5 is increased up to 2500 ꢀ·m, the χ2 misfit remains below numerical studies (Spiegelman and McKenzie, 1987; Davies
the 0.883 that was calculated from the final model. However, and Stevenson, 1992; Furukawa, 1993), in which the prin-
the misfit is evidently small as the resistivity below the 30 km cipal stress axes and magma ascending paths in a back arc
depth in the forearc region decreases. Hence, Fig 12(e) indi- region point towards a volcanic front on the Earth’s surface,
cates the block below 30 km depth is preferably conductive called magma focusing.
in the forearc region, although its resolution is not so good.
Another remarkable result is that a lower crust is likely
to be conductive in the forearc region. Although our data
could not completely confirm the lower crust conductor in
that region (hereafter referred to as the forearc conductor),
Toh (1993) also found the forearc conductor in the Izu-Bonin
arc which is a high-angle subduction zone. The forearc con-
ductor may be a general feature of the resistivity structure of
high-angle subduction zones. A particular feature related to
the forearc conductor is that the extremely negative Bouguer
gravity anomaly is observed in the forearc region (Fig. 2).
Okada (1998) constructed a density structure model from
the Bouguer gravity anomaly data and concluded that it orig-
inated from the low density structure below a 30 km depth in
this area. The depth of the low density structure is roughly
consistent with that of the forearc conductor revealed by the
present study. If the conductor is caused only by the presence
of dehydrated free water, such a negative Bouguer anomaly
maynotbeobserved. Hence, weproposethatdehydratedfree
water and serpentinite altered from the free water as a cause
Fig. 13. Interpretationofthe2-Dresistivitymodel. K.V.GandV.F. represent
the Kirishima volcano group and volcanic front, respectively. See the text
for detail.

M. ICHIKI et al.: RESISTIVITY STRUCTURE OF HIGH ANGLE SUBDUCTION ZONE
547
of the forearc conductor because serpentinite is both low in References
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by referring to the data of serpentine seamount (Ishii et al.,
1992) and the hydrous phase of serpentine (Tatsumi, 1989).
A distinct feature is the resistive block of middle crust in
the forearc region which seems to play the role of a cap rock
trapping free water. A similar result of a resistive block in
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7. Conclusion
The resistivity structure of the high-angle subduction zone
in the southern Kyushu district was obtained by a magnetotel-
luric investigation. We found that the lower crust conductor
probably occupies in the forearc region and that an extremely
conductive block develops beneath the volcanic front. We
conjectured that the lower crust conductor is due to a mix-
ture of serpentinite and dehydrated free water, and that the
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particular feature of the stage of the high-angle subductions
in Kanamori’s (1977) model.
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region was not resolved enough using only the land data.
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Bottom Electromagnetometer) observations to confirm the
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angle subduction zones are required to establish the general
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through the system for extramural research fellowships. A special
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