Bull. Inst. Oceanic Res. & Develop., Tokai Univ.（2007）, 28, 29 − 35

The Electromagnetic Observation and Inter Station Transfer
Function Analysis before and after the 2002 Dyke Intrusion
at Hachijo Island
Jun Izutsu1）, Makoto Harada2）, Toshiyasu Nagao3）, Yoichi Sasai4）
and Koichi Kumagawa5）

Abstract
There are many volcanoes and large earthquakes or earthquake swarms in Izu-Bonin Arc which
stretches to the south in the Paciﬁc Ocean. We have many electromagnetic observation systems in this area.
In this study, we mention about the data observed at Hachijo Island. In August 2002, a dyke intruded in
the N-S direction at a depth of 3 km beneath the Hachijo-Fuji, one of the volcanoes in Hachijo Island. The
3 components magnetic observation was performed in Hachijo Island by Hydrographic & Oceanographic
Department of the Japan Coast Guard. To investigate the possible changes of electric conductivity beneaththe Hachijo-Fuji, we analyzed this 3 component magnetic data before and after the dyke intrusion with themethod of Inter Station Transfer Function（ISTF）. We used data at Kakioka Magnetic Observatory as areference station. The ISTF analysis provides the information on the difference of the electrical undergroundstructure between the observation station（Hachijo Island）and reference station（Kakioka）. We calculatedmonthly mean ISTF elements from 1998 to 2005. We used continuous wavelet transform instead of theconventional Fourier transform to promote accuracy in the calculation. As a result of analysis, we could notﬁnd obvious changes of ISTF components before and after the dyke intrusion in August 2002. The magnetic
observatory was far（about 10 km from Hachijo-Fuji）to detect the inﬂuence of the dyke intrusion.

Introduction
Observations of the electromagnetic changes

generation mechanism and to be of useful tothe monitoring of these activities. These studies
were called

Seismo-, Volcano- and Tectono-

associated with seismic, volcanic and tectonic

Electro Magnetics . It has long been suggested

activities have been performed to clarify thethat the electrical conductivity structure changes

1） Earth Watch−Safety Net Research Center, Chubu University. 1200 Matsumoto-cho, Kasugai city, Aichi, 487-8501, Japan
2） Venture Business Laboratory, Chiba University. 1-33 Yayoi, Inage, Chiba 263-8522, Japan3） Earthquake Prediction Research Center, Institute of Oceanic Research and Development, Tokai University. 3-20-1
Orido Shimizu-ku, Shizuoka 424-8610, Japan
4） Disaster Prevention Division, Tokyo Metropolitan Government Bureau of General Affairs. 2-8-1 Nishi-Shinjuku,
Shinjuku-ku, Tokyo 163-8001, Japan
5） Hydrographic and Oceanographic Department, Japan Coast Guard. 5-3-1 Tsukiji, Chuo-ku, Tokyo 104-0045, Japan（Received : Sept. 19. 2006 ／ Accepted : Jan. 17, 2007）

30

Jun Izutsu, Makoto Harada, Toshiyasu Nagao, Yoichi Sasai and Koichi Kumagawa

by seismicity and volcanism. The geomagnetic

1b shows the distribution of earthquakes in Augusttransfer function is a method to detect and verify

2002.

such changes （Rikitake, 1966；Sano, 1980 and

To investigate the possible changes of electric1982）. Yanagihara and Nagano（1976）reported

conductivity beneath the Hachijo-Fuji, we analyzed

the changes of geomagnetic transfer functions at

3 component magnetic data observed at HJJ

Kakioka Magnetic Observatory over 10 years before

observatory before and after the dyke intrusion

and after the 1923 Kanto earthquake.

with the method of Inter Station Transfer Function

In the central part of the Japanese Islands, Izu-

（ISTF）. ISTF is deﬁned as the response function

Bonin arc which stretches to the south in the

between two sets of three components of magnetic

western Paciﬁ c Ocean is one of the most active

data at a observation station and reference station.

seismic and volcanic areas. Many electromagnetic

We use magnetic data observed at Kakioka Magnetic

observations have been performed intensively in thisObservatory, JMA as a reference station in thisarea, e.g. Izu Oshima Island（Yukutake, 1990）andstudy. The ISTF analysis provides the information

Miyake Island（Nakagawa et al., 1984；Utada et al.,on the difference of the electrical underground

1984；and Sasai et al., 2002）. In Hachijo Island, thestructure between the Island and Kakioka. If there

magnetic observation（total magnetic intensity andwere any conductivity changes at HJJ, the ISTF

3 component magnetic ﬁelds）has been performed

would change before and after the dyke intrusion.

since 1978 by Hydrographic & Oceanographic

In this paper, we calculated monthly mean ISTF

Department of the Japan Coast Guard（HJJ）（Fig.

between Hachijo Island（HJJ）and Kakioka（KAK）

1a）.

from 1998 to 2005.

In August 2002, there was a dyke intrusion in the

In the conventional Fourier transform analysis,

N-S direction 3 km beneath the Hachijo-Fuji, one oftransient changes interfere the overall spectral

the volcanoes in the Island（Kimata et al., 2004）. Fig.

properties and seriously degrade the accuracy

Fig. 1a） Map of the Hachijo Island. HJJ indicates the magnetic observatory of Hydrographic &Oceanographic Department of the Japan Coast Guard.
b） Distribution map of earthquakes occurred around Hachijo Island in August 2002.

31

The ISTF analysis at Hachijo Island.

of the transfer functions. To avoid these effects,

denotes the uncorrelated noise. The diagonalrobust algorithms have been used since 1980s inelements,, indicate the complex-valued

the ﬁ eld of electromagnetic exploration（Egbert

spectral ratio in each component between the twoand Booker, 1986；Chave et al., 1987）. Harada etstations. If the incident wave and the undergroundal.（2004）showed that the accuracy of the ISTF

structure are spatially uniform, the real components

with continuous wavelet transform method wasof the diagonal elements are 1.0. In actual case, theas high as that of robust estimations of transfer

diagonal elements are not exactly 1.0, due mainly

functions calculated by the Fourier transformto structural heterogeneity, and the non-diagonal

method. Therefore, we used the continuous wavelet

elements,

transform method in this study.

The elements,, have non-zero values.
, have the meaning

analogous to the single station transfer functions,

Methodin Eq.（1）.

The three components of geomagnetic ﬁ eld

The ISTF is usually estimated using data taken

variations measured at a point can generally bewhen the ratio ofis high, such as

expressed as ;the nighttime data on days of high solar-terrestrial
geomagnetic activity. The elements of the ISTF

where

is the frequency, andandare the Fourier coefﬁ cients of N-S, E-W,

are determined by making use of the usual powerspectrum approach.

and up-down components of the geomagneticﬁ eld variations.

The mother wavelet is chosen according to theare called thesignal properties and the purpose of analysis. We

geomagnetic transfer functions （Rikitake andchose the Morlet wavelet（Grossman and Morlet,

Honkura, 1985）. Short-term variations in the1984）, because it has a simple relationship between

geomagnetic ﬁ eld are composed of the primary

frequency and scale. The Morlet wavelet uses thesource ﬁeld, secondary induced ﬁeld and noises of

exponential

various origins. The transfer functions depend on

with a Gaussian window function ;as the basis function in combination

the underground electrical structure.
In the geomagnetic mid-latitude regions, including

where the dimensionless parameters

Japan, the geomagnetic ﬁeld ﬂuctuations originatedexpress the time and frequency, respectively. Thisin the upper atmosphere can be approximated bywavelet function meets the admissibility condition

vertically incident uniform plane waves. Under thisatcondition, the relationship of the geomagnetic ﬁeld（

. The waveform of the Morlet wavelet）is shown in Fig. 2.

data observed at a site, and at the remote reference
station may be expressed as follows（Harada et al.,2004）;

or
Here

and

are the Fourier coefﬁcientsof observed geomagnetic ﬁeld data at a station andat the remote reference station, respectively.
is the inter-station transfer function（ISTF）, andFig. 2 The waveform of the Morlet wavelet（
in Eq. [4] ）.

32

Jun Izutsu, Makoto Harada, Toshiyasu Nagao, Yoichi Sasai and Koichi Kumagawa

Based on GPS and tilt-meter data, a model for

Resultcrustal deformation was presented, in which a dyke

We applied the wavelet method to the HJJ andof thickness 3 m intruded in the N-S direction at

the KAK data. We choose the high geomagnetic

a depth of 3 km beneath the Hachijo-Fuji volcano.

activity days（ three hour Kp5 ）for every month.

After several days, the thickness of dyke reduced

The number of such days was usually 3-5 per month.

to 1 m, which suggests that the intrusive magma

There were several months that had no signiﬁcant

moved away to the north（GSI and JHOD, 2002）.

storm. Nighttime 6 hours（ from 21 : 00 to 03 : 00 LT ）On the other hand, very strange extremely low-of the chosen days were segmented into consecutive

frequency earthquakes began to occur at the end

windows with length which would contain enoughof August. Its period is anomalously long, i.e. 7 tonumber of data points to reduce the effects of

12 seconds. A model for such an earthquake wasisolated outliers.

proposed, which was an oscillating vertical magma

The correlation function and the Inter-Station

sheet of 2 km by 4 km at the top depth of 2.6 km.

Transfer Functions were calculated for every

Such very long period（VLP）events continued tochosen time-frequency window. Then, the windows

occur for a few months in 2002（Kumagai et al.,with inter-station correlation function higher than2003）.

0.90 were selected because the transfer functions

The piezomagnetic changes generated by such an

obtained from those windows were expected to

intrusive dyke were computed. Since the dyke top

be higher in accuracy. The ﬁnal transfer functions

was at a depth of 3 km from the ground surface, theof the month were the results of the least-squarecomputed total magnetic intensity changes are at

estimate from these windows.
Fig. 3 shows the six ISTF elements foreach period of geomagnetic ﬂuctuation.
ISTF elements were well determinedover the period range from 30 to 500
seconds.
Fig. 4 shows the time variation of ISTF
elements from 1998 to 2005 in the periodsof 30 seconds, 60 seconds, 120 seconds
and 300 seconds. A dashed line shows thetime of the dyke intrusion event, August
2002. We could not ﬁnd obvious changes
before and after August 2002.

Discussion
By using the wavelet algorithm, ISTF
elements were determined with little
error bars. For the periods longer than
60 seconds ISTF elements show almost
constant value, which implies that there
happened no large-scale changes in theelectric conductivity beneath the Hachijo
Island.

Fig. 3The Inter-Station Transfer Function elements
calculated between HJJ and KAK in August 2002.

33

The ISTF analysis at Hachijo Island.

Fig. 4 The monthly variation of ISTF elements（left : Txx, right : Tzx）for each periods from January 1998
to December 2005. A dashed line indicates August 2002, when the dyke was intruded beneath the
Hachijo-Fuji Volcano.

most less than 1 nT（Nagao et al., 2006）.

to affect the conductivity beneath all of the Hachijo

Therefore our result is quite reasonable because

Island. The magnetic observatory（HJJ）was about

HJJ Observatory is located on the older volcano

10 km far from Hachijo-Fuji, therefore, we consider

about 10 km away from the Hachijo-Fuji volcano.

our result was quite reasonable.

Conclusion

Acknowledgement

We calculated ISTF with using the geomagnetic

We greatly acknowledge the Kakioka Magnetic

data observed HJJ and KAK before and after theObservatory, Japan Meteorological Agency fordyke intrusion beneath the Hachijo-Fuji. In thisproviding us with geomagnetic data at Kakioka.

calculation, we applied the wavelet algorithm to

The authors acknowledge Miss Noriko Suto for thepromote the accuracy of transfer function. Therefore,

cooperation and discussion about ISTF method. We

we could obtain ISTF elements with little errors.

express deep thanks to the referee for giving us

The time variation of ISTF elements was almost
constant value and we could not ﬁ nd obvious
changes before and after the dyke intrusion. Manyvaluable suggestions and comments.

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Jun Izutsu, Makoto Harada, Toshiyasu Nagao, Yoichi Sasai and Koichi Kumagawa

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The ISTF analysis at Hachijo Island.

35

2002 年ダイク貫入前後における八丈島電磁観測データの ISTF 解析
井筒潤 1）・原 田

誠 2）・長 尾 年 恭 3）・笹 井 洋 一 4）・熊 川 浩 一 5）

要旨伊豆小笠原弧は日本でも有数の火山地帯であり， 比較的大きな地震や群発地震なども多い場所である．この地域には数多くの電磁気的観測が行われている．本研究では八丈島で観測された地磁気データについて解析を行った．2002 年 8 月に八丈島北部の火山である八丈富士の地下 3 km に南北方向にダイクが貫入しそれに伴う地殻変動が発生した．八丈島では海上保安庁水路部により 3 成分磁力測定が行われている．本研究ではダイク貫入前後の八丈島磁力データと気象庁柿岡地磁気観測所のデータを用いて，Inter-Station Transfer Function（ISTF）
法を解析手法として，ダイク貫入の地殻変動に伴う地下の電気的構造の変化を調査した．
ISTF 法は二点間の地下構造の違いを表し，我々は 1998 年から 2005 年までの 1 月ごとの代表的な ISTF の値を求め，地下構造の時間的な変化を調査した．ISTF の計算においては従来のフーリエ変換ではなくウェーブレッ
ト変換を用いた．これにより，系統的な誤差を従来よりも小さく抑えることが可能である．
解析結果ではダイク貫入前後で目立った ISTF の変化，つまり八丈島観測点の地下構造の変化は認められなかった．これは八丈島観測点が推定されているダイク貫入の場所よりも 10 km ほど離れており，またダイクの大きさも八丈島全体に影響を及ぼすほどではなかったからである．

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