Translated to English from paper in Japanese, originally published in:
Gijutsu Houkoku, Vol.28, No.3 (No.86), 1-77, 1989 with permission of authors.

Geoelectric Observation used by the Telegraphic
Facilities of NTT CorporationbyToshio MORI
Abstract
Unique geoelectric observations have been conducted in land and on the sea floor in the Kanto
and Tokai districts in Japan, and progress has been made on original research. The geoelectricobservations on land were conducted using the telegraphic facilities of Nippon Telegraph andTelephone Public Corporation (now NTT Company). The geoelectric observations on the sea floor
have been observed by making use of the power-feeding arrangement of Permanent Ocean-Bottom
Seismograph Observation Systems off Tokai and off the Boso Peninsula installed by the Japan
Meteorological Agency. These geoelectric data are fairly high quality in comparison with conventionalgeoelectric data.
A comparison examination of the geoelectric data observed by NTT facilities was carried out
with geomagnetic changes, geoelectric changes, seismic and volcanic activities, tide level changes,
the weather elements, etc. Moreover, examination of the use of geoelectric noises was also
performed.
The geoelectric observations using the electrodes and the underground metallic cables of NTT
have been performed mainly in the Mito and Numazu telephone offices. The observations have thefollowing two advantages in comparison with conventional methods: (1) The contact electric potential
between the electrode and the soil seems very stable. Therefore, we can obtain stable data over along time which is not affected by precipitation. (2) The base lengths for the observation are very
long. The base lengths are 10 to 100 times longer than those for other ordinary observations in Japan.
So, the observed field is affected by a large-scale structure.
The data observed at the Mito group are fairly noiseless, while the data at the Numazu andFujinomiya groups contain large amplitude noises. In order to improve the detectability ofself-potential variations related to tectonic activities, various methods were tried and developed bymaking use of these data. Especially, emphasis was placed on analysis by the method of real-time
detection of anomalous geoelectric changes by removing components induced by geomagnetic
variations with a stochastic difference equation.
As a result of investigating the relations between the observed geoelectric variations andtectonic activities, although no related change was observed in the Mito group, some abnormalities
were found in the Numazu group. Anomalous electric potentials at the Numazu group took place
simultaneously with the start of the earthquake swarm off the east coast of Izu on October, 1985. The
electrical potential changes that took place ranged up to 300 mV in Atami, and 100 mV in Ito andShuzenji. The Izu-Oshima volcano erupted on November 15, 1986. Before the start of the eruption,
the geoelectric potentials at ODW were estimated to have decreased abruptly by 500 mV on
November 6 and attained a maximum decrease of 800 mV on November 9.
Electrical potential changes relevant to tide level changes, and constituents of geoelectric change
unrelated to geomagnetic change and long cycle change, were also detected.
It was shown that the observed electric noises are applicable to investigate the underground40

Toshio MORI

structure and to detect underground electrical characteristic changes. If the earth potential isobserved near the conductivity anomaly boundary, it will be thought possible to detect unusual
sensitivity phenomena relevant to tectonic activities.

Comments on the English translation
The following will be noticed if a paper of about 15 years ago is put into English and read overagain.
Mistakes were made in clarity and in some unsuitable expressions, such as in the names ofplaces which are considered to be common sense by Japanese people, and in expressions where asimplified term was found. Corrections were made for those parts and explanation was added.
Although notations of figures had portions which are not unified, since correction was difficult, thenotation was left as it is, but explanations were added as much as possible.
As it is described in the “Circumstances of observation and acknowledgements,” this paper was
written when research business was being replaced with administrative business. Since it wasthought that it would be difficult to return to research business as before, the data and ideas were
hurriedly adjusted. In addition, since there was no Abstract in the text, it was written at this time.
Finally, the author is thankful to the personnel of the Magnetic Observatory who gave us thisopportunity.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation
CONTENTS
1. Introduction
2. Observation method
3. Outline of the observed records
3-1 Outline of the records at each observation group
3-2 Comparison of the observed record in the same time of a different day
4. Long-term stability
5. Impedance tensor and transfer function
5-1 Impedance tensor at the Shimodate group and at Kakioka
5-2 Impedance tensor at the Mito group and transfer function at Kakioka
6. Time changes of impedance tensors and transfer functions
7. Elimination of the electrical fields induced by geomagnetic variations - Detection of the anomalous
earth potential 7-1 Analysis method by a stochastic differential equation
7-2 For geoelectric fields between Kasama and Mito
7-3 Application to the data of electrical fields at the Mito group and Z at Kakioka
7-4 Spectral analysis
7-5 When using per-minute data
7-6 Use of the transfer function and impedance tensor
8. Relation to tectonic activities
8-1 Relation to the earthquake swarms off the east coast of the Izu Peninsula
8-2 Relation to eruptions of Izu-Oshima volcano
8-3 Other changes
9. Relation to tide, atmospheric pressure and others
10. Application for research of fault structure
10-1 Iriyama fault
10-2 Tanna fault
10-3 Fujikawa fault
11. Anomalous geoelectric change observed near CA (conductivity anomaly) boundary
12. Electrical observation on the sea floor by making use of the power-feeding arrangement of thePermanent Ocean-Bottom Seismograph Observation System off Boso Peninsula (BSOBS)
13. Conclusions
14. Circumstances of observation and acknowledgements
References

41

42

Toshio MORI

1. Introduction
Geoelectric observation is very simple, on theprincipal target, in order to measure the potential
differences between two arbitrary points on the earth.
But it is not so easy to recognize the significant
geophysical signals, because the observed electricalfield involves various other factors: meteorologicalfactors (temperature, rainfall, moisture, etc.), artificial
electrical noises from electric trains, factories, etc.,
troubles in the observation system (electrodes,
insulation of the circuit, recorder, etc.) and so on.
Especially, the contact potential between theelectrodes and the soil are affected by the closeness ofcontact between the surface of the electrode and thesurrounding soil, and also affected by the changes inmeteorological conditions involving temperature,
precipitation, content of soil moisture and so on. So wewere not able to have confidence, especially inchanges that occurred over several hours or more.
Many authors have claimed that “anomalous changes”
in earth potential prior to earthquakes wereprecursors. However, it has been debatable whether or
not those anomalous changes were truly precursors,
because of problems in the reliability of those data.
Unique geoelectric observations have been
conducted in land and on the sea floor, which wereoriginally developed by the author (Mori, 1982, 1985,
1987). The geoelectric observations on land wereconducted by lengths of several 10 km, using thetelegraphic facilities of Nippon Telegraph andTelephone Public Corporation (now NTT Company).
The geoelectric observations on the sea floor have been
observed by making use of the power-feedingarrangement of Permanent Ocean-Bottom SeismographObservation Systems off Tokai (TKOBS) installed by
the Japan Meteorological Agency (JMA). Thesegeoelectric data are fairly high quality in comparison
with conventional geoelectric data because of thestable contact potential. Therefore, we can get very
useful data for geophysical research.
Geoelectric observation is sometimes called earth
current observation. The history of earth current
observation was summarized by Hatakeyama (1940),
Chapman and Bartels (1940), Garland (1960) andothers. It is said that the first earth current
observation was performed in Britain in 1847 using atelephone wire by Barlow (1849). He used thetelephone wires which connected with various points

to Derly, and recognized the existence of earth
currents. In August 1859, telegraphic systems
throughout the world were considerably disturbed byirregular currents induced by remarkable geomagnetic
storms. It is said that research of earth currents were
ignited by it and prospered. The record of thegeoelectric field was performed at Greenwich by Airyfor two years after 1865. Also, the earth current
observations were performed as the work of the FirstInternational Polar Years from 1882 to 1883. In Japan,
Shyda (Shyda, 1886) observed electrical fields by using
the telegraphic line between Nagasaki and Fuzan, andfound clear daily variations in the electrical field. In theearly history of earth current research, observations
were conducted by telegraphic lines longer then 100
km. However, those observations had to bediscontinued because of the disturbing effects of straycurrents which resulted from electrical railways andother artificial sources. After that, routine
observations of electrical fields have been continued
by specific magnetic observatories.
Recently, geoelectric and geomagnetic observations
on land have been actively conducted for research ofunderground conductivity, tectonic activities such as
seismic precursors, etc. Most geoelectric observations
have their base length less than a few hundred meters,
and the observation periods are short except in routine
observatories. The observations have also been
actively conducted on the ocean floor. On the ocean
floor, the lengths of the base lines of the observation
are ordinarily short and the observation periods arealso short, although the contact potential is stable.
Therefore, it is difficult to study all of the events
related to geoelectric variations.
As mentioned above, the specific electricalobservations on the sea floor have been conducted bymaking use of the power-feeding arrangement of
TKOBS installed by JMA. The results of theobservations were reported by Mori (1982, 1987), andstable observation has been conducted over a long
period of time. This observation length is about 110 km
and very long, but there is only one component.
Therefore, we can not discuss current direction. In 1986,
Permanent Ocean-Bottom Seismograph Observation
Systems off Boso Peninsula (BSOBS) were installed by
JMA in the same system of TKOBS, so we can measure
the electrical field on the sea floor. The length is about
80 km. Since Katsuura Weather Station installed theGeoelectric Observation used by the Telegraphic Facilities of NTT Corporation

shore ground close to a direct-current train railroad, theelectrical noise is very large in the day time. However,at midnight, the noise is not so large, so the electricalfield records might be usable.
Many authors have reported anomalous changesof the geoelectric field preceding or accompanying
earthquakes (e.g., Myachkin et al., 1972；Sobolev,
1975; Molnar et al., 1977; Corwin and Morrison, 1977;
Varotsos and Alexopoulos, 1984). However, there is aproblem in the reliability of the data, and it is truly
doubtful in many cases whether it is the precursor ofan earthquake. Consequently, we still do not havegeneral concepts about the anomalous changes in theelectrical fields as precursors of the tectonic activities,
that is, how long, how intense and what type.
The geoelectric observations using the electrodes
and the underground metallic cables of NTT weremade not very long after these observations started.
However, the data obtained from these observations ishigh quality, and especially seem to be epoch-making
in their stability over a long period of time. These data
are very useful for geoelectric research as comparedwith ordinary observations. The previous workers
seem to have made an effort to calculate the electricalfield induced by the observed magnetic variations anddetect anomalous changes according to tectonic
activities such as earthquakes. The author thinks thatthey could not have very reliable data for analysis. In
this paper, emphasis is placed in the analysis on
removing the induced electrical field from theobserved one. That is how valuable data came to beobtained in such analysis. Geoelectric observations
based on the use of long telegraph lines were madeearlier in history, but no detailed analysis was done.
The geoelectric observation which is described in thispaper has not been performed in Japan recently.
In Japan, it is considered that the geoelectricobservations at Kakioka Magnetic Observatory andother branch observatories of JMA are first classobservations, but those observations have also been
affected by heavy precipitations. It will be described in
Section 4 that the observations using NTT facilities
have not been affected by precipitation. The author
had almost given up on long time variations in thegeoelectric data before taking these data, while coming
to hold great expectations for these new reliable data.
Though the observations using NTT facilities havebeen conducted with the good will of NTT, it was heard

43

that the underground metallic cables of NTT would be
replaced with optical fiber cables in the near future.
Therefore, the author felt strongly a pressing need tolearn whether the new geoelectric observations would
be available for research. Therefore, the author has
randomly studied for the purposes which investigate
relations with tectonic activities such as seismic andvolcanic activities. The paper is in an intermediate stage,
so further intense studies are expected.
This observation method has the following two
advantages in comparison with conventional methods:(1) The contact electrical potential between theelectrode and the soil seems very stable.
For conventional observation, the electricalpotential between the electrodes and soil is unstable,
especially due to precipitation. The present
observation is hardly affected by precipitation. It
seems to have originated from the fact that thecontact resistance between the electrode and thesoil is less than 2 ohms, and that the installation
depth is more than 5 meters. We can therefore
obtain stable data over a long time.
(2) The base lengths for the observation are very long.
The base lengths are ten to several tens ofkilometers, and these are 10 to 100 times longer
than those for other ordinary observations in Japan.
The amplitude of the variation of earth potential due
to tectonic activity is roughly proportional to the
distance, while noise due to industries, railways, etc.,
and noise associated with the electrodes themselvesis primarily local. Therefore, we can expect a highratio of signal to noise when we use long electrodespacing. In contrast, the sensitivity for the so-called“effective point” with tectonic activities might beblunt. The observed electrical field is affected by thelarge scale structure.
The detail about the observation due to thefacilities of NTT will be described in Section 2. The
locations used as associated data are listed in the next
page.
These observations have been conducted by
JMA except for the tide level meter at Ooarai (byIbaraki Prefecture) and at Ito (by Geographical
Survey Institute). The manometer at Youkaichibahas been observed for the compensation to thevolume strain meter. The tsunami-meters are
made of a quartz pressure gage attached to theterminal apparatus of TKOBS and BSOBS, andToshio MORI

44

Longitude

Latitude

Kakioka Magnetic observatory (KAK)

36° 13.75’

140° 11.38’

TKOBS

Shore earth (OMZ)

34° 35.73’

137° 35.40’

TKOBS Sea earth（TSI）

33° 45.90’

137° 35.40’

BSOBS

Shore earth (KTU)

35° 08.75’

140° 18.79’

BSOBS Sea earth（BSI）

34° 39.21’

140° 58.68’

Volume strain meter at Fuji

35° 11’138° 44’

Manometer at Youkaichiba

35° 45’140° 32’

Volume strain meter at Higashi-Izu

34° 49’139° 03’

Tide level meter at Ooarai

36° 19’140° 04’

Tide level meter at Ito

34° 58’139° 04’

Tide level meter at Uchiura

35° 01’138° 54’

the pressure values are converted into the sea
level from the bottom.
Throughout this paper, JST (Japanese Standard
Time) is used. JST can be calculated by adding 9 hours
to GMT (Greenwich Mean Time).
2. Observation method
Outline of the observation methods using the
NTT facility is described on the previous reports(Mori, 1985, 1987). Fig. 2-1 shows the observation
networks for the geoelectric field and the location of
Kakioka Magnetic Observatory (KAK) in the Kanto
and Tokai districts of Japan. The electrical fields on thesea floor are measured between shore earth (OMZ and
KTU) and sea earth (TS1 and BS1). The electricalfields on the sea floor are not principally discussed inthis paper, although the electrical fields on land are
affected by oceanic electrical currents and the motionof the sea water. Marine data is also required in orderto study the causes of the geoelectric variations.
The geoelectric fields using the NTT facilities aremeasured between NTT key stations (SMO, MTO,FJM and NMZ in Fig. 2-1) and the connected
sub-stations. The observations between that areconnected with the thick lines have been conducted atpresent in September, 1987. But the stations
connected with dotted lines have not yet had theobservations conducted. In this paper, the observation
networks will be called the “Shimodate group,” “Mitogroup,” “Fujinomiya group” and “Numazu group,”taking the name of the key station for each group. The
locations of the NTT stations for the observation are
shown in Table 2-1, and the distances and the

directions of the observation lines in Table 2-2.
The observations have been conducted withdigital recorders at the Mito, Fujinomiya and Numazu
groups, so we can calculate the mutual electricalpotential differences between two arbitrary stations
belonging to the same group, as shown in Table 2-2.
The input channels of the digital recorder at each key
station are as follows:
Key station

1ch

2ch

3ch

4ch

Input range

MTO

ISI

KSM HIO

ISI*

10 V

ODW

10 V

NMZ

SUZ

ITO

FJM

KOF

MNB FJI*ATM

KOF** 10 V

：using low-pass filter of 30 seconds** ：using low-pass filter of 10 seconds

The observation periods and the methods are as
follows:(1) Shimodate group
The observation was conducted over a distance of
26.8 km from Shimodate (SMO) to Kasama (KSM) anda distance of 15.7 km from SMO to Oyama (OYA). The
observation period was from November 29, 1983, to
March 3, 1984. The electrical fields were recordedwith a two-pen recorder through low-pass filters with acut-off frequency of 0.003Hz (about 1/6 cycle/minute).
It was the test observation for whether or not to get
usable data, for the reasons that such observations
have not been done in recent years in Japan and alsothat the NTT repeater stations are located in thecentral part of an electrically noisy city. Though therecords were not completed because of trouble withthe pen recorder, the purpose has been accomplished.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 2‑1 Observation networks for geoelectricfields and the location of Kakioka Magnetic
Observatory (KAK) in the Kanto and Tokai
regions of Japan.
Electrical fields at the Shimodate (SMO),
Mito (MTO), Fujinomiya (FJM) and Numazu(NMZ) regions have been observed using
electrodes and underground metallic cablesof NTT Company.
Electrical variations between Omaezaki(OMZ) and TS1 off Tokai, and between
Katsuura (KTU) and BS1 off Boso Peninsula
are carried out using the power feedingarrangement
of
the
ocean
bottomseismographs of JMA.

Fig. 2‑2

Schematic diagram of the geoelectric observation system.

Table 2‑1

Location of the NTT repeater stations used for the geoelectricobservation in the Kanto and Tokai regions of Japan

Station
Mito
Ishioka
Kasama
Hitachiota
ShimodateOyama
NumazuSyuzenji
Ito
Atami
OdawaraFujinomiya
Kofu
MinobuFuji

Symbol
MTO
ISI
KSM
HIO
SMO
OYA
NMZ
SUZ
ITO
ATM
ODW
FJM
KOF
MNB
FJI

Lat. N36° 22.50’
36° 11.08’
36° 23.03’
36° 32.36’
36° 18.51’
36° 17.95’
35° 5.91’
34° 58.56’
34° 57.93’
35° 5.47’
35° 15.51’
35° 13.14’
35° 38.17’
35° 22.10’
35° 9.49’

Long. E
140° 28.34’
140° 16.62’
140° 15.57’
140° 31.50’
140° 58.56’
139° 48.12’138° 51.90’
138° 56.92’
139° 5.88’
139° 4.34’
139° 9.44’
138° 37.19’
138° 32.98’
138° 26.34’
138° 41.28’

45

Toshio MORI

46

Table 2‑2 Distance and direction of the geoelectric observation lines

Station group

Line Symbol
ISI-MTOISI-HIO
ISI-KSM
MTO-HIO
KSM-MTO
KSM-HIO

Distance
27.4 km45.1 km22.1 km18.8 km19.0 km29.3 km

Direction
N 40°E
N 29°E
N -4°E
N 15°E
N 93°E
N 54°E

Shimodate group

SMO-KSM
OYA-SMO

26.8 km15.7 km

N 72°E
N 86°E

Fujinomiya group

FJI-KOF
FJI-FJM
FJI-MNBFJM-KOFFJM-MNB
MNB-KOF

54.5 km
9.2 km32.5 km46.7 km23.3 km
31.4 km

N -13°E
N -42°E
N -44°E
N -8°EN -45°E
N 19°E

Numazu group

NMZ-ODW
NMZ-ITOSUZ-ITOITO-ODW
ATM-ODW
ITO-ATM
SUZ-ODW
SUZ-ATM
SUZ-NMZ

32.0 km25.9 km13.7 km32.9 km20.1 km14.1 km36.6 km17.0 km15.6 km

N 56°EN 125°E
N 95°E
N 9°E
N 23°EN -10°E
N 31°E
N 41°EN -29°E

Mito group(2) Mito group
The electrical fields have been observed at the Mito
key station connecting with Ishioka (ISI), Kasama(KSM) and Hitachiota (HIO). The observation using a
multi-pen recorder through the same filters at SMO
was conducted from January 17 to March 6, 1985.
Since March 6, 1985, the observation has been
conducted with a multi-pen recorder and a digitalrecorder as shown in Fig. 2-2. The continuous data
were passed through a 0.5 Hz low-pass analog filter foranti-aliasing, and were converted to 12-bit digital data.
Although data were available every second,
per-minute data were usually obtained by averaging
and stored in a 512 kB PROM (Programmable Read
Only Memory). At the same time, the continuous data
were passed through a 1/30 Hz low-pass analog filter
and recorded on a multi-pen recorder.
(3) Fujinomiya group
Electrical potentials had been observed at the
Fujinomiya (FJM) key station for Kofu (KOF), Minobu(MNB) and Fuji (FJI). The observation period wasfrom June 10 to August 26, 1985. The observation
system is same as in the Mito group in Fig. 2-2. The
Fujinomiya and Numazu groups are located in the
Tokai district which might be affected by the Tokai

earthquake with magnitude 8 class considered likely tohappen in the near future. Also, this area is in atectonically active region including Mt. Fuji, which isan active volcano and the highest mountain in Japan,
and a volcanically and seismically active area. The linefrom FJM to KOF via MNB is selected in order toresearch for volcanic activities of Mt. Fuji. At that time,
NTT Company decided to exchange the undergroundmetallic cables for optical fiber cable in the near future
for connections with FJM and the other repeater
stations. It was unfortunate that the cable from FJM to
KOF via MNB was inundated with heavy rain by thetyphoon on June 30, 1985, and it was not restored. At
result, the observations at FJM were stopped on
August 26, 1985.
(4) Numazu group
Electrical potentials have been observed at the
Numazu (NMZ) key station for Shuzenji (SUZ), Ito(ITO), Atami (ATM) and Odawara (ODW) since August
26, 1985. The observation method is the same as for
the Mito group, except for a part of the filter to a
multi-pen recorder.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

3. Outline of the observed records
3-1 Outline of the records at each observationgroup
The outline of the records in each group isdescribed below.
(1) Shimodate group
Fig. 3-1 shows an example of the analog records forabout 50 seconds without any filtrations at SMO. The
most outstanding noises have a period of 1 second.
These short-period noises have been observed atvarious parts of Japan (Mori and Hasegawa, 1979). The
origin of those noises seems to be from the charge
counters for telephone usage time by customers of
NTT Company. In order to eliminate those
short-period noises, high-cut filters of around 6
minutes have been applied. Electrical variations
observed at the Shimodate group are compared withthe geoelectric and geomagnetic variations at KAK.
The upper part of Fig. 3-2 shows the geoelectricvariations at the Shimodate group and the geoelectricand geomagnetic variations at KAK. The records of thegeoelectric fields at the Shimodate group and KAK
were digitized by a curve-reader because of analog
records. The electrical variations are expressed in

47

mV/km, and a positive sign is taken when thecurrent flows to the north and to the east. OYAMA
denotes the electrical field variations at OYA-SMO,
while KASAMA denotes those at SMO-KSM. EW
and NS at KAK denote the east (N85°E) and north(N5°W) components of the geoelectric field. The
length for the EW base line is 1200 meters, whilethat for the NS base line is 900 meters. X, Y and Z at
KAK denote the geographic north, east anddownward components of the geomagnetic field. X
and Y are converted from the horizontal component
and declination. The electrical variations at
SMO-KSM and EW at KAK are very similar, andwell correlated to geomagnetic variations at KAK.
The greater part of these geoelectric variations are
induced by geomagnetic variations.
The bottom of Fig. 3-2 shows the power
spectrums of the geoelectric fields due to the AR
model (Akaike, 1969). At OYAMA, the power in acycle of several 10 seconds is relatively large, andthe main part seems to be due to artificial electricalnoises of electric trains, because OYA is located near
the Tohoku-railway where the trains are driven by
DC electric power.

Fig. 3‑1 The analog records without any filtrations for about 50 minutes on January 23, 1984, at SMO.

48

Toshio MORI

Fig. 3‑2 Top shows the geoelectric variations at the Shimodate group and the geoelectric and geomagnetic variations at
KAK. OYAMA and KASAMA stand for the electrical field at OYA‑SMO and SMO‑KSM, respectively. KAK EW and KAK NS standfor the east‑west and the north‑south components of the electrical field at Kakioka. KAK H, KAK D and KAK Z standfor the horizontal component, declination and vertical component of the magnetic field at Kakioka, respectively. The
bottom shows the power spectrum due to the AR model.

(2) Mito group
Fig. 3-3 shows the geoelectric fields at the Mitogroup at intervals of 1 second. FC = 2S and FC = 30Sshow the record through a 2 second low-pass filter anda 30 second low-pass filter, respectively. In this figure,there are very remarkable noises with a period of 18
seconds on the record of FC = 2S at HIO-MTO. These
noises also seem to be from the charge counters of
NTT Company. Fig. 3-4 shows an example ofshort-period fluctuations in the geoelectrical fields at
the Mito group and the geomagnetic fields at KAK.
The figure is illustrated by the 10-second averagevalue of a sampling per second, and the data period is 2
hours. The records are of a bay-type event and of Pi2
pulsation with a period of about 60 seconds.
Short-period variations such as Pi2 are observable,
despite the fact that the electrodes are located in sucha noisy area as Mito City. This fact shows that thepresent observation gives us highly accurate data. The
observation stations of the Mito group are locatedalong the Joban Line, which uses AC electricity.
Therefore, the leak current from a train does not affectthe observation.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 3‑3 Geoelectric fields at the Mito group at 1‑second intervals from 12:42 to 13:06 on March 6, 1985. FC=2S, FC=30S
show the record through 2‑seconds low‑pass filter and 30‑seconds low‑pass filter, respectively.

Fig. 3‑4 Example of short period variations (Pi2 and bay‑type events) of the electrical fields between the various pairs
among the Mito group from 21:00 to 23:00 on March 8, 1985. The magnetic fields at Kakioka (KAK) are also shown. The
figure is illustrated by the 10‑second average value of a sampling per second

49

50

Toshio MORI

(3) Fujinomiya group
Fig. 3-5 shows the electrical field in the samplinginterval of 2 seconds. Almost all the fluctuations seemto be artificial noises. The fluctuations at FJM-MNB
are pulse-like variations and different from otherrecords. MNB and FJM are located near the
Itoigawa-Shizuoka tectonic line and the Fujikawa
seismic fault, and those areas might have low
resistivity. Fig. 3-6 shows the electrical fields at the
Mito group and Fujinomiya group and the magnetic
fields at KAK at 1-minute intervals. In this figure, therecord at FJM-MNB is almost straight because of the
few seconds fluctuations as seen in Fig. 3-5. The
records of the Fujinomiya group have very large
artificial noises except for the late night periods, andthe geoelectric variations induced by geomagnetic
variations can be detected only when remarkablegeomagnetic changes take place between 0:00 and
5:00 local time.
(4) Numazu group
Fig. 3-7 shows the magnetic and electrical fields at
KAK and the electrical fields at the Mito group andNumazu group at 1-minute intervals. The bottom 10
records at the Numazu group are very noisy, so we candistinguish the geoelectric variations when a large
geomagnetic disturbance occurred late at night. FJI,
NMZ, ATM and ODW are along the Tokaido Line,

where direct-current trains run. Moreover, the area
has many big factories as compared with Mito.
3-2 Comparison of the observed record in thesame time of a different day
It is interesting to consider whether or not thoselarge artificial noises at the Fujinomiya group andNumazu group are the same noises every day. The topof Fig. 3-8 shows the records between 3:00 and 6:00,
and the bottom shows the records between 8:00 and
11:00 on June 12 and 13, 1985, at intervals of 10
seconds. Those two lines are the first in time from thetop. For example, the third and fourth upper lines ofthe figure show the record on June 12 and that on June
13, respectively, at FJI-FJM. The noises increase fromaround 5:15 every day, and the forms are similar. The
same characteristics of noises can also be seen in theother cases of the Numazu group.
Fig. 3-9 shows the records at the Mito group in
the same manner as in Fig. 3-8. The plotter
sensitivities in the Mito group are 2 to 20 times that of
the Fujinomiya group, as shown in the right sides ofeach figure. At ISI-KSM and MTO-HIO, some noises
are likely on the 2 days, but the noises are fairly smallin comparison with those at the Fujinomiya andNumazu groups.

Fig. 3‑5 Geoelectric variations between the various pairs among the Fujinomiya group at 2‑second intervals from 15:50
to 16:10 in June, 1985. The vertical scale of FJM‑MNB is 5 times as large as others.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 3‑6 Electrical fields between the various pairs among the Mito group (from ISI‑MTO to KSM‑HIO) and Fujinomiya group(from FJI‑KOF to MNB‑KOF) at 1‑minute intervals from June 25 to 27, 1985. The magnetic fields at Kakioka (KAK X, KAK
Y, KAK Z) are shown for convenience of comparison.

51

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Toshio MORI

Fig. 3‑7 Electrical fields between the various pairs among the Mito group (from ISI‑MTO to KSM‑HIO) and Numazu group (fromNMZ‑ODW to SUZ‑NMZ) at 1‑minute intervals from February 20 to 24, 1987. The magnetic and electrical fields at Kakioka(from KAK X to KAK NS) are shown for convenience of comparison.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 3‑8 Geoelectric record on June 12, 1985, and the same time on the 13th at the Fujinomiya group. A thick line shows
the record on June 12, and a thin line shows the record on the 13th. Two are 1 set at a time from the top. The records
from 03:00 to 06:00 are shown by the upper part of the figure at intervals of 10 seconds, and the record from 08:00
to 11:00 are shown by the lower part.

53

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Toshio MORI

Fig. 3‑9 Geoelectric record from 08:00 to 11:00 on June 12, 1985, and the same time on the 13th at the Mito group at intervalsof 10 seconds. A thick line shows the record on June 12, and a thin line shows the record on the 13th. Two are 1 set
at a time from the top.

4. Long-term stability
In order to inspect the long-term stability, thegeoelectric variations at the Fujinomiya, Numazu andMito groups and at KAK, and the geomagnetic
variations at KAK are shown in Fig. 4-1 in the dailymean values from March, 1985, to November, 1986.
The daily mean values averaged the sampling values
for 1 minute, except for the geoelectric fields at KAK.
Those at KAK are the mean values of the 25 hourly
values in the vicinity of every hourly value. The upper
10 lines of the figure are the geoelectric fields at the
Fujinomiya group until August 25, 1985, and since thenthose at the Numazu group. In the top part of the
figure, vertical lines show heavy precipitation, and thenumbers are the continued total precipitations in mm
at Mishima Weather Station near Numazu, and thoseat Mito Local Meteorological Observatory are shownin the middle part.
The electrical variations at the Numazu group arechangeable in comparison with those at the Mito group
because of artificial noises due to direct-current
electric trains and large-scale factories, etc. Typhoons
passed through the observation areas on June 30 andJuly 1, 1985, and on August 4, 1986, and considerable
precipitation was observed at both Mishima and Mito.
After those times, the geoelectric fields at KAK
changed considerably. Precipitation has an adverse
effect on the stability of the contact potential, andthose deviations should be ascribed to precipitation at
KAK. Geoelectric fields of the Mito group, however,show no anomalous changes at those times. Thisshows that precipitation does not change the contact
potentials of electrodes.
The electrical field at FJI-FJM (second line fromthe top of Fig. 4-1) became lower from the beginning ofJuly to the middle of August. The underground cablefrom FJM to KOF via MNB was inundated by heavy rainfrom a typhoon on June 30, 1985, and observations were
stopped after that. Electrical resistivity observation of
the Fujikawa fault in Fujinomiya-shi showed decreasingof the underground electrical resistivity around thesame period (Earthquake Research Institute, Universityof Tokyo, 1987). So the decreasing of the electrical field
at FJI-FJM may not be reflecting the changes ofelectrical conditions near the electrodes, but the widechange of the underground electrical conditions.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

55

Fig. 4‑1 Electrical variations in daily mean values at NTT stations and electrical and magnetic ones at Kakioka. The upper
10 lines are at the Fujinomiya and Numazu group, the middle 6 lines at the Mito group and the lower 5 lines at KAK.
The vertical lines and the numbers (for example 125) on the top and on the middle of the figure show the heavy
precipitation days and the continued total precipitations in mm at Mishima Weather Station (near Numazu) and thoseat the Mito Local Meteorological Observatory, respectively. These precipitations are shown in case the continued total
ones exceeded 60 mm.

After the typhoon at the beginning of August, 1986,
the electrical field at KAK showed anomalous change,and similar anomalous changes at the Numazu group had
begun before the typhoon. As the electrical variations at
the Numazu group seem to be affected by the change ofsea level, as will be shown in Section 9, these anomalouschanges may be due to the variations of the sea level withatmospheric pressure. Furthermore, since tectonic
activities are active in the Tokai district, the influence bytectonic activities may also be included, so the cause
cannot be specified. It is thought that there is at least no
direct influence from precipitation.
5. Impedance tensor and transfer function
The following relation was introduced by Rikitakeand Yokoyama (1955) as a result of analyses for
short-period geomagnetic variations such as bay-type
events:Z = A・H + B・D.
Here, Z, H and D denote the amplitudes of the verticalcomponent, horizontal components and declination in

the geomagnetic variation, respectively. A and B are
complex functions of frequency or period, and arecalled transfer functions.
In the same way, the geoelectric variations are
expressed as
Ex＝Z11・X+Z12・Y
Ey＝Z21・X+Z22・Y.
Here, X represents the northward component, and Y
represents the eastward component, of the geomagnetic
variations. Ex and Ey represent the north-south andeast-west components of the geoelectric variations
respectively, and positive signs are taken when thecurrents flow northward and eastward. Z11, Z12, Z21 and Z22
are called impedance tensors.
In this paper, the direction of the geoelectricvariations observed at the NTT stations are notexchanged for the north or east components. Theseequations are adopted for all periods of the variations,and the transfer functions and the impedance tensors
are calculated by using the least square method (e.g.,Mori, 1987).

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Toshio MORI

5-1 Impedance tensor at the Shimodate group
and at Kakioka
In the Shimodate group, the impedance tensors at
SMO-KSM are calculated because the geoelectric fieldat KSM-OYA has been fairly contaminated by artificial
noises, as described in Section 3-1.
The top of Fig. 5-1 shows the per-minute data of
KASAMA (same as SMO-KSM), KAK EW, KAK H,
KAK D from 8:00 on December 13 to 8:00 on
December 14, 1983. The data of KASAMA and KAK
EW are digitized by a curve-reader. KAK H and KAK
D are the horizontal component and the declination innT at KAK.
The elements of the impedance tensors are
obtained by Fourier transform of geoelectric variationsfor 3 hours. The bottom of Fig. 5-1 shows the
impedance tensors (Z21 and Z22) of SMO-KSM andKAK EW in mV/km/nT. R denotes the real part, and I
denotes the imaginary part. The impedance tensors at

SMO-KSM are almost the same as those at KAK EW,
except for Z21(I).
The impedance tensors at SMO-KSM for fivedays were displayed on Fig. 5-2 in piles. ERRZ21
denotes the standard deviation of Z21, and ERRZ22
denotes that of Z22. Z21(R) shows almost the samevalues, except for December 12 ~ 13 (symbol +).
Z22(I) is approximately zero for all periods.
Fig. 5-3 shows the impedance tensors at KAK EW
in the same manner as in Fig. 5-2. The degree of
scattering in the impedance tensors at KAK EW is
compared with those at SMO-KSM. The degree of
scattering of Z21(I) at SMO-KSM is in comparison withother elements, and those at KAK EW have especiallylarge scattering. Although the origins are not clarified atpresent, artificial noises are considerably large. The
impedance tensors at SMO-KSM and KAK EW become
scattered for periods less than around 20 minutes, andthe origin seem to be artificial noises.

Fig. 5‑1 Impedance tensors (Z21 and Z22) of KASAMA (SMO‑KSM) and KAK EW for KAK H and KAK D in mV/km/nT. R and I denote
the real and imaginary part, respectively. These impedance tensors were calculated by using the upper record from08:00 on December 13 to 08:00 on December 14, 1983.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 5‑2 Impedance tensors at SMO‑KSM on the 5th are displayed in piles. ERRZ21 and ERRZ22 denote the standard deviationsof Z21 and Z22, respectively. The symbols and the analysis times are as follows:○：1983/12/10 08:00 〜 12/11 08:00△：1983/12/11 08:00 〜 12/12 08:00＋：1983/12/12 08:00 〜 12/13 08:00
×：1983/12/13 08:00 〜 12/14 08:00◇：1983/12/14 08:00 〜 12/15 08:00

Fig. 5‑3

Impedance tensor at KAK EW. The legends are the same as in Fig. 5‑2.

57

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Toshio MORI

5-2 Impedance tensor at the Mito group andtransfer function at Kakioka
Geoelectric fields at the Mito group have usually
been recorded to the PROM memory with 1-minutemean values of per-second sampling. However in thissection, per-second records, as on the special
observation, are used for research for the wide
frequency range. Impedance tensors are calculatedusing 10-second, 1-minute and 10-minute averagevalues based on the per-second data. Z transfer
functions are also calculated in the same manner. Here,the impedance tensor and transfer function are
expressed as follows:E（or Z）= ( Au + i Av )・X + ( Bu + i Bv )・Y.
Fig. 5-4 shows the geomagnetic fields at KAK and
the geoelectric fields at the Mito group in 10-minutemean values from 13:00 to 17:00 on March 8, 1985 (thetop of the figure), and the transfer function and theimpedance tensors for the period from 0.8 to 40 minutes.
These are computed using the values after passing
through a digital band-pass filter of 50 ~ 3000 seconds.

Fig. 5‑4 Transfer functions at KAK and impedance tensors at the Mito group calculated through a band‑pass filter of 50〜 3000 seconds. The top shows the geomagnetic variations at KAK and the geoelectric variations at the Mito groupin 10‑second mean values from 13:00 to 17:00 on March 8, 1985

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Au, Av, Bu and Bv are calculated using each groupof sampling data piled up in a figure. Fig. 5-5 shows Au,
Av, Bu and Bv of the transfer function in piles, and Fig.
5-6(a) ~ (f) show Au, Av, Bu and Bv of the impedance
tensor at the Mito group in piles. ERR-A represents
the error of A, and ERR-B represents that of B. The
periods and sampling times used are shown in theupper parts of each figure.
The transfer functions at KAK Z become
scattered in periods shorter than several minutes andin periods longer than several hundred minutes.
Usually Z components in the short period variations

59

don’t have so much of their origin external to the earth,but Z in the daily variations have considerable external
origin. If there is a linear relation among X, Y and Z,
the error of A and B become small. The same manneris in the geoelectric fields at the Mito group.
For impedance tensors, the errors become verylarge in periods shorter than around 10 minutes.
Especially, in the period just around 10 minutes, thesize of the error is remarkable. It might be dependent
on artificial noises. In all of the impedance tensors, atendency is seen that the errors become smaller withlonger periods.

Fig. 5‑5 Transfer functions using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at KAK in piles.

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Toshio MORI

Fig. 5‑6(a) Impedance tensors using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at ISI‑MTO
in piles.

Fig. 5‑6(b) Impedance tensors using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at ISI‑HIO
in piles.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 5‑6(c) Impedance tensors using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at ISI‑KSM
in piles.

Fig. 5‑6(d) Impedance tensors using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at MTO‑HIO
in piles.

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Toshio MORI

Fig. 5‑6(e) Impedance tensors using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at KSM‑MTO
in piles.

Fig. 5‑6(f) Impedance tensors using the 10‑second (○), the 1‑minute (△) and the 10‑minute (＋) mean values at KSM‑HIO
in piles.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

6. Time changes of impedance tensors andtransfer functions
In order to attain the purpose of detecting abruptchanges of underground conductivity related totectonic activities, it is important to get to know how
much time change is usually there. In this section,
time changes of impedance tensors and transfer
functions are researched using geoelectric fields at the
Mito group and geomagnetic fields at KAK.
Fig. 6-1(a) ~ (e) show the time changes ofimpedance tensors and transfer functions calculated byusing 10-minute average values from March to August,1985. Each mark plotted in the figures represents theaverage value for ten days, and the height of the markis the error (±σ).
Changes are slowly seen in all of the components
of impedance tensors and transfer functions, and these63

may be seasonal changes. The impedance tensors of
360-minute period (T = 360 m) in Fig. 6-1(d) arescattered relative to other periods.
Z transfer functions of 60-minute periods are
fairly stable in time. Functions of 120- to 360-minuteperiods become scattered, and those of 720-minuteperiods become stable. That is, Z in short time
variations and in daily variations is well expressed as alinear function of X and Y, but Z transfer functions in
the intermediate periods are variable.
Usually in the short period variations, Z
components don’t have so much origin external to theearth, but Z has a great amount of external origin in its
daily variations. If there are linear relations among X,
Y and Z, the errors of A and B become small. The
same is found in the geoelectric fields at the Mitogroup.

Fig. 6‑1(a) Time variations of impedance tensors at the Mito group and transfer functions at KAK in a 60‑minute period(T=60 m). Height of the mark represents the mean square error (±σ).

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Toshio MORI

Fig. 6‑1(b) Time variations of impedance tensors at the Mito group and transfer functions at KAK in a 120‑minute period(T=120 m). Height of the mark represents the mean square error (±σ).

Fig. 6‑1(c) Time variations of impedance tensors at the Mito group and transfer functions at KAK in a 180‑minute period(T=180 m). Height of the mark represents the mean square error (±σ).

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 6‑1(d) Time variations of impedance tensors at the Mito group and transfer functions at KAK in a 360‑minute period(T=360 m). Height of the mark represents the mean square error (±σ).

Fig. 6‑1(e) Time variations of impedance tensors at the Mito group and transfer functions at KAK in a 720‑minute period(T=720 m). Height of the mark represents the mean square error (±σ).

65Toshio MORI

66

7. Elimination of the electrical fields induced bygeomagnetic variations- Detection of the anomalous earth potential The author aims to detect the precursors ofearthquakes and volcanic eruptions in real time, andto mitigate from these calamities. On the contrast
described in Section 6, if we assume that theelectrical structures of the aboveground and/or
underground of the earth are constant, the electricalfields induced by geomagnetic variations are noises
for the detection of anomalous earth potential related
to tectonic activities. As the induced electrical fields
are affected by these structures, the method of
calculating the induced fields from the structures isalso considered. However, it is impossible to assume
the structures completely. In this section, the methodof detecting the anomalous changes by using therelation with the observed electrical and magnetic
fields is developed further.
Using a stochastic difference equation, Mori
(1987) and Takayama and Mori (1987) developed themethod for subtracting the effect of magnetic changes
such as magnetic storms and daily variations, whichallows real-time detection of earth potential associatedwith tectonic activities. If we assume that the
responses of the geoelectric changes to thegeomagnetic changes at a certain time do not changewith time, we can detect the anomalous changes fromthe observed geoelectric fields by means of calculating
the induced electrical variations by geomagnetic
variations. Though they analyzed using the data ofgeoelectric variations and the geomagnetic horizontal
two components of hourly mean values for about fivemonths, the author expands the analysis ranges.
7-1 Analysis method by a stochastic differential
equation
We assume that the induced electrical variation
E(t) at a certain time “t” is expressed as a finite linear
combination of past, present and future values of thenorthward component X(t) and the eastward
component Y(t) of the geomagnetic variation. That is,
E(t) is assumed to be expressed with a stochastic
differential equation:
E(t)=

K

∑ {a(m)・X(t−m)＋b(m)・Y(t−m)}＋ε(t)Where a(m) and b(m) are coefficients of the impulsive
response for the geomagnetic variations, that is X(t m) and Y(t - m), respectively. ε(t) is an uncorrelatedpart of the geomagnetic variation. Parameters K and L
are lag numbers, and coefficients a(m), b(m) (m ＝ -L~ K) are calculated so as to minimize the AIC (Akaike
Information Criterion) (Akaike, 1973; Sakamoto et al.,1986) expressed as follows:
AIC=−2 log(maximum log likelihood of the model)＋2(number of free parameters of the model)
Here, the number of parameters estimated from data
should be less than 2n1/2. If the difference of AIC
values is larger than one or two, then the difference
is considered to be significant (Sakamoto et al.,1986).
Henceforth in this paper, AIC will indicate thedifference from minimum AIC.
7-2 For geoelectric fields between Kasama and
Mito
Geoelectric variations at the Mito group havebeen analyzed by making use of static models in orderto remove the induced variations. When we calculate
the AIC values for observed geoelectric fields at
KSM-MTO and geomagnetic X and Y components at
KAK, then E(t) = KSM-MTO in equation (7-1) , andthe result is denoted simply by
KSM-MTO(OBS) = (X, Y).
Here, OBS means observation values.
When using X, Y and Z as associated data, thestochastic differential equation is as follows:
E(t)=

K

∑ {a(m)・X(m)＋b(m)・Y(t−m)＋c(m)・Z(t-m)}

m=− L

(7-2)

＋ε(t)

In this case, the result is represented simply by
KSM-MTO(OBS) = (X, Y, Z).
The analyses using hourly mean values of
KSM-MTO and (X, Y) or (X, Y, Z) at KAK are done in
the following intervals:
A : 1985/ 4/ 1 00:00 〜 5/11 00:00
KSM-MTO(OBS) = (X, Y)
B : 1985/ 4/ 1 00:00 〜 5/11 00:00
KSM-MTO(OBS) = (X, Y, Z)
C : 1985/11/20 00:00 〜 12/30 00:00
KSM-MTO(OBS) = (X, Y)
D : 1985/11/20 00:00 〜 12/30 00:00

m=− L

(7-1)

KSM-MTO(OBS) = (X, Y, Z)

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑1(a) The result analyzed using the stochastic difference equation for KSM‑MTO in the case of A. The upper halfof the figure shows the distribution of AIC values in terms of abscissa K(past) and ordinate L(future). The minimum
value marker is shown by ●. The lower half shows the response function for minimum AIC. The impulsive response in
the time domain, the amplitude and the phase of the response functions in the frequency domain are expressed sequentiallyfrom the top in the lower half of the figure.

67

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Toshio MORI

Fig. 7‑1(b) The result analyzed using the stochastic difference equation for KSM‑MTO in the case of B. The legends are
the same as in figure 7‑1(a).

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑1(c) The result analyzed using the stochastic difference equation for KSM‑MTO in the case of C. The legends are
the same as in figure 7‑1(a).

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Toshio MORI

Fig. 7‑1(d) The result analyzed using the stochastic difference equation for KSM‑MTO in the case of D. The legends are
the same as in figure 7‑1(a).

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

In each case of A, B, C and D, the AIC distributions areshown in the upper half of Fig. 7-1 (a) ~ (d). The
minimum value in each figure is marked by a closed
circle (•), and the contour lines are drawn by thevalues of 2, 5, 10, 20, 50 and 100. We select a set of theparameters, K and L, so as to minimize the AIC value,and determine the coefficients, a(m), b(m) (and c(m)),
corresponding to m = -L ~ K. The lower half of Fig.
7-1(a) ~ (d) shows the response functions for theminimum AIC.
In cases A, B, C and D, (K, -L) = (7, -11), (15, -2),
(17, -19) and (17, -8), respectively, for the minimum
AIC. For example, for model A, the electrical field at a
certain time can be represented using the magnetic
fields in the time range of K hours ago to L hours
afterwards. In case B adding the Z component (Fig.
7-1(b)), the response for X is considerably different
from case A (Fig. 7-1(a)), and the coefficients for large
future values K do not become small. Ideal coefficientsin a stochastic differential equation are larger for m =-1 ~ 1 than for other m’s, and K and L are small values.

71

But for B, the coefficients for Z become rather large
values for K larger than 12. So the response function is
not smooth in this case.
For C (Fig. 7-1(c)), (K, -L) = (17, -19), K and L
are large values for the minimum AIC, and theparameter number p = 75. So it cannot be said that themodel is effective by the reason that p is greater than
2n1/2 = 62, but the coefficients are used here. For D(Fig. 7-1(d)), the model is effective, but thecoefficients for K and L are not small values for thelarger K and L, and the gain and phase of the response
are not as smooth as in case B.
Using the coefficients for A ~ D, the estimated(EST) electrical fields at KSM-MTO are calculated
from the observed magnetic fields at KAK;
furthermore the residual (RES) electrical fields arecalculated by subtracting the estimated values fromthe observed electrical fields (OBS). That is, RES =OBS - EST. The results are shown in Fig. 7-2(a) ~ (d).
In these figures, the coefficients obtained for A ~ D
are also applied for another period.

Fig. 7‑2(a) Observed (OBS), estimated (EST) and residual (RES) electrical fields at KSM‑MTO. Coefficients obtained forthe case of A and B are applied for the period March 10 to June 10, 1985.

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Toshio MORI

Fig. 7‑2(b) Observed (OBS), estimated (EST) and residual (RES) electrical fields at KSM‑MTO. Coefficients obtained forthe case of A and B are applied for the period November 1, 1985, to February 1, 1986.

Fig. 7‑2(c) Observed (OBS), estimated (EST) and residual (RES) electrical fields at KSM‑MTO. Coefficients obtained forthe case of C and D are applied for the period March 10 to June 10, 1985.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

73

Fig. 7‑2(d) Observed (OBS), estimated (EST) and residual (RES) electrical fields at KSM‑MTO. Coefficients obtained forthe case of C and D are applied for the period November 1, 1985, to February 1, 1986.

The results read from Fig. 7-2 (a) ~ (d) are madeitemized statements.
(1) The residual fields for the period of A and B(Fig. 7-2(a)) are almost the same whether Z is
considered or not, and the electrical fields induced bygeomagnetic fields such as daily variations andmagnetic storms are considerably separated from theoriginal data. That is, the predominant induced
components in the observed geoelectric variations arealmost completely separated for KSM-MTO.
(2) When the coefficients resulted for A and B are
applied for almost the same period from November,
1985, to January, 1986 (Fig. 7-2(b)), the electrical fields
induced by short period magnetic variations are
considerably separated, but those induced by daily
variations are not so.
(3) When the coefficients resulting from C and D
are applied for almost the same period from November,
1985, to January, 1986 (Fig. 7-2(d)), the result for D, inwhich Z is considered better than C, and the daily
variations are considerably eliminated.
(4) When the coefficients resulting from C and D
are applied for different periods from March to June,

1986 (Fig. 7-2(c)), it looks like that the daily variations
in RES are greater than those in OBS.
(5) The results of (4) may be reflected that thedetermined minimum AIC is not significant because ofa large K+L. Otherwise, it is considered that the data
used for the period was not good, or that it wasaffected by the seasonal effect.
(6) The effect using Z as associated data appears
well for the period including models C and D, but theeffect does not appear for the period including models
A and B.
As shown in Section 5, Z is experientiallyexpressed as a linear function of X and Y for shortperiod geomagnetic variations in the frequency domain.
As shown in Fig. 6-1(a) of Section 6, Z transfer
functions in the 60-minute period are fairly stable intime. Those in the 120- ~ 360-minute range become
scattered, and those of 720-minute periods become
stable. That is, Z in short time variations and in daily
variations is well expressed as a linear function of X
and Y, but the Z transfer function in the intermediate
periods is variable. Adaptation to such a statisticalmodel used in this section seems to induce a goodToshio MORI

74

result when X, Y and Z are mutually independent. In
order that Z not be independent of X and/or Y, theeffect of Z on electrical variations is larger thannecessary, so the responses may become unnatural. So
in the case using Z as associated data, the result is not
so effective. It may be effective to remove the effect ofZ from the residual variation using X and Y. It was notpossible to make it progress there this time, so it
remains a subject for future study.

Table 7‑1 The minimum AIC (K, ‑L) in the periods of A
(1985/4/1 ‑ 5/11), B (1985/11/20 ‑ 12/30) and C(1986/4/15 ‑ 5/15)

A

B

C

KSM-MTO( 7, -11)(17, -19)(19, -2)

ISI-MTO(12, -11)( 7, -16)(13, -11)

ISI-HIO( 4, -2)( 9, -1)( 6, -6)

ISI-KSM( 7, -1)(14, -1)(19, -1)

MTO-HIO(16, -1)( 8, -15)(17, -5)

KSM-HIO( 7, -5)( 5, -1)(19, -6)

Z(12, -6)( 8, -20)(20, -8)

7-3 Application to the data of electrical fields at
the Mito group and Z at Kakioka
The coefficients so as to minimize AIC forgeoelectric fields at all lines of the Mito group and the
Z component of the geomagnetic field at KAK are
calculated in the period around on May, 1986, inaddition to the periods of A and B in the previous
section.
On Z, a stochastic differential equation follows,
the same as on E(t) of equation (7-1).
Z(t)=

K

∑ {a(m)・X(t−m)＋b(m)・Y(t−m)}＋ε(t)m=− LHere, the analyzing intervals forparameters are rewritten as follows:
A：1985/ 4/ 1 00:00 〜 5/11 00:00(7-3)
calculating

N = 961 (40 days)
B：1985/11/20 00:00 〜12/30 00:00
N = 961 (40 days)
C：1986/ 4/15 00:00 〜 5/15 00:00
N = 721 (30 days)
The AIC values for K and L are calculated in the range
K (and L) = 0 ~ 20.
The case of C at KSM-MTO is shown in Fig. 7-3.
AIC takes on a minimum value when (K, -L) = (19, -2),and the response function curves for Y become
considerably oscillating in the short periods. Fig. 7-4 ~7-8 shows typical examples in various combinations.
The parameters (K, -L) for an adopted minimum AIC
in each case are tabulated in Table 7-1. In the cases of
ISI-MTO and MTO-HIO, K+L do not become smallvalues. For ISI-HIO, as shown in Table 7-1, in all
periods, values of K+L become small. For ISI-KSM(Fig. 7-6), values of K+L are small in A and B, andthese responses are similar, but in C, K = 19, and the
response is not smooth. Similar tendencies are seenfor KSM-HIO and ISI-KSM.

Fig. 7‑3 AIC distribution at KSM‑MTO in the interval C andresponse function in the case of minimum AIC.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

75

Fig. 7‑4 AIC distribution at ISI‑MTO in the interval A andresponse function in the case of minimum AIC.

Fig. 7‑5 AIC distribution at ISI‑HIO in the interval A andresponse function in the case of minimum AIC.

The results on Z are shown in Fig. 7-9(a), (b), (b’)
and (c). In the cases of B (Fig. 7-9(b)) and C (Fig.
7-9(c)), the values of K+L in minimum AIC are large,
that is, (K, L) = (8, -20) and (20, -9). However, AIC
indicates a local minimum in (K, -L) = (8, -8) in both B
and C. So (K, -L) = (8, -8) is adopted, and the
responses are calculated. The bottom of Fig. 7-9(b')
and Fig. 7-9(c) shows the responses.
The estimated values (EST) and residual value(RES = OBS - EST) at all the components at the Mitogroup and Z at KAK are calculated using the parameterin Table 7-1 and the geomagnetic X and Y variations at
KAK. The results are shown in Fig. 7-10(a), (b) and (c).

In each figure, RES-A, RES-B and RES-C represent
the residual values for the parameters for the A, B andC periods, respectively.
RES-A and RES-C in Fig. 7-10(a) are almost thesame, and daily and short period variations are
considerably rejected. But in RES-B, daily variations
are almost left behind, as they are still remaining. In
the period B (Fig. 7-10(b)), the amplitudes in RES-B
are smaller than those in RES-A and RES-C. The
period C (Fig. 7-10(c)) is the same season one year
after the period A, and the results have almost thesame tendency to A. So it is said that the parametersat a certain season are usable in the season of another76

Toshio MORI

Fig. 7‑6 AIC distribution at ISI‑KSM in the interval B andresponse function in the case of minimum AIC.

Fig. 7‑7 AIC distribution at MTO‑HIO in the interval C andresponse function in the case of minimum AIC.

year. Although the electromagnetic state of the earth
does not understand how it is changing with seasons,it could be said that the same coefficient can be used at
the same season. Although it asked for the relation
between Z, and X and Y in order to hold the key whichasks for how much influence it has comparatively fromthe earth’s exterior, and from the inside of the earth, atthe present stage, it is unknown.
The change of response of the earth’s current togeomagnetic change is not necessarily based only onchanges of the internal structure of the earth. The
influence of the earth’s exterior is also considered.
The key of at what rate the influence of an earth exterior

and underground structure is included is unknown at
the present stage. It becomes a research subject
whether one can ask for such a coefficient, since it isdesirable to use the same coefficient through one year.
On the contrary, studying seasonal changes of theelectromagnetic state of the earth is also consideredfrom the coefficient from which a season thereforechanges.
7-4 Spectral analysis
Amplitude spectrums of the observed electricaland magnetic fields (OBS) and the residual electricalfields using the coefficients calculated with model A, B

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑8 AIC distribution at KSM‑HIO in the interval B andresponse function in the case of minimum AIC.

and C in Section 7-3 (RES-A, RES-B and RES-C) arecalculated. The period of the data used for analysis is
as follows:
1: 1985/10/1 00:00 ~ 1986/10/ 5 00:00
T = 1hN = 8857 (369 days)
2: 1985/10/1 00:00 ~ 1986/4/26 00:00
T = 1hN = 4969 (207 days)
3: 1986/4/1 00:00 ~ 1986/10/25 00:00
T = 1hN = 4969 (207 days)
Here T is a data interval, and N is the number of data.
The following tidal constituents will be referred
to in this paper.

77

Fig. 7‑9(a) AIC distribution at KAK Z in the interval A andresponse function in the case of minimum AIC.

2Q
Q1
O1
M1
P1
S1
K1

28.0062 h
26.8684 h
25.8193 h
24.8332 h
24.0659 h
24.0000 h
23.9345 h

J1
N2
ν2
M2

23.0985 h
12.6583 h
12.6260 h
12.4206 h

Major lunar elliptic tide
Principal lunar diurnal tide
Minor lunar elliptic tide
Principal solar diurnal tide
Meteorological diurnal tide
Luni-solar declinational diurnal
tide
Small lunar elliptic tide
Major lunar elliptic tide
Major lunar evectional tide
Principal lunar semidiurnal tide

78

Toshio MORI

Fig. 7‑9(b) AIC distribution at KAK Z in the interval B andresponse function in the case of minimum AIC.

Fig. 7‑9(b') AIC distribution at KAK Z in the interval B andin the range (K, L)=(10, ‑10) and response function in
the case of minimum AIC.

S2 12.0000 h Principal solar semidiurnal tide
R2 11.9836 h Minor solar elliptic tide
K2 11.9672 h Luni-solar declinational semidiurnal
tide
Fig. 7-11(a) ~ (c) represent the amplitude spectra
for the various components in the range of 3 ~ 1000
hours for the period “1,” and they are calculated bymeans of the least square method. In all of thesefigures, conspicuous constituents of 10 or fewer hours
are the higher harmonics of the diurnal variation (Sn (n= 3 ~ 6)). The amplitude spectra of 10 ~ 30 hours areshown in Fig. 7-12(a) ~ (h) rearranged for every
ingredient. In these figures, “1,” “2” and “3” on theupper part denote the period, “O” on the left side denotes

the amplitude spectrum of the observed field, and “A,”“B” and “C” on the left side denote those of theresidual fields in the cases of model A, B and C
respectively. Predominant peaks for the tidal
harmonics are noted in these figures.
Fig. 7-13 shows the amplitude spectra of X, Y and
Z at KAK, tide at Ooarai (OOARAI TIDE) andatmospheric pressure at Youkaichiba (YOUKA PRESS),
using hourly mean values from September 8, 1985, toAugust 23, 1986. The spectra of X, Y, Z and PRESS
show large peaks for the tidal constituents of the solar
diurnal and its higher harmonics (S1, S2, S3, S4, S5 andS6). For TIDE, the tidal constituents of O1, K1, M2, S2,
Q1, ν2 and R2 are predominant.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑9(c) AIC distribution at KAK Z in the interval C and response function in the case of minimum AIC (top). The bottom
right shows the AIC distribution in the range (K, L)=(15, ‑15)

79

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Toshio MORI

Fig. 7‑10(a) Observed geomagnetic variations at KAK and geoelectric variations at the Mito group, and the residual
variations from April 1 to May 11, 1985. OBS represents the observed variation, and RES‑A, RES‑B and RES‑C represent
the residual variations in the case of the parameters in the case of A, B and C periods, respectively.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑10(b) Observed geomagnetic variations at KAK and geoelectric variations at the Mito group, and the residual
variations from November 20 to December 30, 1985. The legends are the same as in figure 7‑10(a).

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Toshio MORI

Fig. 7‑10(c) Observed geomagnetic variations at KAK and geoelectric variations at the Mito group, and the residual
variations from April 15 to May 15, 1986. The legends are the same as in figure 7‑10(a).

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑11(a) Amplitude spectra for the observed (OBS) and the residual (RES‑A, RES‑B, RES‑C) fields of X, Y and Z at KAK
in the range of 3‑1000 hours for the period “1” (from October 1, 1985, to October 5, 1986).

Fig.

7‑11(b) Amplitude spectra for the observed (OBS) and the residual (RES‑A) fields of the geoelectric variations at
the Mito group in the range of 3‑1000 hours for the period “1” (from October 1, 1985, to October 5, 1986).

83

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Toshio MORI

Fig. 7‑11(c) Amplitude spectra for the residual (RES‑B, RES‑C) fields of the geoelectric variations at the Mito groupin the range of 3‑1000 hours for the period “1” (from October 1, 1985, to October 5, 1986).

Fig. 7‑12(a) Amplitude spectra for the observed (OBS) fields of X and Y at KAK in the range of 10‑30 hours for the period“1,” “2” and “3.”

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑12(b) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of Z at KAK in the range of 10‑30
hours for the period “1,” “2” and “3.”Fig. 7‑12(c) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of the geoelectric variations at
ISI‑MTO in the range of 10‑30 hours for the period “1,” “2” and “3.”

85

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Toshio MORI

Fig. 7‑12(d) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of the geoelectric variations at
ISI‑HIO in the range of 10‑30 hours for the period “1,” “2” and “3.”Fig. 7‑12(e) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of the geoelectric variations at
ISI‑KSM in the range of 10‑30 hours for the period “1,” “2” and “3.”

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑12(f) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of the geoelectric variations at
MTO‑HIO in the range of 10‑30 hours for the period “1,” “2” and “3.”Fig. 7‑12(g) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of the geoelectric variations at
KSM‑MTO in the range of 10‑30 hours for the period “1,” “2” and “3.”

87

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Toshio MORI

Fig. 7‑12(h) Amplitude spectra for the observed “O” and residual “A,” “B” and “C” fields of the geoelectric variations at
KSM‑HIO in the range of 10‑30 hours for the period “1,” “2” and “3.”Fig. 7‑13 Amplitude spectra of X, Y and Z at KAK, tide at Ooarai (OOARAI TIDE) and atmospheric pressure at Youkaichiba(YOUKA PRESS) using hourly mean values from September 8, 1985, to August 23, 1986.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7-14 shows the amplitude spectra of SEF(electrical field on the sea floor off Tokai), PRESS (sealevel off Tokai), TIDE (coastal sea level at Omaezaki),
H (geomagnetic horizontal component at KAK), Z(geomagnetic vertical component at KAK), D(geomagnetic declination at KAK) and F (geomagnetic
total force at KAK) from January 1 to December 31,
1980 (Mori, 1987). The amplitude spectra of the SEF
show large peaks for the tidal constituents of O1, P1, S1,
K1, N2, M2, S2, R2 and K2.

Fig. 7‑14 Amplitude spectra of SEF (electrical field on thesea floor off Tokai), PRESS (sea level off Tokai), TIDE
(coastal sea level at Omaezaki), H (geomagnetic
horizontal component at KAK), Z (geomagnetic verticalcomponent at KAK), D (geomagnetic declination at KAK)
and F (geomagnetic total force at KAK) from January 1
to December 31, 1980, after Mori (1987).

89

Fig. 7-12(a) ~ (h) lead to the following results:(a) As shown in Fig. 7-12(a), X and Y are
dominated by the diurnal and semidiurnal harmonics(S1 and S2), and Y has the weak peak of M2.
(b) As shown in Fig. 7-12(b), S1, S2 areconspicuous at the observed field of Z, and M2 isconspicuous for a while only in period “3.” S1 and S2 ofthe residual fields of Z are almost removed in somecases, and not in some cases. The amplitudes of M2 arealmost the same in all of the cases. It cannot be
understood that the K1 constituent that differs slightly
from S1 is conspicuous.
(c) As shown in Fig. 7-12(c), S1 remains without
decreasing at all during the periods in the cases of B at
ISI-MTO. In the cases of A and C at period “3,” theamplitude of S1 and S2 decrease, and the daily
variations are almost removed. M2 is conspicuous in allof the cases, and 2Q（28.1h）are conspicuous during theperiods “1” and “3.”(d) As shown in Fig. 7-12(d), S1, S2, O1 and M2 areconspicuous for “O” at ISI-HIO at the periods “1” and“3.” In the residual fields, the diurnal variations suchas S1 and S2 are fairly well removed. So, O1 and M2
which are seldom contained in geomagnetic changes
become conspicuous, and also K1 becomesconspicuous.
(e) The spectra of ISI-KSM (Fig. 7-12(e)) arealmost the same as those of ISI-HIO.
(f) For MTO-HIO in Fig. 7-12(f), S1 and S2 are notremoved in all of the cases.
(g) For KSM-MTO in Fig. 7-12(g), the removal of
S1 and S2 does not work the same as MTO-HIO.
(h) For KSM-HIO in Fig. 7-12(h), removal of S1and S2 work except in the case of B.
It can be read in the residual electrical fields thatthe changes correspond to the constituents which
geomagnetic changes have removed in many cases. It
is one of the subjects that the constituents such as M2,
O1, Q1 and 2Q, etc., seen in the residual electricalfields, depend on the earth tide or the ocean tide.
There are large amplitudes of the 3.5-4 days’
periodic changes in the observed electrical fields at
ISI-MTO and HIO-MTO. These periodic changes arealso seen in Fig. 4-1; furthermore at KSM-MTO, 7-10
days’ periodic changes are seen. Those excellence
cycles are not seen in the geomagnetic fields at KAK,
the electrical field on the sea floor off Tokai and thetidal change at Omaezaki (Fig. 7-4(a)). It is also a

90Toshio MORI

subject of future research to pursue what thesechanges mean.
The constituents of S2, S1, M2, O1, Q1 and 2Q are
calculated using the hourly mean values ofgeomagnetic horizontal components at KAK andgeoelectric fields at the Mito group from 00:00 on
March 8 to 17:00 on July 2, 1985. Fig. 7-15 shows theorbits of these constituents. The number of data used
for calculation of each constituent is the multiple
numbers of the constituent period. That is, thenumbers for S2, S1, M2, O1, Q1 and 2Q are 2641, 2641,
2485, 2583, 2688 and 2802. The top of the figure
shows the orbit of the constituent of the observed
geomagnetic variations. From the 2nd step to thebottom of the figure shows the orbits of theconstituent of the geoelectric changes in the various
three combinations among base lines at the Mito group.
OBS in the figure means the observed field, and RES

is the residual field using the model A in Section 7-2.
The height and width of the cross in the figure show
the scale values, ones at S2 and S1 are 4nT in thegeomagnetic fields, and those at other constituents are
2nT in the geomagnetic fields and 2 mV/km in thegeoelectric fields.
The amplitudes and the predominant directions of
the electrical field among ISI, MTO and KSM(ISI-MTO-KSM) and among HIO-MTO-KSM bring
generally the same results. These predominantdirections are almost the same as those in the shortperiod variations of geoelectric fields resulted by
Takayama and Mori (1987). It can be read that S2 andS1 in the residual field are quite small as comparedwith those in the observed field.
The predominant direction of geoelectric variations
are largely affected by underground conductivity
structure, as will be described in Section 11.

Fig. 7‑15 Polar diagrams of S2, S1, M2, O1, Q1 and 2Q constituents in geomagnetic fields at KAK and geoelectric fields at
the Mito group. The data from 00:00 on March 8 to 17:00 on July 2, 1985, are used.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

7-5 When using per-minute data
Unusual electromagnetic phenomenon reported
to be the sign of an earthquake are found throughout
the cycle and continuation time. Although the analysis
used hourly values in Section 7-2 to 7-4, if it ispossible to analyze using 1-minute values, theabnormalities of a wide cycle are detectable. Therefore,
analyses using values per minute are tried here.
Fig. 7-16 shows geoelectric variations at the Mitogroup and geomagnetic variations at KAK by averagevalues for 1 minute from 05:00 on February 20 to
07:00 on February 21, 1987. Using these data, AIC
values are calculated for KSM-MTO(OBS) ＝ (X, Y)
in the range (K, -L) = (20, -15). The distributions of
the AIC are shown in the top of Fig. 7-19. However,the value which shows the minimum AIC cannot befound in the range. The cause is considered to be thatthe geoelectric responses for geomagnetic variations
do not become small if the cycle does not become
lengthened to 24 hours or more, that is, the responseof a long cycle cannot be expressed with a per-minute91

value. Then, the data are passed through a high-passfilter with the cut-off period of 180 minutes, as shownin Fig. 7-17. Fig. 7-18 shows the geoelectric andgeomagnetic variations which let the filter of Fig. 17
pass to the data Fig. 7-16. The filter was designed
referring to Ashida and Saito (1970).
The AIC distribution is calculated about the casewhich lets the filter pass to the observation value at
ISI-MTO, and shown in the bottom of Fig. 7-19. Fig.
7-20 shows the response in this case. The minimum
value of AIC is set to K = 19, but the value of K+L
does not become small.
It may be necessary to shorten the filter cycle toa slight degree. Or, a better result with it may beobtainable to let a filter pass at only one of the two
observation values. The reason for that is considered
to be that it is better to have lost the coherency
between the two fields in a longer period than acertain period. It is also considered that it is better toseek a relationship so that both should be like that andto seek for a coefficient.

Fig. 7‑16 Geoelectric fields at the Mito group and geomagnetic fields at KAK by average value for 1 minute from 05:00
on February 20 to 07:00 on February 21, 1987.

Toshio MORI

92

Fig. 7‑17

Characteristics of high‑pass filter.

Fig. 7‑18 The geoelectric and geomagnetic variations which let the filter pass shown in Fig. 7‑17 to the data in Fig. 7‑16.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑19 The distribution of AIC by the per‑minute value data of ISI‑MTO from 05:00 on February 20 to 07:00 on February
21, 1987. The top shows the case which does not let a filter pass, and the bottom the case which lets the filter pass.

93

Toshio MORI

94

Fig. 7‑20

The response function on (K, ‑L)＝(19, ‑1) in the bottom of Fig. 7‑19.

Fig. 7‑21 The top shows the impedance tensors at SMO‑KSM in the range from 6 to 200 minutes which read and plotted theaverage curve from Fig. 5‑2. In the bottom, the H and D components of the geomagnetic fields at KAK, the observed
geoelectric field at SMO‑KSM (KASAMA) and the residual geoelectric field which deducted the calculated value fromthe observed one are expressed sequentially from the top.

7-6 Use of the transfer function and impedance
tensorMethods of removing change induced bygeomagnetic variations from observed geoelectric andgeomagnetic variations are tried using the impedance
tensor and transfer function. It may be difficult to
detect an unusual phenomenon in real time in thiscase.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

As an example, the case of geoelectric fields at
SMO-KSM (KASAMA) is shown in Fig. 7-21. The
impedance tensors which read and plotted the average
curve from Fig. 5-2 are shown in the upper part of Fig.
7-21. Fourier transforms of the H and D components ofgeomagnetic fields at KAK and of geoelectric field at
SMO-KSM are carried out, and the impedance tensors
read from curves are applied to each cycle, and theresults of the induced electrical fields are obtained by
Fourier reverse conversion. Here, impedance tensors
in the range from 6 to 200 minutes are used. In Fig.
7-21, the 3rd curve expresses the observed
geoelectric field, and the 4th curve the residual field

95

which deducted the calculation value from theobservation value.
Fig. 7-22 shows the transfer function at KAK and
impedance tensors at the Mito group which read andplotted the average curve from Fig. 5-5 and Fig. 5-6(a)(f). Fig. 7-23 shows the geomagnetic variations at KAK
and the geoelectric variations at the Mito group from
00:00 on May 18 to 00:00 on May 23, 1985, which are
passed through a low-pass filter of 50 minutes, and theresidual fields are shown in Fig. 7-24. Here, theimpedance tensors in the range from 20 to 200
minutes are used. It can read that the daily variations
except Z are considerably removed from the figure.

Fig. 7‑22 Transfer function at KAK and impedance tensors at the Mito group which are read and plotted the average curves
from Fig. 5‑5 and Fig. 5‑6(a)‑(f).

96Toshio MORI

Fig. 7‑23 Geomagnetic variations at KAK and geoelectric variations at the Mito group which are passed through a low‑passfilter of 50 minutes to the 10‑minute data from 00:00 on May 18 to 00:00 on May 23, 1985.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 7‑24 The residual Z at KAK and the electrical fields at the Mito group which removed induced change from the variationsof Fig. 7‑23, here, the transfer function and the impedance tensors in the range from 20 to 200 minutes are used.
The two curves of the upper part of the figure show the geomagnetic X and Y variations, which removed the change ofthe cycle of 20 to 2000 minutes.

97

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Toshio MORI

8. Relation to tectonic activities
One of the great characteristics of the geoelectricobservations using NTT facilities is long time stability
as described in Section 4. Daily means of geoelectricand geomagnetic variations from March 1985 to July
1987 are shown in Fig. 8-1. Vertical and horizontal bars
with Arabic numbers indicate the times when the mainearthquakes and earthquake swarms took place near

the observation networks. Their locations and lists areshown in Fig. 8-2. The horizontal bar in November
1986 indicates the volcanic eruption of the Izu-Oshima
volcano. Some anomalous changes in the geoelectricfields at the Numazu group which appear to relate totectonic activities were recognized, while there is no
anomalous change in the Mito group.

Fig. 8‑1 Daily values of geoelectric and geomagnetic variations from March, 1985, to July, 1987. Vertical and horizontal
bars with Arabic numerals indicate the times when main earthquakes and earthquake swarms took place near the observation
networks, respectively. The locations and the lists of earthquakes and earthquake swarms are shown in Fig. 8‑2. The
horizontal bar on November, 1986, indicates the period of volcanic eruption of the Izu‑Oshima volcano.
Fig. 8‑2 List of earthquakes and earthquake swarms near theobservation networks. The number 3, 7 and 11 areearthquake swarms.
No.
Date
Magnitude
Depth (km)
1
2
3
4
5
6
7
8
9
10
11

1985/8/12
1985/10/4
1985/10/13 ~ 11/10
1986/2/12
1986/6/24
1986/7/9
1986/10/10 ~ 10/29
1987/2/6
1987/4/7
1987/4/23
1987/5/6 ~ 6/4

6.4
6.1

52
78

6.1
6.5
4.1

44
73
15

6.7
6.6
6.5

35
44
47

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

8-1 Relation to the earthquake swarms off theeast coast of the Izu Peninsula
An earthquake swarm, which is indicated bynumber 3 in Fig. 8-1 and Fig. 8-2, occurred off the east
coast of the Izu Peninsula from October 13 toNovember 10, 1985. The largest earthquake withmagnitude of 3.8 occurred on October 29. As shown in
Fig. 8-1, the electrical field at ATM-ODW decreasedaround October 15, 1985, for example. It is estimatedthat the geoelectric potentials at ATM, ITO and SUZ
decreased by 300, 100 and 100 mV in the middle of
October, respectively. As shown in Fig. 8-3, thevolume strain meter at Higashi-Izu (HIGASHI), which
distant is 15 km southward from Ito, shows a gradualcompression during the same period. Changes of thevolume strain meters seen in June to July in Fig. 8-3
are the influence of rain. The geoelectric potential andthe volume strain are independent of each other, andonly the geoelectric potential of these NTT
observation stations near the epicenters and thevolume strain meter at Higashi-Izu began to change just99

at the beginning of the earthquake swarm. If we
presume from these facts, the anomalous changes ingeoelectric potential seem to relate to the earthquake
swarm. Fig. 8-4 shows hourly values of geoelectric andgeomagnetic variations around October 1985. Only thegeoelectric field at ATM-ODW in Fig. 8-4 shows aclear change, but others are not so clear. Also, it is notconsidered that patterns of the daily variations arechanged.
In the past ten years, earthquake swarms haverepeatedly occurred off the east coast of the Izu Peninsula,and earthquake swarms occurred in October 1986 and in
May 1987 (Japan Meteorological Agency, 1987). Whenthe earthquake swarms which are indicated by number 7
and 11 in Fig. 8-1 and Fig. 8-2 occurred, the volume
strain meter at Higashi-Izu showed gradual compressions
at the same time, but there were no anomalous changes
in the geoelectric variations at either time. This ispossibly due to the fact that the magnitude of theearthquake swarm was smaller and that the location was
somewhat different from the previous swarm.

Fig. 8‑3 Top: Volume strain meter (●) by JMA and geoelectric observation stations (□) of NTT. Bottom: Variations ofvolume strain meters during March, 1985, to January, 1986.

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Toshio MORI

Fig. 8‑4 Electrical variations in hourly mean values at NTT stations and electrical and magnetic variations at Kakioka
around October 1985.

8-2 Relation to eruptions of Izu-Oshima volcano
The Izu-Oshima volcano violently erupted on
November 15, 1986. There are abnormal changes inthe electrical variations at NMZ-ODW, ITO-ODW,
ATM-ODW, and SUZ-ODW at the beginning of
October 1986 in Fig. 8-1. Fig. 8-5 shows thegeoelectric variations at the Numazu group from
September to November, 1986, as an elongated part of
Fig. 8-1. The electrical potential at ODW began todecrease on November 6, and reached the minimum
value of -800 mV around November 9. At other pointsof the Numazu group, however, the electrical potential
at ATM could be seen to increase a little bit, but not soclearly. Since the geoelectric data of the Numazu groupfrom November 29, 1986, to January 10, 1987, aremissing because of trouble with the recording system,
it is not clear how the geoelectric potentials at ODW
recovered. Although the power supply was shut off inGeoelectric Observation used by the Telegraphic Facilities of NTT Corporation

order to move the position of the recorder in NTT
Numazu station late in December, since it reset whena wall socket was put in, it is thought that it began towrite double records.
Hourly mean values of the electrical fields at the
Numazu group are shown in Fig. 8-6 in October andNovember, 1986. The amplitudes of the daily
variations are large, and these seem to be due almost
completely to artificial noises. The patterns of thedaily variations and the mean value of the electricalpotential at ODW had been changed since November 6
00:00 or November 5 22:00, 1986. Abnormal changes
on November 4 are electrical changes induced by ageomagnetic storm that occurred with ssc (sudden
storm commencement) at 08:54 on November 4.

101

ODW is located at the extension of the Suruga
Trough, and on the boundary of the Eurasia Plate and
the Philippine Sea Plate. The flow of undergroundwater, which is caused by a change of stress acting on
the boundary, may possibly cause changes in thestreaming potential and/or electrical conductivity.
However, there is a possibility of a fortuitous change in
the geoelectric potential at ODW.
The electrical potential at ODW relative to NMZ
input in the fourth channel of the digital recorder as
described in Section 2. It cannot be somehow that thecircuit of only the fourth channels breaks down on the
1 time target, but there is a very little possibility,
judging from order of the records.

Fig. 8‑5 Electrical variations in daily means at the Numazu group from September to November, 1986.

Toshio MORI

102

Fig. 8‑6

Electrical variations in hourly means at the Numazu group during October to November, 1986.

8-3 Other changes(1) Fujinomiya group
The electrical field at FJI-FJM (second line from the
top of Fig. 4-1 and Fig. 8-1) became low from early in
July to the middle of August. The NTT undergroundcable from FJM to KOF via MNB was inundated byheavy rain by the typhoon on June 30, 1985; after thatobservations were stopped. Continuous observation in
the electrical resistivity of the Fujikawa fault showed
the decreasing of the underground electrical resistivity
generally at the same time (Earthquake ResearchInstitute, University of Tokyo, 1987). So the decreasingof the electrical field at FJI-FJM may be reflecting thechanges of the underground electrical conditions in awide area.
(2) Numazu group
There is a bay-shaped change of -500 mV in theelectrical potential graph at NMZ from the end of
December 1985 to the beginning of January 1986 in
Fig. 4-1 and Fig. 8-1. Also, there is the change of a
volume strain meter at Fuji near NMZ during almost
the same period in Fig. 8-3. Although the similarchanges of the volume strain meter at Fuji changed
several times in the past, it seems that the changesof electrical field and volume strain meter
overlapped by chance.
The electrical fields at the Numazu group appear
considerably changeable during March, 1986, andthese seem to relate to oceanic tide. This will be
described in the next chapter.
9. Relation to tide, atmospheric pressure andothersAs mentioned in the introduction, it is consideredthat geoelectric variations are relative to various
phenomena such as geomagnetic variations, motion of
the sea water, earth tide, conditions of the earth’s
interior, atmospheric condition and so on.
Fig. 9-1 gives the daily mean values of theelectrical field on the sea floor off Tokai (SEF), sealevel by tsunami-meter off Tokai, coastal sea levels,
atmospheric pressures and temperatures. The longtime variations of SEF are affected by the pass of
Kuroshio, which is a strong ocean current off thesouthwestern coast of Japan. The pass is highly
changeable off Tokai district, and also relative to
geomagnetic field changes at the Tokai area (Mori,
1987).

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103

Fig. 9‑1 Daily mean values of the electrical field on the sea floor off Tokai, sea levels, atmospheric pressures andtemperatures during March, 1985, to August, 1986. From top to bottom, electrical field on the sea floor off Tokai(SEF : TOKAI), sea level by tsunami‑meter off Tokai (TSUNAMI : PRESS), coastal sea level at Omaezaki (TIDE : OMAEZAKI),
coastal sea level at Uchiura (TIDE : UCHIURA), coastal sea level at Ito (TIDE : ITO), coastal sea level at Ooarai(TIDE : OOARAI), atmospheric pressure at Youkaichiba (PRESS : YOUKAIC), atmospheric pressure at Mito (PRESS : MITO),
temperature at Mito (TEMP : MITO), atmospheric pressure at Mishima (PERSS : MISHIMA) and temperature at Mishima (TEMP :MISHIMA).

As seen in Fig. 8-1 and Fig. 9-1, there areremarkable long period changes of the geoelectricfields at the Numazu group and sea levels and also
atmospheric pressures around March 1986. In thissection, coherencies between these various data arestudied.
Fig. 9-2 ~ 9-4 show the coherencies, which are
the ratio of the correlated power to total power,
between various pairs. The coherency between
components and the data periods are shown in theupper part of each figure, as for example, “KAK X :KSM-MTO(OBS)” and “1985 4M 1D 0H.”
Coherencies are calculated for the first-stage trial, anddeep consideration has not been carried out. What we
can say at present is in the following memorandums.
(1) Coherency of KSM-MTO for X is high with acycle shorter than 3 or 4 days ((a) of Fig. 9-2), and one
for Y is high with a cycle shorter than one day ((b) ofFig. 9-2). It may be depend on how small the amplitude
is of variations of Y with a cycle longer than one day.
Coherencies for X are low at the period of O1 (25.82h)
and M2 (12.42h) constituents. The cause is that theinduced electrical fields by geomagnetic variations
have small powers at the period of O1 and M2.

(2) Coherency of sea level (TIDE) for
atmospheric pressure (PRESS) is very high with acycle longer than 3 and /or 4 days ((c) of Fig. 9-3).
(3) The coherency of KSM-MTO for TIDE at
Ooarai is high at the cycle of M2 and around 24.5h ((c)
of Fig. 9-2). The cycle of 24.5h may be the mean of theK1(23.93h) and O1(25.82h) constituents.
(4) The coherency of KSM-MTO for PRESS at
Youkaichiba is small, and there is no particular
correlation with the long cycle ((d) of Fig. 9-2). It issaid that the periodic variations of 3 to 10 days in thegeoelectric fields at the Mito group have no relation to
the atmospheric pressure and ocean tide.
(5) The coherency of SUZ-ITO for TIDE at Ooarai
is also high with the cycle of 5.5 days ((b) of Fig. 9-3).
(6) The coherency of ODW-NMZ for TIDE at
Uchiura is high with a long cycle ((b) of Fig. 9-4). Boththe electrical potential at the Numazu group of Fig. 8-1
and the tidal level of Fig. 9-1 were changed sharply inMarch, 1986. It is thought that the coherency isreflecting this.

104

Fig. 9‑2

Toshio MORI

Coherencies of KSM‑MTO(OBS) for KAK X (a), KAK Y (b), TIDE at Ooarai (c) and PRESS at Youkaichiba (d).

Fig. 9‑3 Coherencies of SUZ‑ITO(OBS) for ODW‑NMZ(OBS) (a) and TIDE at Ooarai (b), and of TIDE at Ooarai for PRESS at
Youkaichiba (c).

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 9‑4105

Coherencies of TIDE at Uchiura for ODW‑NMZ in two duration times.

10. Application for research of fault structure
On the research of earth resistivity by the direct
current method, electrical currents of several toseveral tens amperes flow in the earth. The relation
between the waveform of the current flowing in theearth and the waveform of the voltage which isreceived can be sought for information on the
structure. The waveform can be distorted if there is a
discontinuous layer. The presence of such a thing is
confirmed in the active faults, such as the Senya fault(Onoand
Uchida,1982),
Yamasakifault(Electromagnetic Research Group for the Active Fault,
1982) and Kushibiki fault (Mori, et. al., 1983).
In this section, the application to fault researchusing artificial noise is tried instead of passing known
current to the underground. Moreover, there is also afollowing purpose in exploring the possibility ofdetection of change of the electrical character relevant
to tectonic activity. Honkura (1976) has calculated how
much the amplitude and directions change, when theelectrical conductivity changes. Even if the electricalnoise by the direct-current train, etc., changes in time,
it is possible that the response between points is fixed.
It is also one of the subjects how much a responsechanges in time.
As shown in the records of the observed electricalfields at the Fujinomiya and Numazu group, the great
portion of the observed electrical variations is an
artificial noise in industrial regions and the places
where a direct-current train runs. Since a
direct-current train passes several 100 A current in
the earth, we consider the current to be dispatchcurrent and try fault research. It is thought that noises
are observed in the wide area from the result of theobservation of the electrical fields at the Fujinomiya
and Numazu group. So, as the observation areas, theIriyama fault near Fujinomiya and the Tanna fault near
the center of the Numazu observation network arechosen. Moreover, there is also expectation that theposition of the source of the artificial noises might beable to be presumed. Although the observation in the
Fujikawa fault does not come to mind, since it
performs the similar technique, it is introduced here.
10-1 Iriyama fault
Iriyama fault is located on the west side of the
Fuji River as shown in Fig. 10-1, and is an active fault.
Here, the observation was performed with 2 two-pen
recorders in a tentative way on June 12, 1985. A pen
recorder is set in the schoolyard of Yui-Kita
elementary school as the base station. Since 1 Hz of
telephone noises was observed there, there wasconcern because 1 Hz was due to be investigated also
in regard to the noise waveform, but it was notrecognized at other points.
Electrodes have been arranged along an
L-character pattern of a 20 m baseline length, and thecopper-copper sulfate electrodes are used. Fig. 10-2
shows the electrical variations at three points aroundthe Iriyama fault and at the Fujinomiya group. The
recorded papers are cut at the temporary points andthen lined up together so that a vertical axis throughall the papers may indicate the same time. The
rectangular wave-noises are selected for about 30 seconds

106Fig. 10‑1

Toshio MORI

Temporal electrical observation points and the amplitudes of the electrical noises around the Iriyama fault.

before and after 16:00 and 11:20 (arrows in Fig.
10-2), and the amplitude at each observation point isread on the basis of the potential difference between
Fuji and Fujinomiya. The values are written down in
Fig. 10-1. In Funaba-1 and Funaba-2, as comparedwith other points, the amplitudes are small,therefore these points are located in a low resistivity
area, and are in agreement with the position of a
fault. Although most directions of change of current
are north and south, depending on the place, thedirection of east and west also becomes large.
10-2 Tanna fault
Tanna fault also moved at the time of theKita-Izu earthquake (M = 7.0) on November, 26,
1930, and a 2.5-3.5 m left gap level change wasshown (Matsuda, 1972). The observation was

carried out in the Tanna basin shown in Fig. 10-3
and near the center of the Numazu observation
network. This place is located in a convenient
position, although the observed noise judges
whether it is a broader-based noise. Since the Tannabasin is covered by a deposition layer, even if a
dislocation arises at such a place, not being detected
at all electrically is also considered. However, whenthe electrical field is observed in a place of intense
ups and downs, even if the change of the amplitude
and/or the direction are actually observed, it is also
considered that it becomes difficult to judge whether
they are only a thing relevant to geographicalfeature in whether they are observed as a thing
relevant to a dislocation. Therefore, this observationis performed by choosing flat grounds, also including
an experimental meaning.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 10‑2

Electrical records around the Iriyama fault and electrical field variations at the Fujinomiya group.

107

108

Toshio MORI

Fig. 10‑3 Location of the Tanna fault and arrangement of temporal electrical observation points. Closed circles (●) show
the NTT stations of the Numazu group.

The observations were conducted on March 12
and 13, 1986. The number of observing points is 40,
and observation was carried out at intervals of 50 m or
100 m as shown in Fig. 10-3. Observation was
conducted simultaneously with two digital recorders,which used a common ground, and 20 observation
points were divided for measurements on two days,
and two points were carried out in common among
those. The sampling interval is 1 second, and electricalfields at the Numazu group are recorded also at
1-second intervals.
Fig. 10-5 shows the electrical records at the A andB observation groups for 20 minutes in the Tannabasin, and Fig. 10-4 shows the simultaneous record in
the Numazu group. It is thought that the square wave
for 20 seconds where the arrows are placed has asource of a noise near Numazu, and the same change
is observed also around the Tanna fault. The similarsquare wave for 25 seconds at the C and D observationgroups are also selected. Their orbits are drawn in Fig.
10-6. The Tanna tunnels are under the A observationgroup, and the dashed lines of the north and the south

side show the positions of the tunnels of the JR
Shinkansen railway and of the JR Tokaido railway,
respectively. Although the trains of the Shinkansen are
driven by alternating current and those of the TokaidoLine by direct current, an electrical noise shows anunusual change on the tunnel of Tokaido Line. It is notclear whether it is because there is something by which
this unusual behavior reflected the structure of thetunnel itself, and a place from which current tends toleak to some tunnels. We do not yet have a conclusion
about whether other things must be considered.
If the abnormalities near a tunnel are disregarded
and we look at the whole, the amplitudes are large inthe direction of north and east. This will be because
the base of high resistance is shallow in that direction.
Although it is certain that the Tanna fault is near
the position shown in this figure, the abnormalities
considered to be based on it are not seen in electricalpotential change. Since an electrical boundary is notproduced even if a dislocation arises within adeposition layer of low resistance, it is not expectedthat electrical abnormalities appear.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

Fig. 10‑4

109

Secondary electrical field variations at the Numazu group duration 15:40 to 17:00 on March 12, 1986.

Fig. 10‑5 Electrical field variations at the observation lines in and around the Tanna fault in the duration time of 20
minutes on March 12, 1986.

110

Toshio MORI

Fig. 10‑6 The locus of the rectangular electrical noise changes every second for 20 and 25 seconds. The dashed lines of
the north and the south side show the positions of tunnels of the JR Shinkansen railway, which is driven by alternating
current, and of the JR Tokaido railway, which is driven by direct current, respectively. A depth of about 160 m hasthese two tunnels from surface of the earth.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

10-3 Fujikawa fault
It is presumed that the Fujikawa fault is an active
fault mostly prolonged toward the north from near themouth of the Fuji River, and even the Ansei-Tokai
earthquake in 1854 moved (Tsuneishi and Shiosaka,1981). Moreover, it is thought that it is the fault which
will probably cause the Tokai earthquake currently
expected to start this dislocation in the near future. A
good deal of drilling is performed near the Fuji River

111

iron bridge of the Tokaido Line, and the position of thedislocation is generally confirmed. A fall of about 100
m is produced there. However, it is not clear near themouth of the river, and the dislocation is filled in by
presumption.
Observations of the magnetic total intensity by
proton magnetometer, electrical resistivity with
Wenner array, and artificial electrical noise werecarried out near the mouth of the Fuji River.

Fig. 10‑7 Result of the observation of geomagnetic total intensity. The observation was conducted along the thick lines
which meet the beach in the top of the figure. The point of the graph shows the observed magnetic field, and a solidline shows the profile of magnetic total intensity anomaly calculated from the subterranean anomalous magnetized body,which is shown in the bottom of the figure.

112

Toshio MORI

Fig. 10-7 shows the result of the observation ofgeomagnetic total intensity. The observation isconducted along the thick line which meets the beach
because the electromagnetic noises are large in thisneighborhood. The daily and short period variations ofmagnetic total intensity are compensated for using themagnetic variations at KAK. As shown in the middle ofthe figure, there is an anomalous change of around 100
nT. Magnetism, such as the lava near Fuji River, sandof the seashore, and stones and the Kanbara layer,
were measured using an astatic magnetometer. The
magnetic strength of the underground body is assumed
by referring to the above measurement, and theunderground structure as shown in the figure in thelower berth is presumed so that the observed
magnetic total intensity will be suited. In thepresumed model, the stratum inclines toward the east.
The dislocation of an almost perpendicular fall ispresumed from the drilling result near the iron bridge.
Even if a fall produces geographical features by thedislocation only temporarily, it is also considered by
the subsequent landslide that such a geographicalfeature arose. Calculation of a magnetic field isperformed using magnetic permeability of 10-3 emu/cc
by the formula from Mori et al. (1983) but not using
the remaining magnetization.
Observation of electrical noise in the mouth of the
Fuji River was performed with electrodes that havebeen arranged at 50 m intervals, as the black dots ofFig. 10-8 showed. It was carried out, as in the case of
the Tanna fault, by the method of measuring theelectrical potential of other points on the basis of one
point, using copper-copper sulfate electrodes.
Although the example of the potential record is notshown, the noise with a duration time of 5-6 seconds ischosen, and the situation of the change is shown in Fig.
10-8. A similar change was selected although No. 1-10
and No. 10-14 were not the same change, which are
the points observed on another day.
Though the position of the fault presumed by the
result of the observation of the magnetic total
intensity is near No. 1, since it entered into the river,the east side has regrettably not been measured there.
It seems that the amplitudes of the noise have changedin the west from No. 10. Since the measurements
bordering on there were performed on different days,
although firm belief is impossible, it is thought that asurface resistance change will be greatly reflected.
Although 1 more observation line is required inorder to check the observed results of magnetic total
intensity, even if it is measured by the inland side fromit, it is thought that good results will not be obtained
because of large artificial magnetic noises. If the sizeof an unusual region is taken into consideration, it willbe thought that abnormalities are detectable to several
100 m in the offing. So it was considered that themeasurements with a boat would be off by several 100
m, but that was regrettably unrealizable.

Fig. 10‑8 Electrode arrangement for the observation of electrical noise in the mouth of the Fuji River, and the locus
of electrical noise changes every second for 5 − 6 seconds.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

11. Anomalous geoelectric change observed near
CA (conductivity anomaly) boundary
It is well known that induced electrical currents
tend to flow in a preferred direction. Honkura (1976)
calculated how much the amplitude and directions ofgeoelectric variation change, when a conductivity
anomalous region arose in relation to an earthquake. In
his case, a base length for the observation, that is anarrangement of actual observing electrodes, wasignored. Here, when electrodes are arranged aroundan unusual domain, it calculates what potential
difference is observed using the formula which hedrew.
The case where electrical potential is observed
around the unusual domain of a hemisphere (radius =a) as shown in the lower part of Fig. 11-1 is considered.
The small open circles in the figure are the positionsof the electrodes, and the top of the lower part of the
figure looked at the unusual domain horizontally,
whereas the bottom of the figure looked at it from the
top.
Suppose that the uniform electrical field is built
from the distant place of an unusual domain. The
potential in the distance r from the center of thehemisphere is
Vo = A(r + K1a3/r2)cosϕ(r>a)
Vi = ArK2cosϕ(r<a).
Here, A is a constant, and ϕ the angle from thedirection (the x direction) which the external field has
required. If we make Ri the resistance in thehemisphere and Ro the resistance of the outside, K1
and K2 will be given by the following formula:
K1 = (Ri - Ro) / (2Ri + Ro)
K2 = 3Ri(2Ri + Ro) / (1 + K1)
When we actually observe the electrical field, thepotential differences of the directions of x (horizontalaxis) and y (vertical axis) with the base lengths S
centering on the distance D from the center aremeasured. It sets with K = Ri/Ro and a = 1, and whenchanging K, D and S, it calculates what potential
difference (Ex and Ey) is observed.
The top of Fig. 11-1 is the example whichcalculated what values Ex and Ey would become for D= 1.2 and S = 0.2. A solid line shows the calculation
result in K = 0.1, and a dotted line shows in K = 10,
when the direction (ϕ) of external potential is made tochange from 0 up to 90 degrees. Namely, when we
observe the electrical potential immediately outside113

the anomalous region, the predominant direction ofelectrical change is perpendicular to the boundary forthe low resistance anomalous region, and tangent for
the high.
As K = 0.1, calculations were made to learn how
an electrical field would be observed when the value ofD changes when S = 0.01, 0.1 and 1.0, and show theresult to the left of Fig. 11-2. A vertical axis Ex of theabove graph expresses the size of the electrical field of
x directions to the size of an external electrical field.
The lower graph shows the degree of the predominantdirection of the electrical field Ex/Ey. When an
extreme value of S = 0.01 is used, the maximum valueof Ex/Ey becomes ten.
As S = 0.01, calculations were made to learn how
an electrical field would be observed when the value ofD changes for K = 0.1, 0.2, and 0.5, and the results areshown to the right of Fig. 11-2. In this case, also, when
we observe the electrical field near the CA(conductivity anomaly) boundary, the direction of thechanges will show a strong predominant direction.

Fig. 11‑1 Calculated electrical potential (top) around an
unusual domain (bottom). The unusual domain has ahemisphere of a radius r, and Ri and Ro are the electricalresistances of the unusual domain and outside of it,
respectively. Small open circles show the position of
the electrodes. The base length for the observation isS; and D, which is a distance from the center of thehemisphere (O), is the center of the electrodes.

Toshio MORI

114

Fig. 11‑2

Calculated electrical potential around an unusual domain.

The actual example of application of thiscalculation has electrical potential observation in the
Akan lake in the eastern part of Hokkaido (Mori, 1987).
The observation was performed by the north edge of
the Akan caldera. If we assume that the Akan caldera
has low resistance as compared with thecircumference, it can explain why an electricalpotential change which lay in the predominantdirection of north and south was observed.
12. Electrical observation on the sea floor bymaking use of the power-feeding arrangement of
the Permanent Ocean-Bottom SeismographObservation System off Boso Peninsula (BSOBS)
Permanent
Ocean-Bottom
SeismographObservation Systems by JMA is installed in the offingof Tokai, and the offing of Boso Peninsula (Fujisawa et.
al., 1986). As shown in Fig. 2-1, the submarine
systems are supplied with electric power in highly
precise direct current from the power units at

Omaezaki Weather Station (OMZ) and at Katsuura
Weather Station (KTU) through cables. The returncurrent flows between the sea earths (TS1 and BS1)
attached near the terminal apparatus and the seashore
earths (OMZ and KTU) through the sea water and theground. Since the electrical supply voltage iscontrolled to compensate for the current when apotential difference arises between the sea earth andthe seashore earth, if the electrical supply voltage ismeasured, the external potential change will be known.
The case off Tokai is already described (Mori, 1982,
1987). Here, the example of electrical potential change
measured about the case off Boso Peninsula is shownbriefly.
Although it is not directly related to the electricalobservations using NTT equipment, since it is thoughtthat it is necessary to study the relation between theelectrical field on land, and movement of sea water andthe electrical field on the sea floor, it is described forreference.

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115

Fig. 12‑1 Record of the electrical supply voltage (SV) and the tsunami meters (TIDE) in mV by BSOBS. TIDE at KATS 1, KATS
2 and KATS 3 represent the sea level at BS1, BS2 and BS3, respectively.

A part of the results are shown as recorded withthe digital recorder in the Katsuura Weather Stationfrom July to December, 1986, before being put into
regular service. Fig. 12-1 shows the records of theelectrical supply voltage (SV) and the tsunami meters(TIDE), and TIDE at KATS 1, KATS 2 and KATS 3
represent the sea level at BS1, BS2 and BS3,
respectively.
Mustache-like noises are in the SV record inmany cases in the daytime. Since it is thought thatthese noises are due to leak currents by the
direct-current train, the relation between thevariations of SV and the arrival-and-departure time oftrains at the Katsuura railway station is investigated. It
is confirmed that large changes of the SV correspondto the departure of a train as shown in Fig. 12-2. The
state of noises changes from day to day as shown in
Fig. 12-1. The cause for this is considered to be thatthe amount of current which is transmitted and leaks
from a train’s rail changes with conditions, such as
humidity.
Although it is thought that it is difficult to use thedata of a period of time containing that train noise, thedata at night with few noises may be usable aseffective data. Now, the telemeter of the data of
BSOBS is carried out to the Seismological andVolcanological Department of JMA, and it is recorded
there.

13. Conclusions(1) Using the facilities of NTT Company, it isconfirmed that we can obtain highly stable andnoiseless geoelectric data compared to those obtainedby ordinary observations. At Mito, the observed data
are fairly noiseless, while at Numazu, the data containlarge amplitude noises.
(2) The method of real-time detection of theanomalous geoelectric changes accompanying tectonic
activity has been developed. Furthermore, in order toraise accuracy, it is thought to be required to take into
consideration the use of BAYTAP-G (Ishiguro, et al.,1984), seasonal changes of geomagnetic field, etc.
(3) When it is assumed that the impulse response(impedance tensor) of the electrical variation togeomagnetic variation does not change in time and theimpulse response is used for prolonged data, changesof underground electrical conductivity may also appear
in the residual field as natural potential change.
(4) Though in the Mito group, unusual geoelectricchange directly considered to have been related to theearthquake is not detected, a principal method for
reduction of the induced component was established.
However, examination of the abnormalities of shortcycle changes of several hours or less is inadequate.
(5) Anomalous variations seemingly related to theearthquake swarm off the east coast of the Izu
Peninsula were observed in the Numazu group. The

116

Toshio MORI

Fig. 12‑2 Relation between the variations of the electrical supply voltage (SV) of BSOBS and the arrival‑and‑departuretime of trains at the Katsuura railway station.

Geoelectric Observation used by the Telegraphic Facilities of NTT Corporation

earthquake swarm started around October 13, 1985,
when electrical potential changes of 300 mV at Atami
and 100 mV at Ito and Shuzenji took place
simultaneously. The volume strain meter ofHigashi-Izu also changed simultaneously. Stresschange applies to Shuzenji and Higashi-Izu from Atami,
and it is thought that there was an electrical potentialchange corresponding to it. However, on the occasionof subsequent earthquake swarms (as earthquake
swarms, the scales are somewhat small), changes
were not detected.
(6) The Izu-Oshima volcano erupted on
November 15, 1986. Before the start of the eruption,
the geoelectric potentials at ODW are estimated tohave decreased abruptly by 500 mV on November 6
and attained a maximum decrease of 800 mV on
November 9. ODW is located at the extension of the
Suruga Trough, and on the boundary of the Eurasia
Plate and the Philippine Sea Plate. The flow ofunderground water which is caused by a change ofstress acting on the boundary may possibly causechanges in the streaming potential and electricalconductivity. The change of the groundwateraccompanying the stress change of a plate boundary
can be considered to be the cause of this.
(7) The electrical potential changes relevant totide level changes were detected. Around March,
1986, the tide level change was sharp, and theelectrical potential changed in a way that it wasthought that it was related to the electrical potential in
the Numazu group. There was change relevant to atide level with the electrical potential of the Numazu
group around August 4, 1986.
(8) Constituents of geoelectric change unrelated
to geomagnetic change and long cycle change weredetected. Although it is thought that the constituentsare based on the earth tide or a sea tide, in order toseparate these, comparison observation at many points
is required. The earth tide and long cycle change may
express the potential change accompanying stresschange.
(9) The noise by the direct-current train isobserved in several 10 km, as shown by Section 10. By
investigating a time change of response of electricalfields observed between baselines, it is thought that it
is applicable also to detection of an undergroundelectrical change.
(10) As shown in Section 11, the big abnormalities

117

in electrical potential changes should be observed near
the CA (conductivity anomaly) boundary. On thecontrary, if earth potential is observed near the CA
boundary, it will be thought possible to detectsensitively unusual phenomena relevant to tectonic
activities. The pattern of daily changes of the earth
potential before and behind the eruption of the
Izu-Oshima volcano is considered to reflect such astate.
14.
Circumstancesofobservation
andacknowledgements
The author is indebted extensively to the staff of
the Nippon Telegraph and Telephone Public
Corporation (now NTT Company) for theirobservations, and he had many people support thecause which began with this observation.
After the author’s transfer (April, 1980) to the
Seismology Volcanology Research Department ofMeteorological Research Institute (MRI), he was soon
blessed with the opportunity to analyze the geoelectricobservations on the sea floor by making use of thepower-feeding arrangement of the Permanent
Ocean-Bottom Seismograph Observation Systems(OBS) installed by the Japan Meteorological Agency(JMA). It was shown that the earth potential
observation using the electrical supply voltage of the
OBS is effective and useful for the research ofearthquake prediction, oceanography, etc. (Mori, 1982).
Mr. Matsumoto, who belongs to the same researchsection as the author, taught that NTT has adopted thesame system as OBS for the electrical supply systemfrom Ito to Izu-Oshima and Miyake-jima. The author
had Mr. Matsumoto and Mr. Fujisawa, of the
Earthquake Division of JMA, introduce Mr. Aoyagi,
who is taking charge of system manufacture of the
NEC Company, and after that visited the Engineering
Department of the Kanto Telecommunications Bureau
with Mr. Matsumoto and Mr. Aoyagi (July, 1983). The
author explains the affair of the geoelectricobservation off Tokai to Mr. Hagiwara of theEngineering Department, and allowed for an
experimental observation for the submarine cablebetween Ito and Izu-Oshima.
Although it was decided to perform test
observations from the beginning of October, 1983, in
Ito, when the test record of the time of installation of
NTT Corporation electrical supply system was118

Toshio MORI

considered, it became clear that the current supplied
was seldom stable. Moreover, since the supply of
current was performed from both Ito and Izu-Oshima,even if it was recorded only in Ito, it turns out that it islikely to be almost meaningless. It seemed thatfurthermore, temperature changes were also received.
Moreover, since the cable was prolonged also to
Miyake-jima, it became a conclusion that it is better tocarry out simultaneous measurements also in
Miyake-jima. Therefore, although it was thought thatdata with utility values could be taken when
performing simultaneous measurements at threeplaces, Ito, Izu-Oshima, and Miyake-jima, sincemeasurement equipment and traveling expenses were
not able to be prepared, observation had to be given up.
Having considered that Miyake-jima volcano erupted
on October 3, 1983, and Izu-Oshima volcano erupted
on November 15, 1986, it was very regrettable.
The author visited the Kanto Telecommunications
Bureau and said that he could not but give up
observations and expressed his gratitude for all thecooperation he had received. At that time, the author
had Mr. Hagiwara and Mr. Ban teach that there is acable which is used for geoelectric observation also on
land.
Although we had performed many extraordinary
geomagnetic and geoelectric observations, the best we
could do was to record up to half of the values ofgeoelectric observations during an observation period.
Geoelectric observations have been continuously
afflicted by artificial noises and the drift by change ofcontact potential between electrode and soil. Even if
an author reads or hears a paper saying that an unusual
phenomenon of earth current came out before an
earthquake, therefore, do not feel that thosestatements can be trusted as they are. (However, theauthor once observed a geoelectric record which isstill regarded as the sign of an earthquake.) Even if thecopper-copper sulfate electrode and the lead-lead
chloride electrode, which are said to have little drift,
were used for geoelectric observations, the records
were not shown to be the same as daily changes when
they were observed at the same point (Mori, 1984). So
we are beginning to think that geoelectricobservations will be almost meaningless for anything
besides inferring an underground structure. But if thelong baseline of NTT Corporation can be used, theauthor thinks that it will be value to carry out test

observations ashore.
Although noises, such as thunder, were causes foranxiety when observing using the line stretched in theair, it is allowed to use the subterranean buried cablewhich is on a direct route between telephone offices.
And it was allowed to observe geoelectric fields by theroute of Mito-Kasama-Shimodate near the MRI.
Moreover, the author had borrowed the low-path filterof 10 seconds to several tens of minutes from Mr.
Yoshino of the Earthquake Research Institute,
University of Tokyo, recently. Since it was expectedthat the 1-second noise of a charge counter and a city
artificial noise enter when the ground of a telephone
office was used as an electrode, observations could not
be made without the filter, but the timing was very
good.
Since it became clear that Kasama was an
uninhabited relay station just before the observation
start, the pen recorder was installed in the Shimodate
relay station, and it was decided to measure theelectrical potential of Kasama and Oyama on that basis(November 29, 1983). Although the recordersometimes caused trouble, Mr. Tomaru and persons in
the Shimodate relay station very cooperatively
maintained it. Although the obtained record was
fragmentary, it was big harvest that checked that therecord had almost no drift, and there were few artificial
noises beyond anticipation in spite of having the NTT
relay station located at the urban center. However,using only the record of a pen recorder, since there
was a limit in the analysis, the observation wasinterrupted in four months.
Since the prospect which can purchase one digitalrecorder in March, 1985 was attached, it offered again
to want to perform test observation to Mr. Hagiwara.
He pleasantly consented to test the observations. It
was then decided to measure the electrical potential
between Ishioka, Kasama, and Hitachiota on the basisof Mito so that the directivity of the earth current
could also be discussed. The reason for choosing thatwas based on the following reason. Since it is a new
method, it was necessary first to compare with thedata of the Magnetic Observatory, and this area waschosen since there are probably comparatively few
noises because of an alternating current train.
Observation was started from January, 1985, using themulti-pen recorder. Also adding to the March record by
the newly-purchased digital recorder, electricalGeoelectric Observation used by the Telegraphic Facilities of NTT Corporation

potential came to be able to carry out digital inclusion.
The head, Mr. Miyazawa, the Examination Division
chief, Mr. Tsukui, and other personnel also of the Mito
Telephone Office got very much interested in earth
current observation, and they cooperated very muchwith the trouble of a recording form and the pen of apen recorder and entry of time, etc.
Nippon Telegraph and Telephone Public
Corporation became Nippon Telegraph and Telephone
Company (NTT) from April, 1985; however it wasconvenient to make geoelectric observations anyway.
Since the prospects of purchasing the 2nd digitalrecorder seemed much stronger, an offer was made tocarry out observations, starting with continuing
observations in Mito and another point at the Kanto
Telecommunications Bureau in March, 1985. Therewas very good cooperation, although Mr. Toyoda and
Mr. Terashima became negotiation partners, instead of
Mr. Hagiwara, on the Kanto Communication Bureauside at this time. Since the Fujinomiya telephone office,which was the next place desired for groundobservations, was under the jurisdiction of the Tokai
Telecommunications Bureau, they introduced Mr.
Mimura of the Facilities Department, the Tokai
Telecommunications Bureau.
When the plan to do observations in Fujinomiya
was formed in April, 1985, the person in charge of theTokai Telecommunications Bureau had replaced Mr.
Serizawa at this time. As a result of expressing to him
by telephone our desire to carry out the start ofobservations, he pleasantly consented. In addition, theorganization of NTT was changed, the Mito telephone
office came under the jurisdiction of the Kanto Total
Branch, and the Fujinomiya telephone office cameunder the jurisdiction of the Tokai Total Branch from
April.
When it was determined that observation in
Fujinomiya would start from June 10, 1985, theundergroundmetallic
cable
of
the
Fujinomiya-Minobu-Kofu route was already due to betransposed to the optical fiber cable at this time.
Although the author had agreed on waiting until thecable was replaced for good conditions to observe, thecable was flooded out by the heavy rain from thetyphoon on June 30. Since Mr. Serizawa proposed thecontinuation of observation at another point, theauthor chose Numazu and is also continuing
observations there now.

119

Although the Numazu, Shuzenji, Ito, and Atami
telephone offices belonged to the jurisdiction range of
the Tokai Total Branch and Odawara belonged to the
Kanto Total Branch, both the total branches
cooperated willingly. After that the division system of
NTT was changed further, and the Mito, Kasama, andHitachiota telephone office came to belong to a Central
Network Branch. I had Manager Kawasaki of theEngineering Department of the Kanto Total Branch
introduce the Central Network Branch, and had him
become the negotiation partners to Mr. Seki and Mr.
Noguchi of the First Equipment Part.
Personnel movement of NTT was very intense,
and the negotiation person in charge was replaced inseveral months. However, irrespective of that, there
was no trouble in the observation. Since the Tokai
Total
Branchwaslocatedin
Nagoya-shi,correspondence about the observation was performedby telephone and letter. The author took advantage of
his attending the general meeting of the SeismologicalSociety of Japan in Gifu University in October, 1986, tocall on the Tokai Total Branch in Nagoya-shi and wasallowed to explain the observation results, etc., toabout ten personnel. At this time, it was conversely
encouraged by Mr. Ide and Mr. Yamaguchi of theTechnology Planning Part, who had become thenegotiation partners.
In the Numazu and Mito telephone office,geoelectric observations have been performed withdigital recorders and the monitor record was made bymulti-pen recorders. Using the digital recorders aremost convenient, while the paper sendings and thepens of the multi-pen recorders sometimes get out ofcondition. Although it was kept in mind so that trouble
might seldom be made for the personnel of NTT, theycorrected the trouble of paper sendings or pens inpractice. They show interest also about the records
and fill in time, etc., independently. We are especiallyindebted to The First Transmission Radio Engineering
Division Section chief, Mr. Suzuki in Numazu, and The
General Planning and Policy Division Section chief, Mr.
Hinohara in Mito.
The third laboratory for studying the tectonic
activities relevant to an earthquake was installed in
the Seismology Volcanology Research Department of
MRI in October, 1984. The author will belong to thethird laboratory from the first laboratory and will takecharge also of the research on electromagnetism

120

Toshio MORI

thereafter. The following three persons will cooperate
in this research, Mr. Takayama, who transferred fromthe Seismological Observatory of JMA, Dr. Yoshikawa,
who was employed from the Earthquake ResearchInstitute, University of Tokyo and Mr. Koizumi of thesecond laboratory. Especially, the analyses of theremoval of induced electrical potential change and thedislocation inquiry have the large contribution by Mr.
Takayama. Moreover, the author had Dr. Katsumata
and Dr. Seino of the first laboratory and Mr. Takahashi
and Mr. Sato of the Planning Office cooperate in themaintenance of apparatus, etc.
The author was transferred to the Kakioka
Magnetic Observatory in April, 1987, and belongs toalso the MRI will continue observation further
simultaneously with MRI. The observation is takenover pleasantly by the personnel of the Magnetic
Observatory. When summarizing this article especially,
Dr. Ozima cooperated very much.
Finally the author is deeply thankful to theabove-mentioned people.
In addition, the author’s place was changed in the
Seismological and Volcanological Department of JMA
in October, 1987. This paper is described based on thememorandum described in September, 1987.
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