Geophysical Journal International
Geophys. J. Int. (2010) 180, 858–870
doi: 10.1111/j.1365-246X.2009.04461.x
Anomalous pre-seismic transmission of VHF-band radio waves
resulting from large earthquakes, and its statistical relationship to
magnitude of impending earthquakes
T. Moriya, T. Mogi and M. Takada
Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
E-mail: [email protected]
Accepted 2009 November 21. Received 2009 November 9; in original form 2008 December 19
GJI Seismology
SUMMARY
To confirm the relationship between anomalous transmission of VHF-band radio waves and
impending earthquakes, we designed a new data-collection system and have documented the
anomalous VHF-band radio-wave propagation beyond the line of sight prior to earthquakes
since 2002 December in Hokkaido, northern Japan. Anomalous VHF-band radio waves were
recorded before two large earthquakes, the Tokachi-oki earthquake (M j = 8.0, M j : magnitude
defined by the Japan Meteorological Agency) on 2003 September 26 and the southern Rumoi
sub-prefecture earthquake (M j = 6.1) on 2004 December 14. Radio waves transmitted from a
given FM radio station are considered to be scattered, such that they could be received by an
observation station beyond the line of sight.
A linear relationship was established between the logarithm of the total duration time of
anomalous transmissions (Te) and the magnitude (M) or maximum seismic intensity (I) of
the impending earthquake, for M4-M5 class earthquakes that occurred at depths of 48–54 km
beneath the Hidaka Mountains in Hokkaido in 2004 June and 2005 August. Similar linear
relationships are also valid for earthquakes that occurred at different depths. The relationship
was shifted to longer Te for shallower earthquakes and to shorter Te for deeper ones. Numerous
parameters seem to affect Te, including hypocenter depths and surface conditions of epicentral
area (i.e. sea or land). This relationship is important because it means that pre-seismic anomalous transmission of VHF-band waves may be useful in predicting the size of an impending
earthquake.
Key words: Earthquake interaction, forecasting, and prediction.
I N T RO D U C T I O N
In the recent decades, electromagnetic precursors to seismic events
have been reported over wide frequency ranges from all over
the world. Two kinds of anomalous phenomena exist: anomalous
emission and anomalous propagation of electromagnetic signals
(Varotsos & Alexopoulos 1984a,b; Gokhberg et al. 1995; Hayakawa
& Fujinawa 1994; Hayakawa 1996; Nagao et al. 2002). These phenomena span a wide range of frequencies, from DC to HF bands.
Kushida & Kushida (1998, 2002) introduced an empirical earthquake prediction method based on monitoring anomalous VHFband radio waves transmitted from an FM radio station beyond the
line of sight. Sakai et al. (2001) showed that anomalous propagation of VHF-band radio waves emitted from a broadcasting station
in Sendai City were related to earthquakes with magnitude greater
than 5 that occurred in the area between Sendai and the Tateyama
observatory in Chiba Prefecture. Fukumoto et al. (2002) confirmed
that the anomalous propagation events were the result of scattering of VHF-band radio waves immediately prior to earthquakes, by
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documenting reception at an observatory that was beyond the line
of sight of the transmission location. The received intensities of
scattered waves were stronger when the antenna was at a shallower
angle, which implied that the scattering body was in the middle
atmosphere rather than in the ionosphere (Pilipenko et al. 2001).
Fujiwara et al. (2004) also reached the same conclusion using a
more rigorous method, and recorded no scattered waves when antennae were oriented vertically. Hayakawa et al. (2007) described a
generation mechanism of atmospheric disturbances resulting from
changes in geochemical quantities associated with earthquakes and
Yonaiguchi et al. (2007) discussed that the effect of long-range VHF
wave propagation is usually due to meteorological radio ducting.
To further confirm the quantitative relationship between anomalous transmission of VHF-band radio waves and earthquakes, a
new data-collection system was designed. The TNK observatory
began collecting data in 2002 December and finally six observatories have been operating since 2004 December 25, in Hokkaido,
Japan (Table 1). Each observatory has been tuned to three to eight
FM radio stations; on numerous occasions, anomalous VHF-band
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Table 1. Locations and start dates for six observatories. TKB was abandoned in November 2003 due to
power line trouble.
Code
Latitude (N)
Longitude (E)
Altitude (m)
Beginning date
TNK
HSS
ERM
TKB
TES
AKP
44.46.5
42.57.1
42.09.0
44.56.7
43.29.0
43.01.0
142.05.0
141.14.0
143.09.4
141.58.1
144.24.2
144.50.3
60
215
40
240
220
80
2002 December 4
2003 February 7
2003 March 18
2003 July 16
2003 September 3
2004 December 25
radio waves possibly related to earthquakes have been received from
stations beyond the line of sight.
The Tokachi-oki earthquake (M j = 8.0, M j : magnitude defined
by the Japan Meteorological Agency [JMA]) and the southern
Rumoi sub-prefecture earthquake (M j = 6.1), occurred in Hokkaido
in 2003 September and 2004 December, respectively. These large
earthquakes produced anomalous transmissions of VHF-band radio wave that were clearly recorded at an observatory in Hokkaido.
Further the seismic activity in the Hidaka area, central Hokkaido,
was documented after the Tokachi-oki earthquake, and many less
than M5-class earthquakes occurred in the immediate vicinity. Precursory anomalous transmission events associated with the seismic
activity in the Hidaka area were recorded at Erimo observatory.
The anomalous transmission events were observed just before all of
M4-M5-class earthquakes and some of M3 class earthquakes. We
established a statistical relationship between the logarithm of the
observed total duration time of anomalous transmissions and the
magnitude or maximum seismic intensity of the impending earthquake. This paper describes the new data-collection system and
results of observation.
I N S T RU M E N TAT I O N A N D
O B S E RVAT O RY S E T T I N G
Fukumoto et al. (2002) and Fujiwara et al. (2004) suggested that
anomalous VHF-band radio waves received from an FM radio station at a location beyond its line of sight were scattered in the atmosphere during transmission. For this study, a system was devised to
record the temporal intensity changes in the electromagnetic field
over 76–90 MHz of FM radio bands using a highly sensitive radio.
An FM digital-tuning radio (Panasonic RF-U99) manufactured by
Matsushita Electric Industrial Co., Ltd. was modified as follows.
The wide-band (0.3 MHz) intermediate frequency (IF) ceramic filter was substituted with a narrow-band filter (0.013 MHz) to avoid
conflicting reception of closely spaced frequencies (Fig. 1). The
frequency interval of any FM broadcasting allotment is 0.1 MHz,
but given that FM radio waves have rather wide side-band spectra,
mixed reception cannot be completely avoided. The IF pre-amplifier
was added to the circuit to improve overall sensitivity and to increase
the gain of the pre-amplifier in the 8–14 dB range. The output signal
was drawn from the auto-gain control (AGC) terminal, consisting of
an integrated circuit (TA8132AF) containing the IF amplifier and
FM demodulator, amplified by 8 dB through a DC amplifier. This
modification resulted in very high sensitivity and stability (lower
sensitivity limit was –120 dBm; Fig. 2).
Three to eight modified radio receivers were installed at
each of six observatories in Hokkaido (Fig. 3; Table 1): HSS
(Sapporo), ERM (Erimo), TES (Teshikaga), TNK (Nakagawa),
TKB (Horonobe) and AKP (Akkeshi). The ambient electromagnetic noise level was an important factor in observatory selection.
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Journal compilation Five horizontal-element Yagi-Uda antennas connected to each radio
were oriented toward each FM radio station at an elevation angle
of about 15 degrees. For each observatory, the target FM radio stations, their frequencies and directions are indicated in Fig. 3 and
Table 2. We desire a target FM radio station to satisfy the following
conditions.
(1) The line between the observatory and transmitting station
must cross a seismically active region.
(2) The transmitting station should not operate with duplicate
frequencies with any other broadcasting station in Japan. A duplicate frequency is allowed only if the station has small output power
and is located sufficiently faraway.
Antennas were not always directed exactly to the target FM radio
stations because FM waves in this range have very short wavelengths (3.3–3.9m) and can be scattered by topography and buildings around the observatories. The target FM radio stations and
antenna azimuth sometimes adjusted at the initial stage of observation to find appropriate direction according to seismic active area
and noise conditions.
The intensity variation envelope of received anomalous waves
was digitized at 1 Hz and recorded in a data-logger. The data were
then transmitted by dial-up or ISDN through public telephone lines
to the Sapporo Campus of Hokkaido University. When an anomalous VHF-band radio wave was observed, the name of the observatory and appearance and duration times were documented. The
amplitudes of the electric field changes were not collected because
the sensitivity curves of the receiver radios were slightly different
depending on the AGC circuit in each receiver radio, and because
transient and static responses of the AGC were quite different. The
amplitude of the anomalous wave was affected by a variety of unquantified parameters, such as reflectivity of a scattering body or
distance between the observation site and a scattering body. Earthquake data (occurrence time, source location, magnitude or intensity), as collected by the JMA, were then compared with the
temporally associated anomalous wave data.
CHARACTERISTICS OF THE
A N O M A LO U S S I G NA L S
The anomalous VHF-band radio wave temporally associated with an
earthquake is referred to here as an “EQ echo.” Fig. 4 shows typical
examples of EQ echoes that take place before an earthquake. The
shape of the EQ echoes is similar to the CD anomaly documented
by Kushida & Kushida (1998). An EQ echo typically shows abrupt
intensity increases by about 5 to 10 dB and continues for several
minutes to several hours, including multi-pulse forms. The echoes
appear once to several times per day, from several weeks to a few
days before the earthquake occurrence and do not appear on other
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T. Moriya, T. Mogi and M. Takada
Figure 1. Diagram of the receiver. Bold squares indicate modifications added to the RF-U99 Panasonic radio.
Figure 2. Sensitivity curve of the modified RF-U99 Panasonic radio.
time. Other types of anomalous changes that have been recorded
have clearly different waveforms from that of EQ echoes as shown
later.
Several types of events that produce anomalous reception that is
not earthquake-related have been identified: (1) meteors, (2) thunderstorms and (3) the appearance of a sporadic E layer in the ionosphere (Es, Smith & Matsushita 1962). Each anomalous wave or
noise has a characteristic waveform that can be easily distinguished
(Figs 5a–c). Monitoring of noise in the 80.8–81.0 MHz band, in
which there is no allotted FM radio station in Japan, was helpful in
distinguishing EQ echoes from noise.
Fig. 5(a) illustrates meteor reflections from a Gemini meteor
shower recorded at TNK on 2004 December 13. Such phenomena
often appear in the records of long-distance FM radio stations. These
anomalies are of short duration and generally cannot be measured
accurately at 1-second sampling intervals.
Fig. 5(b) shows the appearance time of thunder pulses that moved
from the northwestern station (TNK) to the southern and eastern
stations (ERM and TES). The waves caused by the thunder contained many pulses and persisted for several hours. The appearance
time of the pulses shifted as the storm moved from west to east.
Duration of the thunder-generated pulses is as short as those of
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Figure 3. Locations of observing stations (closed circles) and target FM radio broadcasting stations (triangles). The source area of the 2003 Tokachi-oki
earthquake is also shown (shaded ellipse).
the meteor reflections and cannot be measured accurately with one
second data sampling. These characteristics make meteor reflections
and thunder pulses easily distinguishable from EQ echoes.
The appearance of a sporadic E layer (Es) was intermittently
noted during the daytime in summer. An example of the strong signals resulting from an Es is shown in Fig. 5(c). Such signals were
noted for almost all channels and at all observatories. Given that
signals considered to be EQ echoes show up only on certain FM
radio stations, the three effects are readily distinguished, although
large signals of these effects may mask an EQ echo if both take
place concurrently. There are many disturbances of ionosphere. Indeed, it is sometimes difficult to differentiate between seismic and
non-seismic anomalous events by only one observation. Usually,
however, EQ echoes are quite local phenomena observable in the
records from only a specific station, whereas, disturbances of ionosphere tend to induce anomalous changes on records from widely
scattered many FM stations.
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2009 The Authors, GJI, 180, 858–870
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Journal compilation To confirm the relation between anomalous waves and earthquakes, it was found best to locate the observatory as close as
possible to the target FM radio station, as long as it is beyond the
line of sight. The shortest distance between FM radio station and
observation site was 30 km (between the HOO radio station and the
ERM observatory, see Figs 3 and 7). Even in such a case, distinguishing EQ echoes from other noise still requires monitoring of
anomalous waveforms sourced from other FM radio stations located
farther away, as well as monitoring of noise in vacant frequencies.
E Q E H O E S F O R T W O R E C E N T L A RG E
E AT H Q UA K E S I N H O K K A I D O
The data presented here are for EQ echoes associated with two
recent, large earthquakes in Hokkaido. The first is the Tokachioki earthquake of 2003 September 26, (M j = 8.0, focal depth =
45 km), that occurred below the seafloor. This earthquake produced
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Table 2. FM radio stations producing anomalous radio waves, with station codes and tuned frequencies, and azimuths of
antennas deployed to receive them at six observation stations.
Station code
Channel
Target FM station
1ch
2ch
3ch
Kushiro-B
Chiba
Fukushima
1ch
2ch
3ch
4ch
5ch
6ch
7ch
8ch
fm stn. code
Frequency (MHz)
Antenna no. azimuth
KUSb
CBA
FUK
88.5
78.0
85.3
N1 S 50 E
N2 S
N2 S
Nayoro
Kushiro-B
Akita
Maebashi
Nakashibetsu
Urakawa
Hachinohe
Obihiro
NYR
KUSb
AKI
MAB
NKS
URA
HCI
OBI
88.2
88.5
86.7
86.3
89.9
86.1
78.4
78.5
S1 N 10 W
S2 E
S3 S 10 W
S4 S
S2 E
S5 S 05 E
S4 S
S5 E 05 W
1ch
2ch
3ch
4ch
5ch
6ch
Asahikawa
Nakashibetsu
Hiroo/Haboro
Akita
Shizunai
Ashoro
ASA
NKS
HOO/HBR
AKI
SZN
ASY
76.4
89.9
83.8
86.7
84.0
89.7
R1 N 10 W
R2 N 40 E
R3 E
R4 S 35 W
R5 N 10 W
R2 N 40 E
1ch
2ch
3ch
4ch
5ch
6ch
7ch
Urakawa
Niigata
Hakodate
Nagoya
Hiroo
No station
Nakashibetsu
URA
NIG
HAK
NGY
HOO
NKS
86.1
77.5
88.8
77.8
83.8
81.1
89.9
T1 W 45 S
T1 W 45 S
T1 W 45 S
T1 W 45 S
T2 W 55 S
T2 W 55 S
T3 E 55 S
1ch
2ch
3ch
4ch
5ch
6ch
7ch
8ch
Kushiro-A
Kushiro-A
Obihiro
Kushiro-A
Urakawa
Hachinohe
Chiba
Akita
KUSa
KUSa
OBI
KUSa
URA
HCI
CBA
AKI
86.4
86.4
78.5
86.4
86.1
78.4
87.0
86.7
B1 E
B2 E 23 S
B2 E 23 S
B3 E 45 S
B4 S 68 S
B5 S
B5 S
B6 S 23 W
1ch
2ch
3ch
Abashiri
Teshikaga
Nemuro
ABA
TKG
NMR
83.1
89.5
76.3
P1 N 20 w
P2 N 10 W
P3 E 10 N
TNK
HSS
ERM
TES
TKB
AKP
EQ echoes, recorded at the TES observatory, from the HOO FM
station (160 km away). EQ echoes were recorded on September 4,
the first day of the TES observatory’s operation, which was 22 days
before the earthquake. These echoes persisted until 18 September,
after which there were 9 quiet days prior to the earthquake. Fig. 4
illustrates an example of the EQ echoes documented at TES. The
cumulative total duration of the EQ echoes was about 1000 minutes.
The second example is the southern Rumoi sub-prefecture earthquake of 2004 December 14, that occurred in the upper crust (M j =
6.1, focal depth = 7 km). A protracted train of EQ echoes on the
83.8 MHz channel began at the ERM observatory on 2004 November 30. Fig. 6 shows examples of the EQ echoes received at ERM
on December 2, 11 and 12. By December 13, the cumulative duration of the EQ echoes exceeded 900 minutes. These EQ echoes
were documented at ERM on the HOO channel, but no significant
seismic event took place beneath the Hidaka Mountains near HOO.
In fact, a large earthquake (M j = 6.1; maximum intensity a “strong
5”) occurred in Rumoi sub-prefecture (Fig. 7), just after the inception of some EQ echoes without a quiet period. These echoes were
probably not in the signal from the HOO station, but instead, from
an FM radio station in Haboro (HBR), which transmits at the same
frequency (83.8 MHz) as the HOO station, with output power of
100 W (Fig. 7), and is 20 km away from the earthquake’s epicentre and 280 km away from the ERM observatory. Four other FM
radio stations in Hokkaido also broadcast at 83.8 MHz (Fig. 7).
Seismic activity is generally very high in the vicinity of HOO station, but not so high near the Ashibetsu and Engaru stations, except
for deep earthquakes at 150–200 km. The problem of distinguishing
anomalous activity in signals from FM radio stations that transmit
at identical frequencies should be addressed in future work.
After the Rumoi earthquake, EQ echoes continued intermittently
until December 24. An earthquake of M j = 4.5 occurred on December 19 beneath the Hidaka Mountains, and some of the EQ echoes
observed prior to December 19 might have been related to this
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Figure 4. Examples of EQ echoes transmitted from the HOO FM radio station and recorded at TES on September 4, 22 days before the 2003 Tokachi-oki
Earthquake (M j = 8.0).
earthquake. After this earthquake, however, EQ echoes continued
on December 21 and 24. Although no M j > 4 earthquake occurred
in either the Hidaka or Rumoi areas until 2005 January 23, repeated
very shallow M j < 3 aftershocks occurred in the source area of
the Rumoi earthquake, which may explain this EQ echo activity.
As described in the next chapter, EQ echoes seem to be especially
sensitive to shallow seismic events.
S TAT I S T I C A L C H R A C T E R I S T I C S O F
T H E T O TA L D U R AT I O N T I M E O F E Q
E C H O E S F O R E A RT H Q UA K E S
O C C U R R I N G B E N E AT H T H E H I D A K A
M O U N TA I N S
At the ERM observatory, documentation of anomalous VHF-band
radio wave transmission from the HOO FM radio station began on
2004 May 19. The EQ echoes were observed before all of the (M j =
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Journal compilation 5.6–3.1) 16 earthquakes that occurred in the Hidaka Mountains in
2004 June and 2005 August. To describe the statistical attributes of
these EQ echoes, a time chart was proposed as Fig. 8, with Ta representing the time between the first and last echoes, Tb the quiet time
between the last echo and the earthquake and Te the total duration
of EQ echoes documented. The total duration (Te) of the echoes
and the quiet period (Tb) were analysed for earthquakes that occurred beneath the Hidaka Mountains, using the data emitted from
the HOO FM station and received at the only ∼30 km distant ERM
observatory (Fig. 9). The 1000–2000 m high Hidaka Mountains in
between prevent the direct propagation between these sites. Such
short-distance data are useful for reducing the uncertainty in identifying which earthquake is associated with the EQ echo and for
obtaining highly reliable data.
High seismicity in the crust and upper mantle beneath this area
is well known from seismometric data. Seismic events resulting
from the collision between the northeast Japan arc and the Kurile
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T. Moriya, T. Mogi and M. Takada
Figure 5. (a) Example of anomalous FM signals resulting from Geminid meteor reflections, documented at TNK on 2004 December 13. (b) Examples of
thunder and lightning records recorded at TNK, HSS, TES and ERM. Over time, activity moved from the northern station (TNK) to eastern (HSS and ERM)
and southern (TES) stations. (c) Examples of electric field strength variation caused by a sporadic E layer and recorded at TNK, HSS, TES and ERM on
2004 June 18. EQ echoes from the HOO FM station are evident between 18:00 and 20:00 at ERM.
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Figure 5. (Continued.)
arc have also been studied using 3-D tomography and controlledsource seismic surveys (e.g. Miyamachi & Moriya 1987; Iwasaki
et al. 2004).
From 2004 June to 2005 August, EQ echoes were documented
before all of the 16 earthquakes of magnitude between 5.6 and 3.1
that occurred beneath the Hidaka Mountains at depths between 106
and 27 km (Fig. 9). None of the six earthquakes, with magnitudes
from 4 to 4.9, that occurred beneath the Pacific Ocean off the Hidaka
Mountains during the same period showed EQ echoes.
EQ echoes on the HOO channel are typically expressed by a clear
increase and decrease of the electric field intensity of about 15 db
(Fig. 10). The values of Log (Te) versus magnitude (M) for the 16
earthquakes, with hypocenter depth shown as subscript, are plotted
in Fig. 11. The relative error in hypocenter depth for these events
is estimated to be 1–2 km (Harada 2004). A linear relation between
Log (Te) and M can be clearly seen for the earthquakes at the depth
of 48–54km. This relationship is approximated by the following
equation:
Log10(T e) = 1.15M − 3.45.
(1)
Pearson’s product-moment correlation coefficient for eq. (1) is
0.969. Shallower and deeper earthquakes tend to shift to longer and
shorter values of Log (Te), respectively (Fig. 11). This suggests that
the total duration time of the EQ echoes, Te, is indeed an important
parameter related to the magnitude of the impending earthquake.
Given, however, that the focal depth of an impending earthquake
cannot be estimated beforehand, its magnitude cannot be predicted
precisely from Te.
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Journal compilation It has been noted that Log (Te) has a clearer linear relationship
with the maximum intensity I of earthquake recorded in the Hidaka
Mountains as shown in Fig. 12. As will be discussed later, it seems
reasonable that the relationship is clearer because the factors causing
EQ echoes are likely some surface manifestations, which may be
related with seismic intensity more directly than magnitude. The
relationship is characterized using the following equation:
Log10(T e) = 0.68I + 0.25.
(2)
Pearson’s product-moment correlation coefficient for eq. (2) is
0.942. It should be noted that eq. (2) (and Fig. 12) holds for all the
16 earthquakes, whereas eq. (1) is for the only earthquakes with
focal depth of 48–54 km. Deviation from the line may be caused
by irregularity in the spatial distribution of intensity meters, or by
ground conditions at the locations of the meters. Thus, Log (Te)
may have a more distinct correlation with the maximum seismic
intensity than it has with magnitude, possibly making prediction
of the maximum intensity more accurate than magnitude. Eq. (2)
is held independently from depth of hypocenter and M of which
estimation depends on earth’s model. Eq. (2) suggests that Log (Te)
have a direct correlation with the intensity I (acceleration) of the
seismic wave of impending earthquake. The statistical relationship
between Log (Te) and M or I of the impending earthquake suggest
that the occurrence of EQ echoes may be a reliable earthquake
precursor phenomenon. EQ echo in generally appears clearly as
shown in Figs 4, 6 and 10, and reading error of Te may be small, and
taking logarithm of Te makes the error even smaller. A ‘strong 5’
intensity level (plotted at 5.5-intensity in Fig. 12) was documented
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T. Moriya, T. Mogi and M. Takada
Figure 6. Scattered FM radio waves documented at ERM prior to the southern Rumoi sub-prefecture earthquake, M j = 6.1 (Fig. 7). Blue traces (third channel)
record a temporal change in the electric field at 83.8 MHz, which probably originated from the Haboro FM station rather than from HOO.
Figure 7. Locations of four broadcasting stations with identical transmitting frequencies (83.8 MHz; output power in parentheses) and five observing stations.
The cross indicates the hypocenter of the southern Rumoi sub-prefecture earthquake, M j = 6.1, of 2004 December 14.
not in the Hidaka Mountains, but instead close to the epicenter of
the southern Rumoi sub-prefecture earthquake. This supports that
the statistical relationship reflected in eq. (2) may be applicable to
other areas also.
Tb for the 16 earthquakes that occurred beneath the Hidaka
Mountains was between 10 and 220 hours (approximately 0–9 days)
as shown in Fig. 13, with spans of 10–80 hours being most common.
The relationship between magnitude and Tb was not as clear as the
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Figure 8. Time chart of abnormal VHF waves received before the earthquake. Ta: activity interval of scattered waves; Tb: quiet time between the last EQ echo
and the earthquake, Te: total duration of scattered wave reception.
Figure 9. Location of earthquake epicentres, the ERM observing station,
the HOO and URA FM radio stations and the summit line of the Hidaka
Mountains (broken line). Open circles (O) denote epicentres of earthquakes
associated with EQ echoes. Crosses (X) denote epicentres of sub-ocean
earthquakes (M > 4) without EQ echoes. The focal depth and magnitude of
each earthquake are shown near its symbol.
relationship with Te, and had greater deviation. The maximum Tb
for the 16 earthquakes was 9 days. With respect to the Ta intervals
for the 16 earthquakes, no clear relationship between Ta and M or
I has been identified.
EQ echoes from URA FM station, that is located to the west of
the Hidaka Mountains (Fig. 9), were not recorded at TES (Figs 3
and 7). At HSS observatory that is located in south of Sapporo City,
none of the EQ echoes from URA were recorded (Fig. 3). These
facts seem to suggest that the scattering took place in limited narrow
areas and the scattered wave propagated in a complicated manner
above the Hidaka Mountains in the lower part of the atmosphere.
D I S C U S S I O N A N D C O N C LU S I O N S
The anomalous transmissions of VHF-band radio waves before
earthquakes were recorded using devices that can detect very weak,
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2009 The Authors, GJI, 180, 858–870
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Journal compilation scattered VHF-band radio waves. The anomalous records were documented prior to two large Hokkaido earthquakes and numerous
smaller earthquakes that occurred in the Hidaka Mountains in 2004
and 2005. No anomalous signals synchronized with the origin time
of earthquakes were observed. A clear statistical and quantitative
relationship is presented between Log10 (Te) and the magnitude and
the maximum seismic intensity of M3-M5 class earthquakes.
Te appears to be affected by numerous parameters, such as magnitude, maximum intensity, depth of hypocenter and surface conditions (land, coast or ocean) as suggested by Huang & Ikeya (1998)
and Huang (2005). The linear relationship between Log10 (Te) and
magnitude or intensity (Figs 11 and 12) suggests that this may
be a useful tool for predicting these characteristics of an impending earthquake. The EQ echo usually has a step like onset and end.
Therefore, we can easily count the total duration Te. However, sometimes, especially before large EQ, the EQ echo has not clear onset
and end. Te in such case may have some count error. However, the
unclear onset part is not so long in comparison with Te so that it did
not seriously affect the statistical relationship.
Kushida & Kushida (1998, 2002) proposed that a quantitative
relation exists between the total anomalous VHF-band radio wave
propagation time and the rupture length and they estimated the
magnitude of impending earthquake based on an empirical relationship derived by Utsu (1969). They did not note any empirical
relationship between the total anomalous VHF-band radio wave
propagation time and the maximum seismic intensity or effect of
hypocenter depth. They had only four observation stations for all
of Japan and used obsolete FM tuners, with tuning deliberately
shifted by about 0.1 MHz to target FM service stations; they also
oriented the Yagi-Uda antennas vertically and set their FM tuners
to long-distance FM radio stations. These characteristics may have
resulted in the loss of data related to maximum seismic intensity
and hypocenter depth.
The quiet interval, Tb, between the last EQ echo and the earthquake occurrence was 1–9 days, most commonly in the range of
1–5 days. It is, however, not possible to real time identify which
EQ echo is the very last one and, therefore, to predict precisely
when the earthquake will occur. In spite of this drawback, Tb being
1–9 days can be a highly useful piece of information once the last
EQ echo has been even only guessed. Given that the shortest value
of the quiet period Tb was about 10 hours, this information could be
also used as the advance warning time for an earthquake for which
maximum seismic intensity is estimated from the real time Log10
(Te) data.
EQ echoes may not be produced by the earthquake source itself, but are a secondary effect caused mainly by an interaction
between the ground and the atmosphere. This is consistent with the
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T. Moriya, T. Mogi and M. Takada
Figure 10. Example records of EQ echoes from the HOO FM radio station recorded at ERM before the earthquake in the Hidaka Mountains.
observation that scattering of the VHF-band radio wave propagates
over a limited area and occurs at a low altitude, probably in the
atmosphere, suggesting EQ echoes seem to be independent from
ionosphere disturbance. This would suggest that a denser observation network collecting simultaneously appearing EQ echoes at
different locations would help to delineate both the area affected
by transmission and the epicenter location. It may also be possible
to estimate the magnitude of an impending earthquake based on an
empirical formula (2).
The quantitative relationship between Te and the magnitude of
an impending earthquake implies that the origin of the signals relates to the size of the area ruptured in the fault plane, because
large magnitude earthquakes rupture larger fault plane areas (Utsu
1969). Microfracturing may occur in the area of the fault plane
about to be ruptured. Enomoto et al. (1997) proposed that thermal
exo-electron emission and/or fracto-electron emission from fracturing rocks during the precursory stage of the main shock may
electrify gases that are released at depth. Such electrified gases may
generate electric potential difference along the fault zone, leading
to electric discharge, which produces the seismic electromagnetic
signal. In fact, anomalous pulses of geo-electric current have been
observed from August 27 to October 4 at the Erimo observatory,
associated with the Tokachi-oki earthquake (Enomoto et al. 2006).
Kamogawa & Otsuki (1999) proposed a model to explain how the
higher frequency EM waves propagate in the ground and how the observed longitudinal plasma waves are excited by exo-electrons prior
to the earthquake at the rough surface. There are two commonly
cited mechanisms to explain the generation of electromagnetic
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2009 The Authors, GJI, 180, 858–870
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Journal compilation Pre-seismic transmission of FM radio waves
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Figure 11. Log (Te) for EQ echoes from HOO recorded at ERM versus magnitude (M) of associated earthquakes beneath the southern Hidaka Mountains.
Numerals represent the focal depths in kilometres: • = crust, = plate boundary, = mantle. The line denotes an experimental linear relationship between
Log (Te) and M for earthquakes at the plate boundary.
Figure 12. Log (Te) versus the maximum seismic intensity (I, defined by
the Japan Meteorological Agency). An intensity of 5.5 was recorded for the
southern Rumoi sub-prefecture earthquake. The line illustrates the linear
relationship between Log (Te) and I.
signals associated with earthquake; they are electrokinetic model
(e.g. Mizutani et al. 1976) and piezoelectric effect (e.g. Huang
2002). In addition to them, Freund (2000) reported that, in laboratory experiments, mobile positive-charged holes appeared on the
rock surface under stress gradient conditions, and that this might
imply the appearance of a positive-charged area at the earth’s surface before earthquakes occur. This candidate mechanism is likely
to generate electric charges associated with microfracturing at the
source area of earthquakes and may transport the charges to the
surface. The accumulated electric charges generate an atmospheric
electric field near the surface and charged scattering body in the
atmosphere, causing anomalous FM radio transmissions. Pulinets
et al. (1997) described how radon and metallic aerosol emanate and
form an atmospheric electric field. Although many electromagnetic
precursors have been observed, the validity of the above mechanisms has not yet been confirmed.
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Journal compilation Figure 13. Tb (quiet period in hours before the earthquake shown in Fig. 7)
versus M for earthquakes in the Hidaka Mountains.
The characteristics of EQ echoes provide direct evidence that
earthquake precursor events exist. Although the geophysical cause
of these radio wave events is not clearly understood, pre-seismic EQ
echoes indisputably take place and may be useful for forecasting
earthquake occurrences.
AC K N OW L E D G M E N T S
The authors are grateful to Y. Kushida, who stimulated our interest.
We are also indebted to Prof. M. Kasahara, Institute of Seismology and Volcanology, Hokkaido University, for his encouragement
throughout our work. We would also like to thank Prof. I. Yamamoto
of Okayama University of Science, H. Baba of Tokai University and
T. Yoshida of Hiroshima City University for their help in the laboratory and fieldwork. We are grateful to Y. Koizumi and M. Uehara
of Tohoku University, H. Hojo and A. Namihana of Hokkaido University and M. Yamada of Nagoya University for their cooperation
in the field work. We gratefully acknowledge Prof. Emeritus S.
Uyeda of University of Tokyo for his encouragement and advice
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T. Moriya, T. Mogi and M. Takada
on improving the manuscript. We appreciate Kotomi Nishiwaki of
Hokkaido University for organizing the data.
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