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Possible precursor to the March 11, 2011, Japan earthquake:
ionospheric perturbations as seen by subionospheric very low frequency/
low frequency propagation
Masashi Hayakawa1,2,3,*, Yasuhide Hobara2,4, Yoshihiro Yasuda3, Hiroki Yamaguchi5, Kenji Ohta6,
Jun Izutsu6, Tohru Nakamura7
1University of Electro-Communications, Advanced Wireless Communications Research Center, Tokyo, Japan
2University of Electro-Communications, Research Station on Seismo-Electromagnetics, Tokyo, Japan
3University of Electro-Communications, Hayakawa Institute of Seismo-Electromagnetics Co. Ltd., Tokyo, Japan
4University of Electro-Communications, Graduate School of Informatics and Engineering, Tokyo, Japan
5Earthquake Analysis Laboratory, Information Systems Inc., Minato-ku, Tokyo, Japan
6Chubu University, Kasugai Aichi, Japan
7Kochi University, Akebono Kochi, Japan
ANNALS OF GEOPHYSICS, 55, 1, 2012; doi: 10.4401/ag-5357
ABSTRACT
This study reports on a possible very low frequency/low frequency
(VLF/LF) subionospheric precursor to a recent earthquake in Japan. As the
epicenter of this large Japanese earthquake on March 11, 2011, was located
just on the great-circle path from one of our VLF/LF network stations
(Chofu) to the NLK US transmitter, we examined the propagation
characteristics mainly associated with the signals from the NLK transmitter,
as observed at three of the stations in Japan (Chofu, Kasugai and Kochi). On
March 5 and 6, 2011, a remarkable anomaly was found on the path from
NLK to Chofu, which is highly likely to have been a precursor to this
earthquake. The anomaly in the night-time average amplitude at Chofu was
characterized by a serious decrease in the signal that exceeded −4v(v:
standard deviations). The anomaly was found on the same days on the other
propagation paths (from NLK to both Kasugai and Kochi), although it was
less enhanced. Finally, this propagation anomaly is extensively discussed
with respect to the geomagnetic activity, and we also compare this anomaly
with the properties related to the former 2004 Sumatra earthquake that had
nearly the same magnitude as this March 11, 2011, earthquake.
1. Introduction
Very low frequency/low frequency (VLF/LF)
subionospheric propagation data have recently been used
extensively to monitor lower ionospheric perturbations
associated with earthquakes [e.g., Hayakawa 2007,
Hayakawa 2009, Hayakawa 2010, Chakrabarti 2010]. There
have thus been a substantial number of studies published on
such seismo-ionospheric perturbations. A recent study by
Hayakawa et al. [2010] established a statistically significant
correlation of ionospheric perturbations detected as VLF/LF
subionospheric propagation anomalies with earthquakes
with large magnitudes (>6.0) and shallow depths. This was a
statistical study on the basis of long-term (seven years) data
recorded in and around Japan. That study can be considered
as further confirmation of our previous statistical studies
[Gokhberg et al. 1989, Rozhnoi et al. 2004, Maekawa et al.
2006, Kasahara et al. 2008], which examined shorter periods
of data, of the order of a few years.
Case studies of different large earthquakes are also of vital
importance for investigations into the temporal and spatial
characteristics of seismo-ionospheric perturbations. As was
recently described in a review by Hayakawa [2009], these case
studies include: (i) the Izu peninsula March 1997 earthquake
swarm; (ii) the September 25, 2003, Tokachi-oki earthquake;
(iii) the October 23, 2004, Niigata-chuetsu earthquake; (iv) the
1999 Chi-chi earthquake in Taiwan; and (v) the 2004 Sumatra
earthquake; among others. The present study concerns the
latest earthquake in Japan, on March 11, 2011.
2. The 2011 Tohoku earthquake
There was an extremely large earthquake (magnitude,
9.0) under the sea bed in the Pacific ocean off the Tohoku
area of Japan, which is formally named as the 2011
earthquake off the Pacific coast of Tohoku. This earthquake
took place at 14:46:18 LT on March 11, 2011, with its
Article history
Received August 9, 2011; accepted September 29, 2012.
Subject classification:
Earthquake prediction, Ionospheric perturbation, VLF/LF propagation.
95
Special Issue: EARTHQUAKE PRECURSORS
epicenter at the geographic coordinates (36˚6.2N,
142˚51.6E), as shown in Figure 1, and with a depth of ca. 20
km. This earthquake was a very typical oceanic earthquake
of the plate type, which are very different from fault-type
earthquakes, such as the Kobe earthquake, the Niigata-
chuetsu earthquake, and so it was of great concern.
3. The Japanese VLF/LF network
The Japanese and Pacific network for subionospheric
VLF/LF propagation was established just after the 1995
Kobe earthquake, within the framework of the former
NASDA frontier project (Hayakawa et al., 2004). The main
observatories involved at present are: (1) Moshiri (MSR) in
Hokkaido; (2) Chofu (CHF) and (3) Kasugai (KSG) near
Nagoya; and (4) Kochi (KCH) and (5) Tsuyama (TYM),
Okayama, as shown by the stars in Figure 1 (except for
TYM). Some additional observatories are planned to be built
shortly. At each station, we normally detect the signals from
the two Japanese transmitters with call signs of JJY (in
Fukushima; 40 kHz) and JJI (in Miyazaki, Kyusyu; 22.2 kHz),
as shown by the diamonds in Figure 1. We can also detect
the signals from a few foreign transmitters (e.g., NWC in
Australia; NPM in Hawaii; and NLK in the US). The details
of this VLF/LF network can be found in Hayakawa et al.
[2004] and Hayakawa [2007, 2009, 2010].
4. Observational results and analysis method
Figure 1 illustrates the path from JJY to MSR, along
with its corresponding 5th Fresnel zone, as the wave
sensitive area, and the three paths from NLK (Seattle, USA)
to the other Japanese observatories (CHF, KSG and KCH).
Furthermore, the 5th Fresnel zone for the propagation path
from NLK to CHF is shown, which is the wave sensitive area
for this path and which is much larger than that for the path
from JJY to MSR, as the NLK–CHF propagation distance is
much larger than that for the JJY–MSR path. These wave
sensitive areas mean that any earthquakes that take place
within the wave sensitive area can have a certain significant
influence on the signals received at the observatory, which
is termed a propagation anomaly (either in amplitude or in
phase, or for both).
The analysis in this study follows the night-time
fluctuation method [e.g., Hayakawa et al. 2010], which
focuses only on the night-time amplitude data. We
monitored the temporal evolution of amplitude A(t) at a
current time t during the local night-time on a particular day,
while <A(t)> was estimated as the average amplitude at the
same time t during the period from one day to 30 days before
the current day. Then, we estimated the difference dA(t) =
A(t) —<A(t)>. Using this difference, we can estimate the
most important parameter, the 'trend', as the night-time
average amplitude (mean of the dA(t) during the local time).
The second parameter is the dispersion, which is
characterized by how much the amplitude fluctuates around
the average, and the third parameter is the integration of
dA(t) <0 during the night, the night-time fluctuation. All of
these parameters are normalized by their corresponding
standard deviations over —30 days to —1 day before the
current day. Further details can be found in Kasahara et al.
[2008] and Hayakawa et al. [2010].
For the definition of the night-time period, we take the
UT period of UT = 11 h —19 h for the propagation path from
JJY to MSR. However, the definition of night-time is
considerably complex for the east-west long-distance
propagation from NLK to the Japanase stations (such as
CHF). By considering the sunrise and sunset both at the
transmitter and at the receiving observatory (i.e., the
terminator times) [Hayakawa et al. 1996] and also by
checking the actual diurnal variations for the relevant NLK–
CHF path, we took UT = 9h to 14h for the night-time for the
NLK-CHF path (i.e., the only period when the propagation
path was completely in the dark).
4.1. No precursor propagation anomaly for the JJY–MSR path
As shown in Figure 1, a previous earthquake known as
the 2005 Miyagi-oki earthquake (August 16, 2005; magnitude,
7.2) occurred very close to the wave sensitive area of the
JJY–MSR propagation path (with the same magnitude as the
foreshock on March 9, 2011). Here, we observed very
significant precursor ionospheric perturbations on this
propagation path [Muto et al. 2009].
However, the epicenter of this March 11, 2011,
earthquake was shown to be located considerably far from the
HAYAKAWA ET AL.
96
Figure 1. Relative locations of the JJY transmitter and the Moshiri (MSR)
station, with the wave sensitive area. Also, the great-circle paths for the
NLK (US transmitter) to Japanese receiving stations of Chofu (CHF),
Kasugai (KSG) and Kochi (KCH) are shown. The corresponding wave
sensitive area is only shown for the NLK–CHF path. The epicenters of the
three relevant earthquakes are shown: (i) the August 16, 2005, Miyagi-oki
earthquake (2005/8/16); and (ii) the March 9, 2011, foreshock, and the
March 11, 2011, main shock earthquakes (2011/3/9,3/11).
97
wave sensitive area of the JJY–MSR path, as this earthquake
occurred ca. 150 km away from the coast line. This might
suggest that we would not expect any perturbation on the JJY–
MSR path for this earthquake. Figure 2 illustrates the temporal
evolution of the propagation characteristics over the JJY–MSR
path during the time period up to this March 11, 2011,
earthquake. We pay attention only to the period from March
1 to March 9, so before the earthquake. In this period there
was definitely no time interval in which the trend showed a
decrease, together with the simultaneous increases in the
dispersion and the night-time fluctuation parameters.
Hayakawa et al. [2010] indicated that a propagation anomaly
is characterized by a decrease in the trend, with simultaneous
increases in the dispersion and the night-time fluctuation
parameters. The experimental observation of no precursory
anomaly appears to be very consistent with the initial
theoretical expectation, as judged from the position of this
earthquake relative to the propagation path.
4.2. Significant propagation anomalies associated with the
propagation paths for the US transmitter NLK
Figure 1 suggests that the propagation paths from
Japanese receiving stations (CHF, KSG and KCH) to the US
NLK transmitter are favorably located with respect to the
epicenter of this March 11, 2011, earthquake. In particular, the
NLK–CHF path passed just above the earthquake epicenter,
and the corresponding wave sensitive area for this NLK–CHF
path is shown as a dotted line. Two other propagation paths,
from NLK to KSG and from NLK to KCH, were also favorable
for us to note any corresponding ionospheric perturbations.
In response to these theoretical expectations, Figure 3a-c
illustrates the actual temporal evolutions of the propagation
IONOSPHERIC PERTURBATION FOR THE 2011 JAPAN EARTHQUAKE
Figure 2. The temporal evolution of the propagation characteristics for the
path of JJY–MSR during the period from January 1 to March 12, 2011. Pay
attention only to the period from March 1 to March 12 to define any
precursor to the March 11, 2011, earthquake. Top: trend (average night-time
amplitude). Middle: dispersion (as a measure of the amplitude fluctuation).
Bottom: night-time fluctuation, as defined by the night-time integration of
the area of dA(t)<0. The gray areas indicate periods with no observations.
Figure 3. Temporal evolution of the propagation characteristics for the
three propagation paths: (a) NLK–CHF; (b) NLK–KCH; and (c) NLK–KSG.
The period from January 1 to March 12, 2011, is shown. Pay attention to
the period from March 1 to March 12, to define any precursor to the March
11, 2011, earthquake. (a, b, c) Top: trend (average night-time amplitude).
Middle: dispersion (as a measure of the amplitude fluctuation). Bottom:
night-time fluctuation, as defined by the night-time integration of the area
of dA(t)<0. Any significant anomaly should be indicated in (a), (b) and (c).
characteristics for these relevant paths; Figure 3a shows the
NLK–CHF path, Figure 3b, the NLK–KCH path, and Figure
3c, the NLK–KSG path. In each panel of Figure 3 the three
physical parameters are shown: from top to the bottom, the
trend, dispersion and night-time fluctuation, and these
parameters are all normalized by their corresponding
standard deviations (v). With the propagation path of NLK–
CHF in Figure 3a, we can pay attention to the period before
the earthquake on March 11, 2011. We can clearly note a
significant propagation anomaly on the two days of March 5
and 6, 2011. In particular, the propagation anomaly on
March 5 is characterized by a remarkable decrease in the
trend (exceeding —3v, or even more), together with
simultaneous increases in the second parameter (dispersion)
and in the night-time fluctuation. The corresponding
anomaly can also be recognized in Figure 3b for the
propagation path of NLK–KSG. The anomaly for the path
of NLK–KCH in Figure 3b is particularly evident because the
most important parameter, the trend, showed a significant
decrease, reaching the —2vlevel. On the other hand, the
anomaly for the path of NLK–KSG in Figure 3c is less
enhanced on the same days of March 5 and 6, 2011, although
the response to this earthquake is very evident.
5. Summary and discussion
By making full use of our Japanase VLF/LF network,
we have defined the following observational results for this
large March 11, 2011, earthquake:
1. No definite anomaly was observed for the JJY–MSR
path, as the earthquake epicenter was well away from the
wave sensitive area of this propagation path.
2. There was a significant propagation anomaly on
March 5 and 6 (about 5-6 days before the earthquake)
especially for the NLK–CHF path. This anomaly is
characterized by a remarkable decrease in the trend, together
with clear enhancements in the dispersion and the night-time
fluctuation. The same anomaly was also observed for the
other two paths: NLK–KCH and NLK–KSG.
3. This earthquake was a consequence of an oceanic
earthquake that was due to the movement of the plates, so
that the result in the present study is the first report for any
large plate-type earthquake in the sea.
We also discuss further the above summary. First of all,
we ask whether the anomaly on March 5 and 6, 2011, was
really a precursor to the March 11, 2011, earthquake. The
most serious point for this problem is that the geomagnetic
activity might have some influence on our summary. Figure 4
illustrates the temporal evolution of geomagnetic activity
measured according to RKp (the daily sum of the Kp index)
during the relevant period, from late February to around
March 10. As can be seen from Figure 4, the geomagnetic
activity on March 5 and 6, 2011, when the anomaly was
observed, was relatively quiet (of the order of RKp = 10-15),
although we note some enhancement of the geomagnetic
activity in late February and around March 10. Thus, it is
likely that the geomagnetic activity has nothing to do with
this earthquake. As summarized as point (2) above, we found
a significant propagation anomaly on the path of NLK–CHF.
This means that the earthquake responsible for this anomaly
might be located at any position along the path of NLK–CHF,
as we have no propagation paths crossing these NLK–Japanese
observatories. After having examined the earthquakes over the
whole area of this propagation path, we were not able to find
any earthquakes on the path during the propagation path.
Thus, the propagation anomaly on March 5 and 6, 2011,
shown in Figure 3 is very likely to be a precursor to the
March 11, 2011, earthquake. The lead time for this earthquake
was about 5-6 days, which appears to be consistent with the
corresponding statistical results for land earthquakes in and
around Japan reported by Hayakawa et al. [2010].
It is interesting for us to compare these characteristics
of the ionospheric perturbations for this earthquake with
those of an earthquake with nearly the same magnitude; i.e.,
the 2004 Sumatra earthquake with a magnitude of 9.1 [Horie
et al. 2007a, b]. On the basis of the ground-based observations
in Japan [Horie et al. 2007a, b] for the propagation paths
associated with the Australian NWC transmitter, and also on
the satellite observations of whistler-mode signals from the
same transmitter [Molchanov et al. 2006], we found that the
radius of the ionospheric perturbations was of the order of
2.5 Mm for the Sumatra earthquake. This earthquake was
also an oceanic earthquake of the plate type, so the same
type as the March 11, 2011, Tohoku earthquake. However,
the most important difference between these two
HAYAKAWA ET AL.
98
Figure 4. Temporal elevation of the geomagnetic activity (Kp index) during the relevant period for the March 11, 2011, earthquake.
99
earthquakes is that the Sumatra earthquake occurred very
close to the land area of Indonesia (so we would expecting a
lot of influence on the land area), whereas the Tohoku
earthquake occurred far from the coast line (ca. 150 km), and
thus right in the ocean. If we assume the same spatial scale
for this March 11, 2011, earthquake as for the Sumatra
earthquake, we would expect any significant perturbations
to extend even to the path of JJY–MSR, but Figure 2 indicates
that there was no anomaly at all on this path. This means
that the spatial scale of ionospheric perturbations for the
March 11, 2011, earthquake was considerably smaller, as
compared with that for the 2004 Sumatra earthquake. Also,
we can assume that this is probably closely related with the
fact that the earthquake occurred completely in the sea, so
that there are no such significant effects on the ionosphere
due to the presence of the sea water.
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*Corresponding author: Masashi Hayakawa,
Advanced Wireless Communications Research Center,
University of Electro-Communications (UEC), Chofu, Tokyo, Japan;
email: hayakawa@whistler.ee.uec.ac.jp.
© 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights
reserved.
IONOSPHERIC PERTURBATION FOR THE 2011 JAPAN EARTHQUAKE
