Ionospheric Anomalies Appear Before the 12 May
2008 Mw7.9 Sichuan Earthquake
J. Y. Liu1,2, Y. I. Chen3, C. H. Chen1, C. Y. Chen1, M. Nishihashi4, K. I. Oyama1, and
K. Hattori4
1Institute
of Space Science, 2Center for Space and Remote Sensing Research,
National Central University, Taiwan
3Institute
of Statistics, National Central University, Taiwan
4Graduate
School of Science and Technology, Chiba University, Japan
The total electron content (TEC) of Global ionospheric map (GIM) and
electron density profiles probed by six micro satellites of FORMOSAT3/COSMIC (F3/C) are for the first to simultaneously observe seismo-ionospheric
anomalies during an M7.9 earthquake in Sichuan, China on 12 May 2008. It is
found that ionospheric electron densities above the fore coming epicenter
anomalously decrease in the afternoon period of day 6 to 4 and the late evening
period of day 3 before the earthquake.
Results further show that the
ionospheric F2 layer significantly descends about 50-80km and the GPS TEC
anomalously reduces about 1650km and 3300km in radius from the epicenter in
the latitudinal and longitudinal direction, respectively.
An earthquake with magnitude Mw7.9 occurred in eastern Sichuan, China
(30.986°N, 103.364°E, Depth 19km) at 06:28:01 UT on Monday 12 May 20081. The
global ionosphere map2 (GIM) of the total electron content (TEC) constructed with
1000s of worldwide ground-based receivers of the global positioning system (GPS)
1
and routinely published in a 2-hour time interval is used to observe anomalous
signatures induced by the earthquake. Here, the TEC is integrated the ionospheric
electron density between a ground-based receiver and a GPS satellite at about
20200km altitude. The spatial resolutions of the GIM on the ±87.5oN latitude and ±
180oE longitude are 2.5° and 5°, respectively. Therefore, each map consists of 5040
(=70x72) grid points (Fig. 1). Similar to a Geostationary Meteorological Satellite
hourly observing clouds for the metrological weather, the GIM now is employed to
observe signatures of the lithospheric, atmospheric, and ionospheric weather (such as
thunderstorm, ionospheric storm, and earthquake). For each grid point, we compute
the median of the 1-30 days before the earthquake (12 April to 11 May 2008) and
difference of each observation day to the median. Figure 1 displays the GIMs at
08:00 UT and at global fixed local time of 15:00LT on day 6 before the earthquake (6
May 2008), the associated median, and the extreme reduce difference on 6 May 2008.
It is interesting to see that GPS TECs above the fore coming Sichuan epicenter on 6
May 2008 significantly and extremely decrease. Note that the median represents the
background while the extreme reduction stands for possible earthquake related
anomaly.
We focus on temporal variations of the GPS TEC above the epicenter isolated
from the GIM database during the earthquake. Figure 2 displays that the ionospheric
GPS TEC significantly decrease in the afternoon period on 6, 7, and 8 May (6-4 days
before the earthquake) as well as in late evening of 9 May (3 days before). To find
spatial distributions of these anomalies, sequences of GIMs at 06:00 and 14:00 UT
(13:00 and 21:00 LT at Sichuan) of the 30-day period (12 April to 11 May 2008) are
examined. Figure 3 shows that the decrease GPS TECs and the extreme reduction
2
differences generally appear around the Sichuan epicenter at 06:00UT on 6, 7, and 8
May 2008, as well as at 14:00UT on 9 May 2008. The local time of 06:00 and
14:00UT at the Sichuan area are the afternoon period of 13:00LT and the late evening
period of 21:00 LT, respectively. Taking into account local time effects, sequences
of GIMs at the fixed global times of 13:00 and 21:00LT on the four days are also
studied by the same process. We again confirm that the decrease GPS TECs and the
extreme reductions appear around the epicenter in the afternoon period of day 6-4 and
late evening period of day 3 before the Sichuan earthquake (Fig. 3).
Six microsatellites of the joint Taiwan-US satellite constellation mission, termed
FORMOSAT-3/COSMIC, were successfully launched in to a circle low Earth orbit at
01:40 UT on 15 April 20063. Each satellite houses a GPS occultation experiment
(GOX) payload (space-based GPS receiver) applying a powerful technique of the
atmospheric radio occultation4 to globally derive the vertical profile of electron
density in the ionosphere. Figure 4 reveals the electron density profiles observed
over the epicenter during the afternoon period of 13:00-17:00LT on 6, 7, and 8 of
May and the late evening period 21:00-24:00LT on 9 May as well as their associated
references of the median, lower quartile, and upper quartile of the 15-day period from
21 April to 5 of May 2008, which is day 7-22 before the earthquake. The satellite
results confirm that the electron density of the ionospheric F2-peak NmF2 in the
afternoon period on 6, 7, and 8 of May and in the late evening period of 9 May are
significantly less than their the median and lower quartile. Moreover, it can be seen
that the F2-peak height descends from about 300 km down to 200s km altitude which
is approximately 50-80 km lower than the associated median, on the 4 anomalous
days.
3
Large earthquakes are often preceded or accompanied by signals of different
nature: electric, magnetic, electromagnetic, or luminous, although seismic waves are
the most obvious manifestation5,6.
Recently, ionospheric NmF2 and GPS TEC
anomalous variations appeared before earthquakes have received considerable
discussions7-10. The NmF2 is the greatest electron density of the ionosphere while the
TEC is the integration of the ionospheric electron density, and the NmF2 and TEC
generally are highly correlated11. A statistical investigation demonstrates that the
abnormal decrease of the ionospheric NmF2, in term of the corresponding plasma
frequency foF2, in the afternoon period of 12:00-18:00LT occurs significantly within
1-5 days before 184 M5.0 earthquakes during a 6-year period of 1994-1999 in the
Taiwan area12.
An extension work shows that the ionospheric GPS TEC
pronouncedly reduces in the afternoon period of 12:00-18:00 LT and especially
evening period of 18:00–22:00 LT within 5 days prior to 20 M6.0 earthquakes in
Taiwan during September 1999 -December 200213. A study of GIM data during 160
worldwide M7.0 earthquakes in January 1999 - May 2008 confirms that the GPS
TECs above the epicenters significantly decrease in the afternoon period day 1-5
before the earthquakes14. The existing results suggest that the GPS TEC decrease
anomalies in the afternoon and evening periods of 6-9 May 2008 are related to the
Sichuan earthquake.
The statistical study of 184 M5.0 earthquakes shows that the reduction anomaly
of the ionospheric electron density appears more often before larger earthquakes, but
less likely away from the epicenter12. On the other hand, the spatial analyses reveal
that the ionospheric GPS TECs centered on the epicenters notably decrease day 3-4
before the 20 September Mw 7.6 Chi-Chi earthquake15 and day 5 prior to the 24
4
December 2004 Mw9.3 Sumatra earthquake16. It can be seen that the GPS TECs
once again significantly decrease about 10-15° in latitude and 15-30° in longitude
from the epicenter in the afternoon and evening periods on day 3-6 (6-9 May 2008)
before the Sichuan earthquake. Since 1° is corresponding to about 110 km, the
anomalous size of the Sichuan earthquake is 1650km (=110km/ ° × 15 ° ) in the
latitudinal and 3300km (=110km/°×30°) in the longitudinal direction, respectively.
On the other hand, in the lithosphere the earthquake preparation area can be estimated
by R =100.43M, where R is the radius of the earthquake preparation zone, and M is the
earthquake magnitude17. For the Mw7.9 Sichuan earthquake, we obtain R = 2495 km,
which is about the similar size in the latitudinal but two times greater in longitudinal
direction of the anomaly size observed in this paper. On the other hand, the F3/C
GOX observes that tremendous amount of the electron density disappears from the
upper part of the ionosphere on those four anomalous days. Comparing to the
associated median, the electron density of NmF2 decreases more than 30-50% and
the height of HmF2 descends about 50-80 km on the anomalous days. Note that it
shall need enormous power and/or energy to modify the ionosphere in the area of
1650km in the latitudinal and 3300km in the longitudinal direction.
Very colorful earthquake clouds appear around the epicenter several tens
minutes before the Sichuan earthquake onset18,19.
This indicates that the
electromagnetic environment has been significantly changed and seismoelectromagnetic signals (SESs) have been activated around the fore coming epicenter
area during the earthquake preparation period. Due to abundance of electrons, the
ionosphere above about 100km becomes highly conductive20. Here the lithosphere
and the ionosphere could act as a concentric conducting sphere and shell, and
5
therefore any electromagnetic anomalies, such as currents, charges, vertical electric
field etc., appearing on Earth’s surface generated during the earthquake preparation
period can easily and quickly map to and affect the ionosphere by modifying the
spatial distribution of the electron density within.
Although the SESs were not
directly measured, the observed colorful earthquake clouds indicate either vertical
electric field and/or broadband electromagnetic emissions on the Earth’s surface
being important.
The dip angle of the Earth’s magnetic field around Sichuan
earthquake (30.986°N, 103.364°E) is about 45°, and therefore an upward plasma flow
(polar wind like) due to upward penetration electric fields20 and/or a plasma
expansion due to electromagnetic emissions can easily result in the anomalous
decrease of the electron density in upper part of the ionosphere during the earthquake
preparation period.
Acknowledgements
This research was partially supported by the Ministry of
Education Grant 91-N-FA07-7-4, the iSTEP project, to National Central University.
6
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Figure legends
Figure 1 | The GIMs observed 6 days before the Sichuan Earthquake. The left
and right columns are GIMs at 08:00 UT and global fixed local time of 15:00LT,
respectively.
The top panels are observed on 6 May 2008 (day 6 before the
earthquake), the second top panels are the median of the period of 12 April - 11 May
2008 (1-30 days before the earthquake), and the third top panels denote the extreme
minima of the 30 day appear on 6 May 2008. The color denotes the difference of the
TEC observed on 6 May 2008 from the associated 30-day median. It can be seen that
the ionospheric GPS TECs around the Sichuan epicenter marked by the circle dashed
circle significantly reduce. The bottom panels are the magnified of the difference
between 6 May 2008 and the associated median. The red grids denote the 30-day
extreme minima. The GIM grid points lie between ±87.5oN and ±180oE with 2.5°
and 5° grid intervals in the latitudinal and longitudinal directions, and therefore each
map has 5040 (=70x72) grid points in total.
Figure 2 | A time series of GPS TEC right above the Sichuan epicenter extracted
from GIMs during the 2008 M7.9 Sichuan Earthquake. The red curve is the
observation while the two dark curves are the low and upper bounds. The low bound
(LB) and upper bound (UB) are constructed by the 1-15 pervious days moving
median (M), lower quartile (LQ), and upper quartile (UQ). Here, LB=M-1.5(M-LQ)
and UB=M+1.5(UQ-M). It can be seen that the GPS TEC anomalously reduces
during 06:00-10:00UT (the afternoon period of 13:00-17:00LT) on 6, 7, and 8 May as
well as 14:00-17:00UT (the late evening period of 21:00-24:00LT) on 9 May 2008.
The blue dots denote the extreme minima of the 30 day period before the earthquake.
9
Figure 3 | The ionospheric GPS TEC anomalies appear on the GIMs 6 to 3 days
prior to the Sichuan Earthquake. The left and right columns are GIMs of the
universal time and global fixed local time, respectively. The top three panels are the
difference between the observations and the associated medians at 06:00UT and
global fixed 13:00LT on 6, 7, and 8 May 2008 (from top to down). The color denotes
the difference of the TEC on the observation day from the associated 30-day median,
and the red grids represent the 30-day extreme minima. The bottom panels are at
14:00UT and global fixed 21:00LT on 9 May, 2008.
It can be seen that the
ionospheric GPS TECs around the epicenter significantly reduce.
Figure 4 | The ionospheric electron density above the epicenter observed on 6 to
3 days before the Sichuan Earthquake by FORMOSAT3/COSMIC satellites.
The panels from top to down are the electron density vertical profiles observed during
13:00-16:00LT on 6, 7, and 8 as well as 21:00-24:00LT on 9 May 2008, respectively.
The red curves represent the observed profiles while the solid and dashed curves in
each panel are the associated median and upper/lower quartiles at the same local time
period during 21 April – 5 May 2008 (7 to 22 days before the earthquake). It
confirms that the electron density significantly decreases on 6 to 3 days before the
earthquakes.
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Figure 1.
11
P
P
P
P
←M7.9
Figure 2.
12
Figure 3
13
Figure 4
14