The present paper reports the analysis of GPS based TEC measurement corresponding to 5 earthquakes (M > 6.0) that have occurred in 2009 and 2010 during low solar activity period and quiet geomagnetic conditions. The No. of satellites registering high TEC at low elevation angles is observed prior to these earthquakes. The earthquakes are 1) Loyalty Island (22 o36'S, 170 o54'E) M-6.6 on 19 January 2009, 2) Izu Island (32 o48'N, 140 o36'E) M-6.6 on 12 August 2009, 3) Ryukyu Islands (23 o30'N, 123 o30'E) M-6.7 on 17 August 2009, 4) Haiti (18 o24'N, 72 o30'W) M-7.0 on 12 Jan 2010, 5) Chile (34 o12'S, 71 o48'W) M-6.9 on 11 March 2010. To identify anomalous behavior of TEC, we compute interquartile range associated with median TEC so as to get upper and lower bound of TEC variation. The results show occurrence of anomalies in VTEC 1 - 13 days prior to the earthquake. The VTEC shows enhancement for all the earthquakes. Enhancement in No. of satellites registering high TEC at low elevation has been observed prior to earthquakes.
Extensive researches have resulted in the recognition of many earthquake precursors in the field of the lithosphere, atmosphere and ionosphere. The effect of pre-seismic activity on ionosphere can be investigated by using the ionosphere electron density. The variation of electron concentration related to earthquake has been reported by Ondoh [
The present paper reports the analysis of TEC data obtained from five different IGS Stations. These data are for the period corresponding to five different earthquakes that have occurred in different parts of the world.
The atmospheric refractivity is proportional to the pressure and the temperature gradient of the lower atmosphere. Prior to an earthquake, vertical profile of humidity, pressure and temperature changes [
In this paper, we take 5 earthquakes into consideration: Loyalty Island, Izu Islands, Ryukyu Islands, Haiti and Chile.
The RINEX data obtained from GPS receivers, contain the C1 (C/A code pseudo range, in meters, on L1 frequency), P2 (P code pseudo range, in meters, on L2 frequency), L1 (L1 carrier phase, in cycles, on L1 frequency) and L2 (L2 carrier phase, in cycles, on L2 frequency) with a time resolution of the 30 s.
The slant total electron content (STEC) estimated, from an IGS data set in RINEX format, as
where
STEC converted into VTEC using a suitable mapping function of different ionosphere pierce point (IPP) locations. The mapping function S(E) is defined as,
Sr. No | Epicenter (Lat, Long) and MAGNITUDE of Earthquake | Date | Location of IGS Station | Distance from Epicenter (KM) |
---|---|---|---|---|
1 | Southeast of the Loyalty Island (22˚36'S, 170˚54'E) M 6.6 | 19/1/2009 | Noumea France (22˚12ʹS, 166˚24ʹE) | 464 |
2 | Izu Islands, Japan region (32˚48'N, 140˚36'E) M 6.6 | 12/8/2009 | KSMV, Japan (35˚54ʹN, 140˚36ʹE) | 344 |
3 | Ryukyu Islands, Japan (23˚30'N, 123˚30'E) M 6.7 | 17/8/2009 | TNML, Republic of China (24˚42ʹN, 120˚54ʹE) | 296 |
4 | Haiti (18˚24'N, 72˚30'W) M 7.0 | 12/1/2010 | SCUB, Cuba (20˚00ʹN, 75˚42ʹW) | 380 |
5 | Liberatador O Higgins, Chile (34˚12'S, 71˚W) M 6.9 | 11/3/2010 | VALP Chile (33˚001S, 71˚36ʹW) | 135 |
where
To identify anomaly in ionospheric TEC, we compute in this paper the median
The analytical study of 5 earthquakes listed in
This paper presents the results of variation in VTEC, prior to 5 earthquakes in 2009 and 2010 as summarized in
The epicenter, date and magnitude of the Loyalty Island earthquakes are shown in
Sr. No. | Epicenter (Lat, Long) and Magnitude of Earthquake | Date | Enhancement or Depletion in TEC Prior to EQ | No of Satellite Passes Prior to EQ |
---|---|---|---|---|
1 | Southeast of the Loyalty Island (22˚36'S, 170˚54'E) M 6.6 | 19/1/2009 | Enhancement 3, 9 and 12 days | Increase7, 6, 5 and 3days 1(d) |
2 | Izu Islands, Japan region (32˚48'N, 140˚36'E) M 6.6 | 12/8/2009 | Enhancement 2, 4, 6 and 7, Depletion 9 and 10 days 2(a) | Increase 7, 6, 4 and 2 days 2(d) |
3 | Ryukyu Islands, Japan (23˚30'N, 123˚30'E) M 6.7 | 17/8/2009 | No enhancement 3(a) | Increase 7 days 3(d) |
4 | Haiti (18˚24'N, 72˚30'W) M 7.0 | 12/1/2010 | Enhancement 2 and 3 days 4(a) | Increase 2, 3 and 5 days 4(d) |
5 | Liberatador O Higgins, Chile (34˚12'S, 71˚W) M 6.9 | 11/3/2010 | Enhancement 7 days 5(a) | Increase 7 days 5(d) |
TECU (16 TEC is selected as mean TEC for peak of 25 days) were observed 7, 6, 5 and 2 days prior to earthquake.
It is seen from
As indicated in
It is also observed increase in VTEC greater than the upper bound 7 days prior to earthquake on 11 March 2010 at Chile (
In order to check the behavior of TEC variation without an earthquake preparation period, the VTEC profiles for
the earthquake free period are obtained corresponding to one of the five IGS station SCUB in Cuba. The choice of station is based on the fact that, there was no continuous earthquake free period in other four stations for the year 2009 and 2010. It is observed that the TEC did not show significant variation during this period as shown in
There is no common opinion in scientific society on physical mechanism to explain observed ionospheric variation associated with earthquakes. Variation in ionospheric TEC prior to an earthquake has been reported by many researchers ( [
atmosphere-ionosphere system appears due to the stress of the rocks [
During seismic active periods due to modification in lower atmospheric variables like temperature, pressure and humidity the atmospheric refractive changes, which causes the variation in the number of satellite passes ob-
served thereby changing in the field of view (FOV) of the receiver. To understand the evolution of the FOV of the receiver (or transmitter) of the GPS system, the dynamic situation of the lower atmosphere should be examined. An increase in the propagation horizon of VHF signal has been reported by many researchers [
As ray curvature C changes with refractive index gradient, its relation to the earth’s curvature does also changes therefore the concept of “effective earth’s radius”
where a = 6371 km and
The relation of N with temperature, pressure and water vapor is given below
For the normal lapse rate
1) It is observed that we find anomalous variation in TEC for all 5 earthquakes included in the paper, which is in agreement with those reported by earlier literature, and hence it can be observed that ionosphere behavior changes, a few days prior to a major seismic shock within or far from the earthquake preparatory zone.
2) It is observed that 2 - 7 days prior an earthquake, high VTEC values are observed from a large number of satellites, indicating an enhancement in the FOV of the receiver. It means that we see satellite passes appear before earthquake which are not seen under normal situation. This is only possible when the ray path of GPS signal changes. At both IGS stations we have observed the change in the number of satellite passes prior to an earthquake. We determine that changes in atmospheric refractivity lead to change in FOV of satellite signal. The results agree with those reported by [
The authors would like to thank the data centers, IGS for RINEX data, Kyoto for Dst data and NOAA for solar flux data.
Kunvar S. Yadav,Sheetal P. Karia,Kamlesh N. Pathak, (2015) Anomalous Variation in GPS Based TEC Prior to the 5 Earthquakes in 2009 and 2010. Positioning,06,96-106. doi: 10.4236/pos.2015.64010