Effect of Some Major Shallow Earthquakes (M > 6.0, Depth < 30 km) that Occurred in and Around India on the GPS-Based Total Electron Content (TEC) of the Ionosphere

The GPS-based total electron content (TEC) data observed at four low-latitude TEC stations—Lucknow (LCK3), Bangalore (IISC), Hyderabad (HYDE), and Lhasa (LHAZ) are subjected to Quartile-based statistical analysis to study the effect of six major shallow earthquakes (M > 6.0, depth < 30 km) that occurred in and around India in 2017. The results show anomalous enhancements in the TEC data 2–14 days before and 3–15 days after the onset of the earthquakes considered. These pre- and postseismic TEC enhancements are between 1.2–8.7 and 0.7–25.6 TECU, respectively, and percentage TEC enhancements before and after these earthquakes range from 3.82–69.04 and 4.40–95.53%, respectively. The influence of solar activity and magnetic storms on GPS-TEC data have also been examined, and it has been noted that the recorded anomalous TEC enhancements are not associated with these spurious sources except for correlation of TEC enhancements for three with magnetic storms. To confirm the association of observed precursory TEC enhancements occasions with the earthquakes considered in the present analysis probabilities for the pairs of TEC enhancements with the focal depths and precursory times with the distances of epicenters of the earthquakes from the observing stations are computed using t-test. The probabilities for the said pairs were 99.7 and, 70.5% for Lucknow; 47.5 and 98.9% for Bangalore; 95.7 and, 98.5% for Hyderabad; and 71.7 and 99.9% for Lhasa which are fairly large except for the Bangalore TEC observing station for the pair of precursoy time and epicentral distances, confirming the relationship between the TEC enhancements and considered earthquakes. In addition, possible mechanisms for perturbation in the TEC data due to seismic events are also discussed.