Effect of VLF electric field changes associated with major shallow earthquakes (M=5.6-5.9) occurred in Indian subcontinent, on the atmosphere

Using a terrestrial antenna routine observations of the amplitude of vertical component of electric field of seismogenic VLF emissions (frequency, f = 3.012 kHz) is in progress at the Mathura observing station (Lat. 27.49 degrees N, Long. 77.67 degrees E) since 24 March 2011. In this paper, the VLF data obtained from terrestrial antenna have been analysed statistically for the months of April 2012, July 2013, September 2013, and October 2013 for examining the effect of five major shallow earthquakes (M = 5.6-5.9, depth\ 10 km) that have occurred in the Indian subcontinent within circular region of radius 3300 km during these months assuming Mathura observing station as the centre. The mean (m) and two standard deviations around the mean (m +/- 2 sigma) criterion is used for analysing the VLF data. The anomalous VLF amplitude enhancements in the vertical component of the electric field are seen, 1-13 days prior to onset of four seismic events while for one seismic event, the enhancement is co-seismic. Further, the effects of magnetic storms, lightning activity, local power line emissions and local building noises are also studied on the VLF data and it is noticed that these enhancements are not correlated with these spurious noises. For confirming the association of days of VLF amplitude enhancements with seismic activities correlation coefficients and probability (using null hypothesis) for the pair of focal depth and precursory time are computed. The values of correlation coefficient and probability for this pair are found 0.87 and 0.88 respectively. These fairly large values of correlation coefficient and probability indicate a strong association between days of VLF amplitude enhancements and seismic activities. Further, the association of amplitude anomalies with seismic events is also confirmed by using another statistical approach. Also, the possible mechanisms for the generation and propagation of VLF electromagnetic radiations are discussed briefly.