Lithospheric Electromagnetic Emissions Associated with Some Major Earthquakes Occurred in Indian Subcontinent

The data related to lithospheric electromagnetic emissions such as ultra low frequency (ULF, f = 0.01 Hz–10 Hz) and very low frequency (VLF,  f = 3 kHz–30 kHz) emissions are analysed in relation to three major earthquakes (M = 6.8–8.5) occurred in Indian subcontinent during the years, 2011–2013. The equipments used to collect the data are thee component search coil magnetometer installed at Agra station (geographic lat. 27.2° N, long. 78° E) and indigenously developed amplitude measurements using a borehole and terrestrial antenna at Mathura station (geographic lat. 27.5° N, long. 77.68° E) about 70 km North of Agra station. The results show that ULF anomalies occur in the form of amplitude bursts 9 to 16 days prior to main shocks of the earthquakes whereas VLF amplitude anomalies occur coseismic and 3 days before recorded by the terrestrial antenna (the anomalies are weak and do not occur clearly in borehole antenna). These results suggest that ULF emissions (f < 0.1Hz) can travel to the observing station both via crustal and atmospheric regions, whereas VLF emissions can travel via atmospheric region only. The ULF data are compared with those obtained at two other stations of Shillong and Kachchh and malfunctioning of the sensors is ruled out. The effect of magnetic storms and lightning are not found to influence the data also. The precursory periods of 9–16 days are of the same order as 7–15 days reported by Singh et al. (2018) from the study of ionospheric perturbations caused by the same earthquakes. Hence, this study provides an interesting information about lithosphere–ionosphere coupling.