Anomalous subsurface VLF electric field emissions related to Nepal earthquakes (M = 7.8, M = 7.3), and their generation and propagation mechanisms

Measurements of amplitude of vertical components of very low frequency (VLF) electric field emissions have been in progress at Chaumuhan, Mathura (27.5°N, 77.68°E), Uttar Pradesh, India, at the frequency of 3.012 kHz using a borehole antenna since 24 March 2011. In the present study, we have statistically analysed the VLF data from 1 January to 31 July 2015 using the mean (m) and standard deviation (s ) around the mean (m ± 2s ) criterion with respect to two major earthquakes (M = 7.8, 7.3) and their aftershocks that occurred on 25 April and 12 May 2015 in Khudi and Kodari regions of Nepal respectively. Results of the analysis show a spiky enhancement above the m + 2s line in the daily variation of the normalized VLF data, 7–14 days before the onset of main shocks of these earthquakes. These anomalous enhancements in the normalized values of amplitude are examined in light of various other possible causative sources such as magnetic storms, local lightning, solar flares and solar eclipse, rainfall, wind speed, temperature and pressure of the atmosphere. It was found that the VLF amplitude anomalies are not related to these sources. Further, we have developed a theoretical model for generation and propagation mechanisms of VLF emissions associated with the said earthquakes and their aftershocks by assuming an ensemble of elementary radiators randomly oriented and distributed in space and time in the preparation zones of these earthquakes. The computed electric field from the model is in good agreement with the observations.