A multi-experiment approach to ascertain electromagnetic precursors of Nepal earthquakes

Abstract

The experimental data obtained from three different techniques such as (i) subsurface VLF electric field measured with the help of a borehole antenna (ii) total electron content (TEC) of the ionosphere measured with a GPS receiver and (iii) VLF amplitude of subionospheric fixed frequency (f = 19.8 kHz) NWC (Naval Communication Station) transmitter signals monitored with SoftPAL receiver are analysed in relation to two major Nepal earthquakes occurred on 25 April (M = 7.8) and 12 May (M = 7.3), 2015. The VLF electric field data and fixed frequency VLF transmitter signal data are taken from Mathura and Agra stations in India whereas TEC data are taken from three IGS stations, one of which Lucknow (Lck3) is located in India and others Lhasa (Lhaz) and Wuhan (Wuhan) are located in China, all within 800 km from epicenters of two earthquakes except Wuhan which is located at about 2865 km. The results of the analysis indicate that while borehole and TEC data show anomalies in normal variations within 8–15 days prior to the earthquake of 25 April, the same occur within 5–7 days prior to the earthquake of 12 May. The subionospheric variations of VLF amplitude also show an anomaly 4 days before the later earthquake. Occurrence of these anomalies is examined critically in the light of local lightning and magnetic storms. However, these factors are not found to influence the data. A brief description of generation and propagation mechanisms of seismogenic emissions is also presented.