Ionospheric perturbations due to earthquakes as determined from VLF and GPS-TEC data analysis at Agra,India

Employing SoftPAL receiver, amplitude variations of VLF transmitter signals NWC (19.8?kHz) and NPM (21.4?kHz) are analyzed at Agra station in India (Geograph. lat. 27.2°N, long. 78°E) ±15?days from five major earthquakes of magnitude M?=?6.9–8.5 occurred in Indian subcontinent during the years 2011–2013. We apply nighttime fluctuation (NF) method and show that in almost all cases the trend decreases and dispersion and NF increase on the same days corresponding to each earthquake about 11–15?days prior to the main shock. Assuming that the ionospheric perturbations are caused by atmospheric gravity waves (AGW), we also calculate AGW modulation index for each case and find its values increased on the days amplitude fluctuations take place. Its value is decreased in one case only where the perturbations may be attributed to penetration of seismogenic electric field.In order to support the above results we also present GPS-TEC data analyzed by us corresponding to three of the above earthquakes. We study the TEC anomalies (unusual enhancements) and find that in one case the precursory period is almost the same as that found in NF method.