Sub-ionospheric vlf propagation anomalies associated with the earthquake ON 25 March 2013

Ionospheric perturbations in possible association with the earthquake occurred on 25 March 2013 in India-Oceania region (magnitudes ≈ 6.0) is investigated on the basis of subionospheric propagation data from the NWC transmitter(19.8 kHz) on the North West Cape, Australia to a VLF receiving station at Agra, India. The local nighttime VLF amplitude data are extensively investigated during the period, ±15 days of the earthquake, in which the trend (nighttime average amplitude), dispersion and nighttime fluctuation are analyzed. It is found that a clear precursory ionosphere perturbation is detected, about 12 days before the main shock, which is characterised by the simultaneous decrease in the trend and increase in nighttime fluctuation. Possible seismo-ionospheric mechanisms to demonstrate the physics that how ionosphere starts to feel these earthquakes are discussed.