Anomalous behaviour of very low frequency signals during the earthquake events

The seismo-ionospheric coupling has been extensively studied in the recent years adopting various ionospheric parameters, like the critical frequency of F sub(2) and E sub(s) layers and total electron content (TEC). There have been reports of seismic signatures in the amplitude and phase of the very low frequency (VLF) waves when an intense earthquake occurs near the propagation path. In the present paper, the amplitude of VLF waves transmitted from GQD (UK), GBZ (UK) and TBB (Turkey) received in South France during the period 2010-2013 has been studied for the precursory effect of earthquakes on the morning and evening terminator timings (TT). Six earthquake events of magnitude greater than 5 have been considered. The results of the present study show that the evening terminator undergoes many fluctuations during the pre-earthquake period of 5-15 days. But the pre-earthquake fluctuations in the morning terminator are predominant for three events, namely 8 March 2010, 11 April 2010 and 22 May 2012. The VLF day (time between morning terminator and evening terminator) is observed to be longer in the pre-earthquake period for three events.