Anomalous behavior of Very Low Frequency signals during the earthquake events

Abstract

The Very Low Frequency (VLF) radio waves in the frequency range 3-30 kHz propagating in the earth ionosphere waveguide have their entire propagation path lying in the D-region. Since the phase velocity and height of reflection of these waves are very much sensitive to the D- region conductivity, these waves are used to study the D-region. In recent times, the amplitude and phase of these VLF waves have been studied for signatures of earthquakes occurring near their propagation path. In the present paper, the amplitude of the VLF waves transmitted from GQD (UK), GBZ (UK ) and TBB (Turkey) received in South France have been studied for fluctuations in morning and evening terminator timings during the earthquake events. For the event of Oct 12^th, 2013(magnitude=6.6), it is observed that there are predominant fluctuations in the morning terminator timings in the pre-earthquake period.  The VLF day is observed to be long on the earthquake day of 12^th Oct 2013.