VLF nighttime amplitude as a precursory signal from earthquakes

   This paper discusses the results of changes in VLF nighttime amplitude as
   a precursory signal from earthquakes (EQs). The perturbations observed are
   in the form of increase or decrease in average nighttime amplitude of the
   subionospherically propagating Very Low Frequency (VLF, 3-30 kHz) signal
   from Australia at 19.8 kHz (NWC), observed from Tripura, India. The great
   circle distance between them is around 5.6 Mm. The experimental setup
   consisted of consists of four main components: the a omnidirectional
   vertical whip antenna, a preamplifier, a sound card that digitizes the
   measured signal and a software VLF receiver. The preamplifier is a well
   calibrated standard World Wide Lightning Location Network (WWLLN)
   receiver.

   Data were analyzed for a period of one year for 2012. A circle of radius
   2500 km centering the signal hop position (Lat 2.760N, Long 99.150E)
   having earthquake magnitude above 6.5 and depth in the range between 1-30
   km were selected for current analysis. From the U. S. Geological Survey
   earthquake database, three earthquakes were found. We analyzed the
   nighttime VLF signals amplitude for 19 days centering each individual
   earthquake days and found that the nighttime amplitude of VLF signal shows
   a change in signal strength three days prior the impending earthquake. A
   decrement of 5.36 dB with Standard Deviation (SD) of 2.4 dB is observed
   for EQ on 11th November, whereas an increment of 6.3 dB with SD of 2.37 dB
   is observed for rest two EQs. The results are discussed with respect to
   the change in variations of the electron density in the lower ionosphere.
   The maximum change of beyond one sigma is generally found to occur three
   days prior to seismic events, thus supporting the fact that nighttime
   signal amplitude can be considered as earthquake precursory.