A statistical report on atmospheric vertical electric field as a precursory signature for earthquakes observed from North-East India

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   The studies on anomalous variations observed at the Earth's atmospheric
   vertical electric field (VEF) in the form of bay like depression in its
   signal strength have been used as a precursory signature of earthquakes.
   The present paper discusses the first statistical report on VEF variation
   form one of the earthquake prone zones in North-East India, situated at
   the juncture of three tectonic plates. The analysis is performed from July
   2009 to December 2012. Twelve meteorologically fair weather days were
   found where anomalous variations in near surface atmospheric VEF in the
   form of bay like depression in its strength are observed prior the
   earthquake strike. The average VEF bay duration of about 50 - 70 minutes
   and VEF depth of magnitude around 500 - 800 Vm-1 were found. The VEF
   showed anomalous variation around 7-12 hours before an impending
   earthquake. A 34.5 percent probability of earthquake precursor in VEF was
   established with the help of the present analysis. The plot between the
   VEF bay depth and the ratio of earthquake magnitude by depth depicted a
   positive correlation coefficient of 0.74, whereas for the plot between VEF
   bay duration and the ratio of earthquake magnitude to depth, a negative
   correlation coefficient of 0.79 was computed. This highlights the fact
   that during a meteorologically fair-weather day, the atmospheric VEF can
   show some earthquake precursory effects with respect to both VEF bay depth
   and bay duration. The correlations of great circle distance of earthquake
   epicenter to the observation point with the VEF bay depth have been found
   to be of 0.71, but with the VEF bay duration, the correlation is very
   poor. The correlation of time difference of VEF variation and earthquake
   with VEF bay depth is good whereas correlation of time difference of VEF
   variation and earthquake with VEF bay duration is too low. The results are
   discussed with respect to change in ionization in the vicinity of the
   earthquake preparation zone.