Simultaneous perturbation observed on VLF and atmospheric vertical electric field for Sumatra earthquake on 11th April 2012

The paper discusses the results of simultaneous observation of earthquake precursory effect in the form of increase in nighttime amplitude of subionospherically propagating Very Low Frequency (VLF) signal from Australia at 19.8 kHz (NWC) and an anomalous variation in atmospheric vertical electric field (VEF) in the form of bay like depression observed from Tripura. The observation was made for a severe earthquake of magnitude 8.5 at a depth of 10 km which occurred on 11^th April, 2012 at the west coast of northern Sumatra. We analyzed the nighttime VLF signals amplitude for 19 days centering the earthquake day and found that the nighttime amplitude of VLF signal shows an increase in signal strength three days prior the earthquake, whereas the VEF shows variation 8 hrs before the impending earthquake. The results are correlated with the decrease in vertical total electron content (VTEC)variations of the ionosphere in the vicinity of the earthquake epicenter.