ALBERT

Description: We present a critical analysis of the observations and interpretation of VLF sub-ionospheric measurements related to the main Nepal Gorkha earthquake which occurred on April 25, 2015 (M w 7.8) and its major aftershock on May 12, 2015 (M w 7.3). The VLF narrowband signal used is from NWC (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (lat. 25.41° N; long. 81.93° E). Allahabad is located very close (~360 km) to these earthquake (EQ) epicenters. Two widely used analysis, viz., (1) terminator time and (2) night time fluctuation techniques are applied to extract seismic related effects in the NWC narrowband VLF data. The terminator time analysis yields statistically significant shifts of ~45 and ~26 minutes respectively in evening terminator time in the NWC VLF amplitude signal, one day before both the earthquakes. The nighttime fluctuation method shows a consistent, statistically significant, increase in three parameters one day before the earthquake. The observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity, other earthquakes on the signal path, meteorological disturbances such as lightning activity, wind speed, temperature etc. along the transmitter-receiver great circle path. The VLF sub-ionospheric signal analysis results unambiguously point towards the presence of seismically excited atmospheric gravity waves (AGWs) during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere.