Abstract:
We present a critical analysis of the observations and interpretation of VLF subionospheric
measurements related to the main Nepal Gorkha earthquake which occurred on 25 April 2015 (Mw7.8)
and its major aftershock on 12 May 2015 (Mw7.3). The VLF narrowband signal used is from North West
Cape (NWC) (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (latitude 25.41°N,
longitude 81.93°E). Allahabad is located very close (~360 km) to these earthquake epicenters. Two widely
used analysis, viz., (1) terminator time and (2) nighttime 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 min, respectively, in evening terminator time in the NWC VLF amplitude
signal, 1 day before both the earthquakes. The nighttime fluctuation method shows a consistent,
statistically significant, increase in three parameters 1 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; and meteorological disturbances such as lightning activity, wind speed, and
temperature along the transmitter-receiver great circle path. The VLF subionospheric signal analysis
results unambiguously point toward the presence of seismically excited atmospheric gravity waves
during these major earthquakes and their important role in providing the coupling between the seismic
source region and overlying ionosphere.
