The 25 April 2015 Nepal Earthquake: Investigation of precursor in VLF subionospheric signal

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.

On the Co‐Seismic Ionospheric Disturbances (CIDs) in the Rapid Run Ionosonde Observations Over Allahabad Following Mw 7.8 Nepal Earthquake on April 25, 2015

“…Maurya et al (2016) also suggested that there is no extreme tropospheric weather events recorded during Nepal earthquake. In addition, we also analyzed several days prior to and after the earthquake day to identify the peculiar signatures of the ionosphere during the time of earthquake than on control days.…”

“…It is noticed that no strong thermal anomalies were observed in the OLR during the time of Nepal earthquake suggesting that there is no strong gravity wave activity in the troposphere. Maurya et al (2016) also suggested that there is no extreme tropospheric weather events recorded during Nepal earthquake. Also it may be mentioned that Allahabad station is 400 km away from the epicenter of the Nepal earthquake.…”

“…It is believed that mechanical signatures such as acoustic and gravity waves, chemical signatures such as radon gas emission and electromagnetic signatures such as ELF/VLF wave emissions near the earthquake preparatory zones are being used to study the pre or co-seismic signatures in the ionosphere (e.g., Pulinets & Boyarchuk, 2004;Heki, 2011;Maurya et al, 2016). It is believed that mechanical signatures such as acoustic and gravity waves, chemical signatures such as radon gas emission and electromagnetic signatures such as ELF/VLF wave emissions near the earthquake preparatory zones are being used to study the pre or co-seismic signatures in the ionosphere (e.g., Pulinets & Boyarchuk, 2004;Heki, 2011;Maurya et al, 2016).…”

“…Earthquakes are mostly produced at the subduction zones where one plate colloids with other. It is believed that mechanical signatures such as acoustic and gravity waves, chemical signatures such as radon gas emission and electromagnetic signatures such as ELF/VLF wave emissions near the earthquake preparatory zones are being used to study the pre or co-seismic signatures in the ionosphere (e.g., Pulinets & Boyarchuk, 2004;Heki, 2011;Maurya et al, 2016). The ground motion near the epicenter causes air to oscillate producing wave perturbation.…”

Characteristic Variations of VLF/LF Signals during a High Seismic Activity in Japan in November 2016

“…There are a few conventional analysis methods: 1) terminator time method [8] [20] and 2) nighttime fluctuation method [3] [27].…”