Transient Effects in Atmosphere and Ionosphere Preceding the 2015 M7.8 and M7.3 Gorkha–Nepal Earthquakes

We analyze retrospectively/prospectively the transient variations of six different physical
parameters in the atmosphere/ionosphere during the M7.8 and M7.3 earthquakes in
Nepal, namely: 1) outgoing longwave radiation (OLR) at the top of the atmosphere (TOA); 2)
GPS/TEC; 3) the very-low-frequency (VLF/LF) signals at the receiving stations in Bishkek
(Kyrgyzstan) and Varanasi (India); 4) Radon observations; 5) Atmospheric chemical
potential from assimilation models; and; 6) Air Temperature from NOAA ground
stations. We found that in mid-March 2015, there was a rapid increase in the radiation
from the atmosphere observed by satellites. This anomaly was located close to the future
M7.8 epicenter and reached a maximum on April 21–22. The GPS/TEC data analysis
indicated an increase and variation in electron density, reaching a maximum value during
April 22–24. A strong negative TEC anomaly in the crest of EIA (Equatorial Ionospheric
Anomaly) occurred on April 21, and a strong positive anomaly was recorded on April 24,
2015. The behavior of VLF-LF waves along NWC-Bishkek and JJY-Varanasi paths has
shown abnormal behavior during April 21–23, several days before the first, stronger
earthquake. Our continuous satellite OLR analysis revealed this new strong anomaly on
May 3, which was why we anticipated another major event in the area. On May 12, 2015,
an M7.3 earthquake occurred. Our results show coherence between the appearance of
these pre-earthquake transient’s effects in the atmosphere and ionosphere (with a short
time-lag, from hours up to a few days) and the occurrence of the 2015 M7.8 and M7.3
events. The spatial characteristics of the pre-earthquake anomalies were associated with a
large area but inside the preparation region estimated by Dobrovolsky-Bowman. The preearthquake
nature of the signals in the atmosphere and ionosphere was revealed by
simultaneous analysis of satellite, GPS/TEC, and VLF/LF and suggest that they follow a
general temporal-spatial evolution pattern that has been seen in other large earthquakes
worldwide