Figure 10. Propagation of seismo-acoustic waves in 3D space
with time from the Trial seismic source assumed at 25\(^{\circ}\)N 85\(^{\circ}\)E. The
propagation of six rays modeled at six different launch angles is shown.
The first ray is launched at an angle of \(\sim\)58\(^{\circ}\) that is
the threshold angle at 120 km altitude. The rays with launch angles
higher than this refract downward while those lower than this propagate
further upward. Similarly, the second ray is launched at an angle of
\(\sim\)38.8\(^{\circ}\) that is the threshold propagation angle at 150 km
altitude and so on. The inset shows the variation of the threshold angle
and maximum horizontal distance along with the atmospheric altitudes.
(From Bagiya et al., 2019) (Courtesy IIG)
In addition to the transient perturbations, prolonged ionospheric oscillations following large earthquakes (Mw > 8.0) have also been found to provide seismic source information from the ionosphere. Such ionospheric oscillations related to the earth-atmospheric resonance frequencies of \(\sim\) 3.7 and \(\sim\) 4.4 mHz following the 2012-04-11 Sumatra doublet (Fig.11) and 2011-03-11 Tohoku-Oki earthquakes were scrutinized (Nayak et al., 2021, 2022). The Earth’s background free oscillations at \(\sim\) 3.7 and \(\sim\) 4.4 mHz resonantly couple with the atmospheric acoustic modes and thus energy cross-talk between the earth-atmosphere system is maximum at these frequencies. Our studies emphasized that resonant ionospheric signatures during the Sumatra doublet event were related to the seismic source. Therefore, resonant co-seismic ionospheric signatures could provide additional information on the low frequency features of seismic ruptures.