6.3 Observational environment
The giant Sumatra-Andaman earthquake of magnitude Mw 9.1 occurred off the west coast of Sumatra at 00:58:52 UT on 26thDecember 2004 triggered the most devastating tsunami in the Indian Ocean (Lay et al., 2005). Acoustic gravity waves generated by the tsunami perturbed the ionosphere (Liu et al., 2006) when the geomagnetic condition was weak (Astafyeva et al., 2006). The tsunami wave-field was simulated using COMCOT (Liu et al., 1998; Wang and Liu, 2006) by modeling the ocean surface displacement at six fault segments (Hebert et al., 2007) to identify the TIPs induced by the tsunami (Fig. 3). TIPs associated with the simulated tsunami (time lag of ~25 minutes corresponds to vertical propagation of acoustic gravity waves) are considered in this analysis.
Similarly, Nepal-Gorkha earthquake (Fig. 4) occurred on 25th April 2015 at 06:11:26 UTC with the magnitude of Mw 7.8 had ruptured around 160 km with a duration of 55s (Fan et al., 2015) disturbed the ionosphere and generated CIPs under extremely quiet geomagnetic conditions (Kong et al., 2018). CIPs obtained in the near field of the earthquake are considered in this analysis.
6.4 Detecting a liasing and artifacts free TIPs
Aliases and artifacts in TIPs computed using differential and residual methods and its removal by SPLA are illustrated here using observations along the track of COCO07, COCO17, IISC26, and HYDE29 (Fig. 5, 6, 7 and 8). In all the four cases, the TIPs were detected at low elevations which are conventionally discarded to avoid the errors associated with multipath, and slant ray path. But, the coincidence in time and location of the perturbations with the simulated tsunamis (Fig. 7, and 8) ensures that the perturbations obtained from GPS-TEC observations are TIPs associated with the Indian Ocean tsunami. Moreover, in the case of COCO07 (Fig. 5) and COCO17 (Fig. 6) elevation varies only from 5°-35° during 11 hours of continuous observation. Applying, for example, a 30° elevation cut-off in such cases leads to omission of the observations almost in its entirety. Such omissions will eventually lead to missing onshore detection of tsunamis propagating in the open ocean. This emphasizes the importance of removing errors associated with low elevation observations from the TIPs.