Figure 11. Power spectral analysis of Pseudo Random Number (PRN) 32 detrended TEC (dTEC) time series from umlh and pbri and of PRN 16 dTEC time series from umlh during 2012-04-11 Sumatra doublet earthquake. Resonant ionospheric signatures centered at \(\sim\) 4 mHz could be noticed. (From Nayak et al., 2021) (Courtesy IIG)
    Rapidly moving objects excite short-period waves, and slow objects excite long-period waves. This has been confirmed for atmospheric waves excited by vertical crustal movements associated with large earthquakes. We compared atmospheric wave amplitudes excited by ordinary earthquakes and by “tsunami” earthquakes, characterized by slow fault movements. It has been found that the 2010 Mentawai earthquake, a typical tsunami earthquake, excited abnormally large internal gravity waves from ionospheric observations. This is the first slow earthquake signature found in space (Heki et al., 2022).
    In another initiative, the Sumatra 2004 tsunami induced ionospheric signatures, which were detected simultaneously in GPS-TEC \(\sim\) 90 minutes before the arrival of actual tsunami over the Indian east coast, were successfully explained. This work offers an alternative tool to monitor the offshore signatures that travel ahead of tsunami in the ionosphere could potentially be an important early warning tool for the tsunami over coastal regions. These findings, with a bearing on mitigation of hazards in coastal regions, are likely to impact tsunami forecast related research in a significant manner.

9. Paleoseismology

The E-W trending reverse Dauki Fault (DF) in NE India has played a major role in the regional deformation of the adjoining areas and was believed to be active during the Late Quaternary time. Previous paleo-seismological studies conducted on the eastern and western part of the DF, Bangladesh, revealed that the fault ruptured in AD 849-920 and AD 1548 respectively. However, there were no studies on the DF from the southern side of the Shillong Plateau (SP), India. IIG has reported soft sediment deformation structures (SSDS) from five trenches in and around the DF zone, SP. Close to the Dauki village, five trenches in the eastern part of the DF show micro-faulting, sand dykes, disturbed strata, and water escape structures. The detailed investigation of SSDS indicates that the origin of deformation is seismically triggered. The 14C AMS (Accelerator Mass Spectrometry) dating of deformation structures generated by earthquakes suggests that three seismic events occurred between 130 and 920 yr BP, 5415 to 9140 yr BP, and at about 4285 yr BP. This study confirms that DF is indeed active, at least, since the mid-Holocene (Lakshmi and Gawali, 2022).
    The major seismic source in the Kachchh basin, i.e. the Kachchh Mainland Fault (KMF), was studied using various geological investigations during 2019 and 2022. After thorough scrutiny of high-resolution satellite imageries and field investigations, several trenches were dug across prospective sites along the KMF for detailed paleo-seismic study. Based on these investigations, it was concluded that the KMF illustrates an oblique strike-slip fault in the western and central segments. A total of six paleo-seismic events have been identified during the period from 890 – 1980 years BP (Kothyari et al., 2021), out of which five were in the Holocene whereas one was in Late Pleistocene. The slip rate shows variability from lesser values in the western flank (0.08 – 0.04 mm/yr) to progressively increasing values (0.22 – 0.36 mm/yr) towards the eastern flank.

10. Seismological Networks

10.1 National Network