Studies of scintillations and TEC variations with GPS satellite links together with soil radon anomalies preceding Nepal earthquakes of April–May 2015

Ionospheric effects like scintillations and anomalous variations in total electron content (TEC) monitored with Global Positioning System (GPS) satellites of L1 frequency over Kolkata, West Bengal, India, in April–May 2015 were studied together with radon activity in soil recorded by solid-state nuclear track detector (SSNTD) during summer of 2015 in the same city, with a view to identify possible precursory signals for earthquakes that occurred in the Nepal Himalayas during April–May 2015. Weak-to-intense fluctuations even up to saturation levels in some links of GPS satellites and anomalies in TEC were observed in the pre-earthquake days, although 2015 was a medium-to-low solar activity year. Prominent near-simultaneous anomalies of all three precursors were observed prior to the two massive earthquakes of magnitude > 7 that devastated vast areas of Nepal in 2015. The occurrence of anomalies and spurious pulses has been studied in the present work, and the effectiveness of analysing together two different types of earthquake precursors for short-term prediction of high-magnitude earthquakes has been discussed. Moreover, this is the first work on the Nepal Himalayan region in which ionospheric scintillation and TEC have been studied concurrently with soil radon in Kolkata for earthquake precursor research.