Study of Seismic Precursory Nature of Soil Radon with a Network of Monitoring Stations in Eastern India

Radon concentration in soil, a well-known short-term precursor of an earthquake, was monitored for about five months in 2018, simultaneously at three monitoring stations in the eastern Himalayas and the Bakreswar-Tantloi geothermal region of east India. The eastern Himalayas is one of the world’s six most seismically active regions. In contrast, the geothermal area, although considered ‘non-seismic’, has experienced small to moderate earthquakes over the years. The recorded data shows that various physical and meteorological parameters influence the emission of radon gas from soil, resulting in complex nonlinear nonstationary radon time series. A nonlinear two-step technique consisting of empirical mode decomposition and Hilbert-Huang transform was applied to remove influences of periodic factors. However, as this method alone is inadequate to identify the effects of geophysical factors, the networking approach was adopted in conjunction. The study looked for precursors for earthquakes of magnitude 5.0 or above within 500 km epicentral distance from each monitoring station; however, for earthquakes of smaller and larger magnitudes, smaller and larger epicentral zones, respectively, were considered. Apart from several seismicity-induced anomalies found in the radon time series of all three stations, one anomaly was observed almost simultaneously from all three monitoring centres preceding an M 5.3 earthquake that occurred on 12 September 2018, in Kokrajhar, Assam. The epicentre of this earthquake was found to be in the common region monitored by the three centres.