Hydro-geochemical changes in response to multiple earthquakes in Northeast India

Earthquake-induced changes in groundwater composition, commonly known as hydro-geochemical earthquake precursors, have been documented repeatedly across the world. These changes are associated with the slow accumulation of tectonic stress in the Earth's crust before an impending earthquake. This study was initiated on 30th January 2021 to investigate such changes in the groundwater chemistry at two different borewells located at a distance of ~100km from the Kopili fault zone which is the most active seismic zone in Northeast India. Over the six-month ongoing study period, three earthquakes with magnitude 5 (27th March 2021), 5.1 (5th April 2021), and 5.2 (7th July 2021), two earthquakes (within 30 km of sampling locations), with magnitudes of 3.6 (25th February 2021) and 3.2 (19th May 2021), and one major earthquake with magnitude 6.4 (28th April 2021) followed by 56 aftershocks (magnitudes ranging from 2.5 to 4.7) have been reported in this highly active seismic region. Earthquakes with magnitudes of 5, 5.1, 5.2, and 6.4 were reported at distances of 180, 310, 145, and 80 kilometers from the observation wells, respectively. Groundwater samples were collected on a daily to weekly basis from both borewells and analyzed for physicochemical parameters (pH, TDS, conductivity, major and trace elements) to evaluate the hydrogeochemical changes in response to these earthquakes. This study presents time-series data with significant anomalies in the concentrations of B, Be, Li, Cl,­ K, Ca, Na, and TDS around the reported earthquakes. A rapid increase in the concentrations of Li, Cl, and F and a decrease in Na, Ca, and TDS, were observed in response to the reported earthquakes at both observation wells. The concentrations of B and Be kept on increasing after every earthquake so far. However, further in-depth evaluation is required to explain the change in the concentration of certain elements. Since the observed earthquakes in this study period are tightly spaced on time series, long-term and continuous monitoring of hydrogeochemical changes in groundwater are being conducted to identify reliable earthquake precursors.