Groundwater anomalies associated with seismic activity in the north fergana fault zone: Insights from the chartak no. 2 well

Abstract

The paper presents long-term hydrogeoseismological investigations based on monitoring data from the Chartak No. 2 observation well, located within the North Fergana deep fault zone. The study aims to identify hydrogeoseismological anomalies associated with regional seismic activity using data collected during 2002–2025. Gas-geochemical (He, H 2 , O 2 , N 2 , CO 2 , CH 4 ), hydrogeochemical (pH, Eh, HCO 3 − ), and physical groundwater parameters were analyzed. Earthquakes with M ≥ 4.5 and an energy criterion of M/LgR ≥ 2.5 were selected according to the nomogram of Rautian (1971), while volumetric strain (ɛD) was calculated using the I.P. Dobrovolsky model. Seven-day and 91-day median filters were applied to suppress seasonal and short-period variations. The results show that gas-geochemical parameters are the most sensitive indicators of deformation preceding earthquakes, with He, H 2 , CO 2 , pH, and Eh identified as the most informative parameters. Short-term anomalies appeared as sharp spikes and trend reversals, whereas long-term anomalies reflected gradual restructuring of the hydrogeochemical system. These anomalies are interpreted as responses to deformation-induced permeability changes, enhanced deep fluid migration, and disturbances in the water-rock-gas equilibrium.