Anhydrite dissolution dynamics as a hydrogeochemical tracer of seismic-fluid coupling: insights from the East Anatolian Fault Zone, Türkiye

Abstract. Pre-seismic turbidity and salinity anomalies in groundwater were documented at HS04 and HS14 monitoring wells and/or springs along the East Anatolian Fault Zone (EAFZ) following the 2023 Kahramanmaraş Earthquake Doublet (Mw 7.8 and Mw 7.6). By synthesizing hydrogeochemical datasets (2013–2023) with post-seismic responses, we unravel fault-segmented groundwater evolution: Northern Na–Cl and Na–HCO3 type waters result from mixing of mantle-derived magmatic fluids (0 %–7 % contribution) with shallow groundwater, governed by volcanic rock–carbonate dissolution. Central–southern Ca–HCO3 and Ca–Na–HCO3 systems reflect shallow circulation with localized inputs from evaporites (increased SO42- concentration caused by dissolution of anhydrite), ophiolites (Mg2+ anomalies) and seawater. PHREEQC simulation shows that the dissolution–precipitation equilibrium of anhydrite is sensitive to the variation of water–rock reaction intensity in the central–southern segments of the EAFZ. Coseismic permeability changes disrupt the solubility equilibria of anhydrite, driving hydrochemical anomalies. We propose that seismic stress redistribution induces fracture network reorganization, thereby disrupting anhydrite solubility equilibria. Given its tectonic sensitivity and widespread occurrence, anhydrite dissolution dynamics emerge as a potential tracer for hydrogeochemical monitoring in active fault zones. We propose a novel research paradigm wherein regional hydrogeological surveys identify applicable target indicator horizons, enabling continuous monitoring and establishment of region-specific evaluation metrics to ultimately achieve early warning capabilities for geohazard precursors.