Hydrogeochemical Anomaly in the Aegean Region, Western Türkiye: Possible relation to Santorini Island (Greece) Earthquake Swarm

Hydrogeochemical anomalies in groundwater systems are recognized as potential earthquake precursors, yet their detectability varies depending on long-term monitoring techniques as well as geological and structural features of observation sites. This study examines 28-month long spring water record comparing the seasonal variations, rainfall contribution and propose a possible link between precursory signals for February-March 2025 Santorini earthquake swarm which comprised approximately 2,700 events and a cumulative seismic energy release equivalent to Mw 6.1-6.2. Monitoring of the Naz-01 spring in Nazilli, Türkiye, displays optimal level of EC (90 µS/cm) and have an approximately 180 days long positive anomaly reaching up to (180 µS/cm) which revealed marked increases in electrical conductivity as well as major ion concentrations before the onset of seismic activity. However, two other spring waters (Boz-01 and Çine-01) has no distinct anomaly before or during the events. Moreover, while empirical precursor models would predict no detectable response at such distances for individual moderate events, the observed signal may be caused by the cumulative energy release from the swarm, which lies within the expected impact range for a single, larger equivalent source. The results suggest that structural connectivity between the source region and the monitoring site enabled efficient stress transfer and fluid-rock interaction, triggering measurable geochemical anomaly at Naz-01 spring. Conversely, seismic events of similar or greater magnitude occurring in structurally decoupled neighboring micro-plates (blocks) did not produce detectable anomalies at Boz-01 spring, contrarily Çine-01 as well. These observations emphasize that near fault located springs seems to be much more susceptible to the hydrogeochemical anomalies and tectonic connectivity is a way to show the reasoning why presence/absence of the anomalies in spring waters. These findings highlight the importance of monitoring the spring waters in a multi-disciplinary way to improve our understanding to the reliability of earthquake precursor detection with multiple monitoring sites and various monitoring techniques.