Groundwater system responses to the 2016 ML 5.8 Gyeongju earthquake, South Korea

The September 12, 2016 Gyeongju earthquakes (ML = 5.1 of foreshock and ML = 5.8 of mainshock) had significant effects on groundwater systems along the Yangsan Fault System composed of NNE-trending, right-lateral strike-slip faults in Korea. Hydrological changes induced by the earthquakes are important because no surface ruptures have been reported and few earthquakes usually occur in Korea. The main objective of this research was to propose a conceptual model interpreting the possible mechanisms of groundwater response to the earthquakes based on anomalous hydrogeochemical data including isotope concentrations with lithostratigraphic classification. For this, annual monitoring data (groundwater level, temperature, and electrical conductivity) and collected data (hydrochemical parameters, radon-222, and strontium isotopes) were used. Groundwater level anomalies could be attributed to the movement of the epicentral strike-slip fault. Radon concentration data showed the potential of groundwater mixing processes. Strontium anomalies could be related to the lithology and stratigraphy of the bedrock, reflecting the effect of water–rock interaction. Using a Self-Organizing Map (SOM) statistical analysis, associations of hydro-geochemical characteristics among groundwater wells were interpreted. By combining the grouped results of the SOM with lithostratigraphic unit data, 21 groundwater wells were classified into four groups, each corresponding to different hydrogeological behaviors. A new comprehensive conceptual model was developed to explain possible mechanisms for the hydrological and geochemical responses in each group, which have been respectively identified as water–rock interaction, mixing of shallow and deep aquifers via sea water intrusion, bedrock fracture opening related to strike-slip fault movement, and no response.