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Hydrogeochemical Evidence of Earthquake‐Induced Anomalies in Response to the 2017 MW 5.5 Pohang Earthquake in Korea

Significant changes in groundwater systems were observed after the 2017 Mw 5.5 Pohang earthquake. Groundwater level changes cannot be explained solely by poroelastic responses to the earthquake. In this study, we analyzed hydrogeochemical data, including environmental isotopes (18O, 2H, 87Sr/86Sr, and 222Rn) of water samples as well as time‐series data of groundwater level, temperature, and electrical conductivity. Principal component analysis (PCA) and physical modeling of pore pressure changes were also used. Some anomalies in time‐series data could be explained by the distributions of pore pressure changes based on modeling. However, the other wells, which did not have the correlation between time‐series data and modeling results, exhibited abnormal patterns in hydrogeochemical and isotope data. Stiff diagrams showed differing patterns of temporal change among groundwater wells. These diagrams and isotopic anomalies revealed possible mechanisms underlying the aftereffects of earthquakes, including saline water mixing, water‐rock interactions, deep fluid upwelling, and bedrock fracture opening. These anomalous phenomena were related to the clusters derived by PCA and depended on the distance to the epicenter, faults, and sea water. This study highlights that a single measurement is insufficient to reliably interpret groundwater responses, whereas a combined hydrogeochemical and physical interpretation may provide critical information for clarifying anomalies related to earthquakes.