Evaluating the responses of alluvial and bedrock aquifers to earthquakes (ML5.1 and ML5.8) using hydrological and environmental tracer data

Abstract

In Gyeongju, South Korea, local magnitude ML5.1 and ML5.8 earthquakes occurred on 12 September 2016; ML4.5 aftershocks and >500 aftershocks with ML > 1.5 were observed over the next 3 months. Responses of the aquifers were compared using hydrological and environmental tracer data (noble gases, δ18O, δD, 3H, and 13C). To assess the hydrologic response to the earthquake activity by the shallow (alluvial) and deep (bedrock) aquifers, time series data from the national groundwater monitoring wells were compared. Groundwater-level changes were not observed in most alluvial wells, while groundwater level and electrical conductivity (EC) increased in the confined igneous rock for several days to months after the earthquake activity. Noble gas anomalies in groundwater were closely related to the epicentral distance, lithology, and aquifer type. The relatively low concentration of 3H (<0.8 TU) and depleted values of δ18O and δD in the alluvial and bedrock aquifers suggest they were affected by upwelling of deep and old water. Elevated values of δ13C and 222Rn were observed in wells close to the epicenter. Groups resulting from cluster analysis using environmental tracer data were closely related to the responses of the earthquake on the aquifers of different types (alluvial and bedrock), lithologies, and distances from the epicenter. Groundwater-level change and geochemical response after the earthquake activity showed different correlations depending on aquifer and fault types. Combined use of groundwater level, EC, and environmental tracer data in groundwater can be useful to understand the origin and preferential flow paths of water during and after earthquakes.