Assessing Coseismic Groundwater-Level Responses to the 2016 ML 5.8 Gyeongju Earthquake: Implications for National Groundwater Monitoring Networks

The M_L 5.8 Gyeongju earthquake occurred on 12 September 2016 at 20:32:54 (KST), with a focal depth of approximately 15 km, along the Naenam Fault near Gyeongju, southeastern Korea. This event represents the strongest instrumentally recorded earthquake in southeastern Korea and has raised significant concerns regarding seismic hazards and associated hydrological responses in the Korean Peninsula. In this study, we investigate coseismic groundwater-level responses associated with the Gyeongju earthquake using data from the Korean National Groundwater Monitoring Network (NGMN). A total of 24 monitoring wells located within a 50 km radius of the epicenter were analyzed to assess the detectability and spatial characteristics of groundwater-level changes induced by the earthquake.

 

Most wells did not exhibit obvious groundwater-level responses at the time of the earthquake. This limited detectability is primarily attributed to two factors: (1) the earthquake occurred between scheduled measurement times of the National Groundwater Monitoring Network, which records groundwater levels only at hourly intervals, thereby preventing the capture of rapid coseismic fluctuations; and (2) coincident rainfall events likely masked subtle earthquake-induced groundwater-level changes, making it difficult to reliably identify wells exhibiting true coseismic responses. Nevertheless, after applying a moving-average filter to remove short-term noise, subtle but systematic groundwater-level changes were identified at four monitoring wells. Among these, three wells showed groundwater-level rises, while one well exhibited a groundwater-level decline.

 

To explore the physical mechanisms underlying these observations, static coseismic crustal strain fields were simulated using the Okada-based earthquake strain model. The resulting volumetric strain at a depth of 100 m was calculated and spatially mapped, and the magnitude and sign of the inferred poroelastic pressure responses to these volumetric strains were quantitatively compared with the observed equivalent groundwater-level changes at the monitoring wells. The results reveal a clear correspondence between the sign of coseismic strain and observed groundwater-level changes. Wells located in contraction-dominated regions experienced groundwater-level rises, whereas the well located in an extension-dominated region exhibited groundwater-level decline. This spatial consistency contrasts sharply with a previous study based on Coulomb stress change analyses, which reported no systematic relationship between earthquake-induced stress changes and groundwater-level variations in Korea.

 

Our findings provide compelling evidence that coseismic groundwater-level changes in Korea can be physically linked to elastic strain induced by earthquakes, even within a monitoring network not originally designed for high-frequency seismic-hydrologic studies. These results highlight the importance of identifying sensitive wells and upgrading the selected stations with high-resolution pressure transducers and minute-scale sampling intervals. Such improvements would significantly enhance the capability of the national monitoring system to capture earthquake–groundwater interactions and provide valuable data for assessing seismic hazards in southeastern Korea experiencing increasing seismic activity with events of M_W ≥ 5.0.