## 4 Discussion
### Temporal Variability and Observational Challenges in GNSS-Derived Strain Field Interpretation  
In particular, the marked temporal variation in strain fields between the 2018-2020 and 2019-2020 periods is a noteworthy observation, suggesting that stress accumulation in this region is not governed solely by static tectonic processes but is also influenced by dynamic mechanisms that evolve over time. One plausible explanation for this variability is the occurrence of aseismic slip along segments of the fault, which can relieve strain without generating detectable seismic events. Additionally, episodic fluid migration along structural weaknesses--such as fault zones or lithological boundaries--may induce localized crustal weakening and facilitate differential deformation. These transient mechanisms, when interacting with pre-existing structural heterogeneities, can lead to short-term fluctuations in both the magnitude and orientation of strain. Therefore, a comprehensive understanding of the temporal evolution of strain in such complex fault systems requires long-term GNSS monitoring in conjunction with integrated analyses of seismic activity and hydrogeological conditions.  
High-precision GNSS networks are capable of detecting displacements on the order of millimeters per year, making them particularly effective for monitoring inter-seismic strain accumulation, even in tectonically stable intraplate settings. Notably, geodetic strain field models offer enhanced spatial granularity compared to traditional hazard maps based on historical seismicity. This allows for the identification of aseismic but critically stressed zones that may be overlooked by earthquake catalog-based assessments. In particular, using GAMIT/GLOBK (10.7), millimeter-level precision in displacement measurement is achieved, enhancing the detection of subtle strain accumulation. Case studies from intraplate domains such as North China, Italy, and southwestern Japan have demonstrated that concentrated geodetic strain is often spatially correlated with future large-magnitude events, underscoring the utility of this approach in predictive modeling [1; 2; 70; 71].  
Nevertheless, the use of elastic strain fields for seismic hazard assessment is subject to several limitations. First, the elastic assumption inherently simplifies the rheological behavior of the crust, neglecting time-dependent viscoelastic or inelastic deformation that may significantly influence long-term strain evolution [31; 32]. In regions undergoing slow slip, fluid migration, or postseismic relaxation, this simplification may lead to misinterpretation of the actual stress state. Second, the resolution and accuracy of strain field estimates are highly dependent on the spatial distribution of GNSS stations. In areas where the network is sparse or uneven--such as mountainous terrains or offshore regions--strain interpolation can introduce artifacts and obscure localized deformation signals [2].  
Accordingly, this study improved the spatial resolution of the GNSS network by utilizing existing GNSS CORS data [72] and conducting three additional campaigns of geodetic point observations. However, these supplementary surveys are resource intensive, requiring considerable time and manpower, which poses limitations for long-term monitoring. While the study compares deformation fields between the 2018 to 2020 and 2019 to 2020 periods, incorporating more frequent temporal intervals, such as quarterly or semiannual snapshots, could enhance the ability to detect transient deformation signals and distinguish them from long-term trends. To enable high-precision monitoring of the elastic strain field in the Gyeongju and Pohang regions, where multiple nuclear power plants are concentrated, it is essential to establish a dense GNSS observation network by installing additional permanent CORSs along the Yangsan Fault zone.