## 4 Discussion
### Localized Strain Anomalies and Fault Interaction Effects on Seismic Hazard  
The application of elastic strain fields derived from GNSS observations represents a significant advancement in the quantitative assessment of seismic hazards. By interpreting relative displacements among geodetic baselines through the framework of elastic dislocation theory, it becomes possible to infer subsurface stress accumulation and delineate zones with elevated rupture potential. The eigen-decomposition of strain tensors enables the extraction of principal strain axes and maximum shear magnitudes, which provide insight into tectonic stress orientations and fault kinematics. In regions where the inferred stress directions align with mapped fault geometries, such as the Yangsan Fault, this correspondence strengthens the case for ongoing tectonic loading and heightened seismic risk. Beyond shear stress orientations, an examination of the first principal strain (\(\epsilon_{1}\)) magnitudes reveals that several triangular elements--specifically Triangles 6, 10, 11, and 16--exhibit notably elevated extensional strain. This anomalous behavior indicates localized zones of heightened stress accumulation, which may not be uniformly distributed along the fault system. Among these, Triangle 16 is particularly significant, as it lies at the intersection between the Yangsan and Ulsan Faults, a region of complex fault-fault interactions. The coexistence of oblique fault geometries in this area likely facilitates distributed deformation and transient stress reorganization. Moreover, this triangle coincides with a well-defined lithological boundary, as is evident from regional geological maps. The western side is composed predominantly of Mesozoic crystalline basement rocks, including granodiorite and granite (e.g., yyTK\({}_{2}\), ayK), whereas the eastern side contains Cretaceous sedimentary formations (e.g., K\({}_{2}\), AK\({}_{2}\)), with localized deposits of Neogene strata (e.g., O\({}_{2}\), Q) along the eastern fault margins. Such lithological contrasts introduce mechanical heterogeneity into the crust, which can act as strain concentrators by amplifying differential deformation across material interfaces [69]. The combination of structural discontinuities and lithological contrasts thus provides a plausible explanation for the elevated strain accumulation and angular deviations in strain orientation observed in these regions. These findings underscore the importance of integrating geological and structural context into the interpretation of geodetically derived strain fields, especially when identifying critical segments for seismic hazard evaluation. In this context, Triangle 16--located at the structural boundary between the Yangsan and Ulsan Faults--demonstrates a notably high maximum shear strain of 2.325 \(\upmu\)strain/yr and a shear stress direction (\(\theta\_\gamma\)max) of 87.59\({}^{\circ}\), indicating a dominant NE-SW oriented shear regime. When compared to neighboring triangles, such as Triangle 17 (\(\theta\_\gamma\)max = 33.32\({}^{\circ}\)) and Triangle 8 (\(\theta\_\gamma\)max = 42.56\({}^{\circ}\)), the substantial angular deviation in shear orientation (over 45\({}^{\circ}\)) observed at Triangle 16 suggests localized reorganization of the stress field, likely  
Figure 11: Conceptual workflow illustrating the process of converting GNSS displacement data into elastic strain and stress evaluations, which subsequently inform seismic hazard identification. This framework emphasizes the role of geodetic observations in detecting crustal deformation and supporting tectonic risk assessments. induced by complex fault-fault interactions. This directional discrepancy underscores the spatial heterogeneity of deformation patterns and supports the interpretation of Triangle 16 as a critical site for assessing fault reactivation potential. Such pronounced discrepancies in shear orientation are of particular interest for seismic hazard analysis, as they may indicate abrupt mechanical transitions that influence fault rupture behavior.