## 3 Results
### Strain Tensor Analysis and Spatial Characterization of Deformation
High-resolution monitoring in these areas is essential for detecting early signals of deformation, improving regional seismic hazard forecasting, and refining the understanding of fault dynamics along the Yangsan Fault and its adjacent segments. The magnitude and direction of principal strain were visualized in Figures 8 and 9, based on the data derived from Tables 1 and 2. Figure 8 illustrates the principal strain ellipses for the two observation periods, 2018-2020 and 2019-2020, helping to clearly understand the regions of strain concentration and regional deformation patterns. Each ellipse represents the magnitude and orientation of the maximum and minimum principal strain axes, allowing for a comparison of strain pattern differences between the two periods. The red lines in the figure represent the locations of the Yangsan Fault (left) and Ulsan Fault (right), providing context for the spatial distribution of strain relative to these key tectonic features. This visualization helps to identify localized strain accumulation areas and provides insights into how deformation evolves over time. Figure 8: Maps of principal strain magnitude and direction derived from GNSS-based strain tensor analysis for two observation periods: (1) 2018–2020 (left) and (2) 2019–2020 (center). The rightmost panel illustrates the triangulated geodetic network used for strain computation. Black bars indicate the orientations and magnitudes of maximum and minimum principal strain axes, with bar lengths scaled to strain intensity (\(\upmu\)strain/yr). The red lines denote the Yangsan Fault (western trace) and the Ulsan Fault (eastern trace). Strain accumulation is notably concentrated between the two fault zones, particularly in the central segment of the Yangsan Fault. The network includes 23 geodetic control points (blue squares) and 10 CORS (black circles). Figure 9 visualizes the maximum shear strain vectors, highlighting the distribution of shear deformation in regions centered around the Yangsan Fault. The red lines again indicate the Yangsan Fault (left) and Ulsan Fault (right), emphasizing how shear deformation is concentrated in these areas. The direction and magnitude of the strain vectors are used to identify areas where deformation is focused and to show the changes in deformation patterns during the two observation periods. The visualization particularly highlights the deformation zones around the Yangsan Fault, offering a clearer understanding of regions with higher seismic potential. Figures 8 and 9 illustrate the spatial distribution of principal strain and maximum shear strain for the periods 2018-2020 and 2019-2020. These maps highlight regions of strain concentration along the Yangsan Fault and its surrounding areas, showing distinct differences in strain magnitude and orientation between the two observation periods. Figure 8 presents the magnitude and direction of principal strain, visualizing areas of concentrated deformation. High strain values, such as those observed in Triangle 6 (2.498 \(\upmu\)strain/yr and 3.482 \(\upmu\)strain/yr) and Triangle 10 (2.367 \(\upmu\)strain/yr), indicate significant deformation likely related to active faulting along the Yangsan Fault. These regions suggest the potential for strain accumulation, which could elevate seismic hazard risk. Negative strain values, such as those found in Triangle 11 (\(-5.588\)\(\upmu\)strain/yr), indicate compressive stress, which may reflect the reactivation of faults or areas with increased potential for seismic events. In this context, attention must be given to the shear stress direction at Triangle 16, located at the structural boundary where the Yangsan and Ulsan faults diverge. Notably,  
Figure 9: Maps of maximum shear strain magnitude based on GNSS-derived strain tensor analysis for two observation periods: (1) 2018–2020 (left) and (2) 2019–2020 (center). The rightmost panel shows the triangulated geodetic network employed for strain estimation, with triangle IDs indicated.