## 3 Results
### Regional Analysis of Coordinate Differences
In contrast, the Bernese vs. KGD2002 comparison shown in Figure 8 reveals higher variability in directional spread. Notably, Chungnam displays a strong bias toward the east-southeast (mean azimuth \(\approx\) 103.8\({}^{\circ}\), concentration \(\approx\) 0.36), while Gangwon shows a wider azimuth range with less concentration (\(\approx\)0.14), indicating locally differing influences in station geometry or processing constraints. Figure 7: Polar histograms showing the azimuthal distribution of \(\Delta\)N, \(\Delta\)E residuals between the GAMIT solution and the official KGD2002 coordinates, grouped by administrative region. Each subplot corresponds to one of nine provinces, illustrating the directional spread of horizontal discrepancies. Residual vectors were projected in the local NEU frame, and their azimuths were computed from atan2(\(\Delta\)N, \(\Delta\)E). Most regions exhibit near-isotropic patterns, while some (e.g., Jeju and Gyeongnam) show mild directional concentration. This directional discrepancy may partially stem from the fact that the KGD2002 frame itself was originally realized using solutions from the GAMIT/GLOBK software. As demonstrated in Section 2.5, comparisons between 10-year velocity fields independently computed using GAMIT and Bernese reveal regional differences in estimated crustal motion of up to -30 mm/year. These differences can likely be attributed to variations in satellite orbit modeling, geophysical parameter selection, and stochastic strategies applied in each processing software. However, consistent directional biases observed in regions such as Gyeongbuk cannot be fully explained by software-induced variations alone. Rather, these patterns are more plausibly linked to the cumulative effects of datum deformation and datum holding, whereby the static KGD2002 frame--fixed to an early epoch--fails to accommodate present-day geodynamic conditions. This interpretation is further supported by significant seismic activity concentrated in the Gyeongbuk area, including the 2016 Gyeongju earthquake (Mw 5.8) and the 2017 Pohang earthquake (Mw 5.4), which provide independent geophysical evidence for ongoing crustal deformation in the region. Figure 8: Polar histograms showing the azimuthal distribution of \(\Delta\)N, \(\Delta\)E residuals between the Bernese solution and the official KGD2002 coordinates, grouped by administrative region. Each subplot illustrates the directional distribution of horizontal discrepancies within a given province. In certain regions—particularly Chungnam and Gangwon—the residuals exhibit noticeable clustering toward specific directions (e.g., east–southeast), potentially reflecting local network geometry or processing-specific influences associated with the Bernese software. The GAMIT vs. Bernese residuals (Figure 9) show more pronounced directional clustering in some regions, particularly along Korea's eastern coastal provinces such as Gyeongbuk and Gangwon. These areas display mean azimuths of 85.1\({}^{\circ}\) and 91.2\({}^{\circ}\), respectively, with moderate concentration indices (\(\approx\)0.27-0.31). Such alignment may reflect systematic biases related to model differences or reference frame realization inconsistencies between the two processing strategies. Overall, the polar histogram analysis reinforces and refines the magnitude-based interpretations. While most regions exhibit broadly dispersed residual vectors, certain areas--particularly those affected by tectonic activity--show recurring azimuthal patterns. These directional biases may be partially attributed to the continued use of the outdated KGD2002 reference frame, which no longer reflects the current geodynamic conditions. Over time, the accumulation of datum deformation within a static frame can lead to systematic discrepancies in GNSS-derived velocities. Figure 9: Polar histograms illustrating the azimuthal distribution of \(\Delta\)N, \(\Delta\)E horizontal coordinate differences between the GAMIT and Bernese solutions.