## 2 Materials and Methods
### Long-Term Crustal Velocity Field of the Korean Peninsula: Impact on Static  
_Datum Realization_  
Although the Korean Peninsula is generally regarded as a tectonically stable intraplate region, recent GNSS observations reveal consistent horizontal motions at the millimeter-per-year level. Figure 3 illustrates the horizontal crustal velocity field derived from approximately 10 years of CORS GNSS observations (2015-2025), expressed in a Eurasia-fixed reference frame. The velocity vectors indicate that the peninsula is undergoing coherent motion toward the east-southeast at an average rate of approximately 30-32 mm/year, with directional azimuths generally between 110\({}^{\circ}\) and 130\({}^{\circ}\). This motion is broadly consistent across the region and reflects stable but measurable tectonic displacement. While these velocities may appear small in a global context, they accumulate significantly over time. For example, 30 mm/year motion results in a 3 cm shift over just one year and nearly 30 cm over a decade. Such shifts are highly relevant when evaluating the adequacy of a static reference frame like KGD2002, which is fixed at epoch 2002.0 and does not account for ongoing crustal motion. Figure 3: Time series of east (**left**) and north (**right**) coordinate components from five GNSS stations (SUWN, WULJ, MUJU, KWNJ, and PUSN) in the Korean Peninsula. Each subplot compares daily position estimates processed by GAMIT (blue) and Bernese (red) software packages. Linear trends represent horizontal crustal motion, with velocity estimates (in mm/year) annotated in each plot. To mitigate the effects of these accumulated displacements, this study applied a strong-constraint adjustment strategy using 17 national continuously operating reference stations (CORSs) whose positions were computed in the KGD2002 frame. The velocities of these CORSs were rigorously analyzed and held fixed during the network realization, enabling a comparative analysis with the static reference frame KGD2002, which is fixed at a specific epoch (2002.0) and does not account for ongoing crustal motion [13]. Figure 4 presents a comparative visualization of horizontal crustal velocities at five selected GNSS sites across the Korean Peninsula: SUWN, WULJ, MUJU, KWNJ, and PUSN. The velocity vectors are derived from two empirical solutions--GAMIT/GLOBK and Bernese--based on 10-year time series analyses and were compared with three global tectonic models, ITRF2020, GSRM v2.1, and APKIM2005-IGN, computed using the UNAVCO GAGE Plate Motion Calculator [29; 30; 31; 32]. Figure 4: Horizontal crustal velocity vectors at five GNSS stations (SUWN, WULJ, MUJU, KWNJ, and PUSN) in the southern Korean Peninsula. Vectors represent velocity estimates derived from Bernese (blue) and GAMIT (yellow) processing, as well as three plate motion models: ITRF2020 (purple), GSRM v2.1 (green), and APKIM2005-IGN (red). All vectors are referenced to a fixed Eurasian plate. The arrow in the lower left indicates a scale of 30 mm /year. Overall, the observed GNSS-derived velocities exhibit strong agreement with the global model predictions, particularly in terms of vector orientation and magnitude. Most sites show horizontal motions directed toward the east-southeast at rates of approximately 30 mm/yr. Minor variations in direction and length between the empirical and model vectors are evident, especially in coastal stations such as WULJ and PUSN. These differences are particularly evident at stations such as WULJ and PUSN. However, they are more likely attributed to the influence of the nearby Yangsan Fault system rather than the coastal location itself. The Yangsan Fault is one of the most seismically active fault zones in Korea. Notably, the 2016 Gyeongju earthquake (Mw 5.8) and the 2017 Pohang earthquake (Mw 5.4)--two of the strongest earthquakes ever recorded in Korea--occurred in this region, indicating that localized crustal deformation may affect velocity estimates near active fault structures.