## 3 Results
### Analysis of Coordinate Differences  
To characterize crustal motion around the southeastern fault systems of the Korean Peninsula, GNSS velocity fields were analyzed based on observations from 23 unified control points and 10 satellite reference stations of the 2020 National GNSS Control Network. Horizontal velocities were estimated with respect to three different geodetic reference frames: the Global Strain Rate Model (GSRM v2.1), the International Terrestrial Reference Frame (ITRF2014), and the Asia-Pacific plate-fixed reference frame (APKIM2000). These models provide complementary perspectives on long-term plate motion, intraplate deformation, and regional kinematics across the study area. Figure 7 illustrates the horizontal velocity fields derived from continuous GNSS stations for the periods 2018-2020 (left) and 2019-2020 (right). The velocity vectors are shown relative to three reference frames--GSRM v2.1 (black), ITRF2014 (blue), and APKIM2000 (green)--to provide insight into absolute plate motion and regional kinematics. In addition, magenta arrows represent the inter-epoch displacements between campaign measurements, highlighting localized deviations from long-term trends. Notably, differential motion near the Yangsan Fault indicates transient strain accumulation and/or release within short time intervals. For the 2018-2020 interval, the mean difference in velocity magnitude between the observed GNSS measurements and the model-predicted velocities was calculated  
Figure 7: Horizontal velocity vectors derived from GNSS observations for the periods 2018–2020 (**left**) and 2019–2020 (**right**). Vectors are shown with respect to three geodetic reference frames: GSRM v2.1 (black), ITRF2014 (blue), and APKIM2000 (green). The reference scale for velocity vectors is 10 mm/yr. as 0.08 cm/yr for the GSRM v2.1 frame, 0.20 cm/yr for ITRF2014, and 0.24 cm/yr for APKIM2000. These differences indicate that while the overall agreement between the long-term tectonic motion models and the multi-year GNSS results is relatively good, subtle deviations are present depending on the reference frame used. Among the three models, GSRM v2.1 showed the closest match in velocity magnitude, possibly due to its incorporation of global strain rate data, which may better reflect regional deformation patterns. In contrast, APKIM2000 yielded the largest deviation, likely due to its assumption of a rigid plate-fixed frame, which may not fully account for intraplate strain accumulation in the study area. The corresponding mean azimuthal deviations for the same interval were calculated as \(11.92^{\circ}\) for GSRM v2.1, 10.05\({}^{\circ}\) for ITRF2014, and 12.67\({}^{\circ}\) for APKIM2000. These angular differences suggest that not only the magnitude but also the direction of crustal motion varies depending on the adopted model. The relatively small deviation in azimuth for ITRF2014 may indicate that it provides a more stable directional reference for this region over longer time spans, despite exhibiting a slightly higher magnitude discrepancy compared to GSRM. In contrast, the GNSS-derived velocities for the 2019-2020 interval revealed significantly larger discrepancies from the model predictions. The mean velocity differences during this one-year period were 0.42 cm/yr for GSRM v2.1, 0.53 cm/yr for ITRF2014, and 0.57 cm/yr for APKIM2000, representing a notable increase compared to the 2018-2020 results. This pattern suggests greater instability or transient deformation that is not captured by long-term reference models. Since these models are based on multi-decadal averages, they may not reflect short-term strain events such as localized aseismic slip, slow earthquakes, or postseismic deformation that can occur on shorter timescales. Similarly, mean azimuthal deviations for the 2019-2020 data were found to be 2.37\({}^{\circ}\) for GSRM v2.1, 0.05\({}^{\circ}\) for ITRF2014, and 3.12\({}^{\circ}\) for APKIM2000.