# Monitoring of Possible Activities of Yangsan Fault Zone Using GNSS
## 4 Results--Discussion
### Time Series
#### 4.1.2 Yearly Solution  
Using the ADDNEQ2 module of Bernese Software V5.2, two separate final solutions with global constraints were calculated by combining the movements, one for 2018 to 2019 (Solution 1) and the other for 2020 to 2021 (Solution 2). The result for relative movement with respect to GKPG follows:  
1. Absolute Velocity with IGS Constrained  
The IGS stations were fixed to analyze the trend of the movement of the Yangsan Fault Zone for each period (see Figure 4).  
Figure 4: (**a**) Displacement of station in 2018–2019; (**b**) displacement of station in 2020–2021. Red is the largest station moved, and blue is the least moved station.  
The crust around the Yangsan Fault Zone Solution has a displacement ranging from 32.0 mm to 37.6 mm in the southeast direction in Solution 1. During this period, the POHG station moved the least while the maximum movement occurred at WOLS station. From 2020 to 2021 (Solution 2), the movement direction is similar to Solution 1 with the displacement from 31.9 mm to 34.6 mm (POHG and GCNG are the minimum and the maximum, respectively, see Table 4). This is close to the result of annual movement of the Korean Peninsula [19; 20]. In addition, as a result of comparison for the velocity and direction on each side of the Yangsan Fault Zone, there seems to be no significant difference between two regions. Therefore, it might be concluded that the area around the Yangsan Fault Zone is stable at the moment.  
Among the velocity vectors between 2018 and 2019, the GYU station moved with a magnitude of about 73.9 mm/year in the direction of 132.0\({}^{\circ}\) (in azimuth). This was excluded from the calculation because there might be a potential error with the scale and direction of movement because the observation data were too small (see Figure 5).  
2 Relative Velocity with respect to GKPG  
As a result of the previous study, the displacement of the Yangsan Fault appears to have no significant change in terms of magnitude and direction of the crustal movement. Nevertheless, there might be an internal distortion that is potentially more important in the analysis of the crustal movement. Therefore, additional analysis was performed to clearly confirm the distortion inside the tomography. It was analyzed by calculating the relative displacement of the crust around the Yangsan Fault based on the GKPG station. This study used data from 25 February 2019, to 31 December 2021. The reason for the difference in the study period is that the data of the GKPG, which is a fixed point, were not available for outside the above period.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Velocity [mm/year]** & **Direction/Azimuth [deg]** \\ \hline
2018–2019 & 34.9 & 110.35 \\
2020–2021 & 33.6 & 110.83 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Difference between velocities and directions.  
Figure 5: Time series of GYU Station.  
Figure 6 shows the results of the baseline analysis with GKPG station (red star) fixed in millimeters, which is the displacement calculated excluding the movement of the GKPG station. As a result, most stations around the Yangsan Fault move in the northwest direction at an average of about 0.18 cm. Considering that most stations move in the same direction and magnitude, it is judged that there is no distortion of the crust inside the Yangsan Fault. Based on the results of this study, it is not possible to conclude whether the Yangsan Fault is active or not. This is because the period analyzed in this study is relatively short to conclude. However, it would be possible to confirm the activity of the fault if continuous research with a longer data span is conducted in this regard.