# GPS Results from Long Time Monitoring of Geodynamic Processes in South-Western Bulgaria
## 4 Comparison of the Results with Previous Studies  
To evaluate the result, we combined the results for the obtained horizontal velocities of points from Sofia's geodynamic network with the results for the point velocities from the EPN densification project. EPN Densification is a joint venture of agencies and institutions from European countries, which operates and/or performs regular processing of the data from dense national GNSS networks. The primary goal of EPN Densification is to realize a continental-scale, homogeneous, high-quality position and velocity product. The generated results are extremely reliable and freely available for the geosciences community [20]. Figure 3 shows the horizontal velocities of points from Sofia geodynamic network along with the results for the point velocities from EPN densification project.  
Figure 3: Horizontal velocities of the point from Sofia geodynamic network (red arrow) and from the EPN densification (light blue arrow).  
Previously deployed EPN stations (Figure 3) cover a broader area, whereas our network is covering the area of SW Bulgaria, focusing on the territory with the densest population in Bulgaria--around Sofia. Our network is especially aiming to provide new, reliable data on present-day geodynamics, that may be used in the evaluation of the geological risk like seismicity, landslides, debris-flow, landslides, etc. Our results, compared to the results of EPN that cover the same area, are, in general, quite comparable. However, in some areas, some differences are seen. This is especially observed in the points south of Sofia, where the results from the previous measurements are not uniform with our data. This concerns, especially, station SOFA. At the same time, station PLA1 has almost a full concurrence of the results. As seen in this nodal locality, only a few stations were available, whereas in the results of the present study, a dense network of stations is now available. The results are very homogeneous and provide a reliable base for further studies.  
The results of both networks supplement each other and are a base to evaluate the reliability of obtained velocities. From this point of view, the compliance and reliability are very high.  
The comparison of velocities from the two different networks allows some important conclusions on the role of some faults to be made, especially where EPN points cover bordering areas not covered by our observations. This is obvious in the SE part of the studied area (Figure 3). The sharp difference between stations RAZ1, DOB1, GDE1, and SATO from the west and stations SMOL and ROZH from the east clearly demonstrates the activity of the NW-SE trending fault, known as the Dospat fault. To the west, is formed an area with well-expressed higher velocities moving to the south. The difference in velocities suggests a significant amount of extension along this fault.  
In the NE part of the area, we do not have our own measurements. The velocities of the points of EPN (RIBI, TRYA, MIRK, and KARV) tend to show visibly lower velocities compared to our westerly located points (VITI, VERY, and MUHO). It remains unclear if these results are due to network systematic errors or whether they reflect geodynamic differences.