INFLUENCE OF INTRAPLATE DEFORMATIONS ON THE STATE GPS NETWORK IN THE REPUBLIC OF BULGARIA

The Republic of Bulgaria is located on the periphery of the Eurasian tectonic plate, and part of it falls within the seismically active Aegean extensional region [1]. The study of modern crustal movements is important in terms of elucidating the geodynamic behaviour of the country on a regional and global scale. It should be taken into account when establishing and maintaining geodetic networks for high precision applications. It should be noted that for the main carrier of the Bulgarian geodetic system - the State GPS network, only the velocities of the points of its Basic class are derived and published. Intra-plate displacements show an increase from about 1 millimetre per year in Northern Bulgaria to about 5 millimetres per year in Southern Bulgaria - mostly in the geodynamically very active South-West region [2]. For the present study, we have selected an area in the southeastern part of Bulgaria, a region with relatively homogeneous horizontal velocities with respect to Eurasia. The area is 1500 sq. km, with the presence of points from the basic class (9 points) and secondclass (28 points) GPS network. Class I level surveys are currently being carried out in the area, and the coordinates of the points in the State GPS network will be used as fixed points to determine the geodetic coordinates and heights of the bench marks from the level surveys. The aim of the study is to show the effect of the manifested intra-plate deformations on the coordinates of the State GPS network, as well as the need to develop a strategy/methodology to account for point displacements over time and calculate them, taking into account the passive nature of the State GPS network of the Republic of Bulgaria.