GNGTS 2018 - 37° Convegno Nazionale

226 GNGTS 2018 S essione 1.2 In 2011 a Memorandum of Understanding between EUREF (www.euref.eu ) and CEGRN based on the ETRS89 implementation and densification of the velocity field, both of common interest, was signed at the 2011 EUREF Symposium of Chisinau, Moldova (Euref Symposium 2011). In 2014 the ten weekly campaigns (1996-2013) were re-processed following the EUREF’s guidelines for densification and stacked in a combined solution using consistently precision orbits, clocks, antenna models and processing standards (so called IGb08 reprocessing). This solution was validated by the EUREF Technical Working Group (EUREF 2015 Resolutions, 2015; Caporali et al. , 2015) as amultiyear network combination consisting of a set of coordinates, velocities (for sites with 4 or more years of tracking) and network variance-covariance matrix. CEGRN has started receiving, since mid 2017, a large number of high quality SINEX files from Agencies processing new GNSS permanent stations. The CEGRN network has consequently evolved into a very dense GNSS network that presently consists of 1242 different sites (latest CEGRN cumulative solution released on July 2018), covering Central Europe from Lithuania to Makedonia and from Switzerland to Ukraine. The data, from raw data files to final velocities, are made available to the relevant Working Groups of EUREF (http://euref. eu/euref_gb_workinggroups.html) on ‘European Dense Velocities’, ‘EPN Densification’ and ‘Deformation Models’ for validation and comparison with independent analyses. This paper is based on the last and most complete cumulative solution of the CEGRN network (1996-2017), and aims at investigating velocity profiles across major tectonic structures in Central Europe. This is made possible by the availability of positions and velocities of a regional scale network least squares adjusted in a consistent way according to state of the art processing guidelines (Bruyninx et al. , 2018). Velocity data base and profiles. Eight Analysis Centers (AC) are involved in the delivery of the velocity database for CEGRN: the Military University of Technology (MUT) in Warsaw, Poland, for the backbone European Permanent Network, the CEGRN Analysis Center (CEG) and the University of Padova (UPA), delivering network adjustments for the core CEGRN network and the Italian Permanent Network; the Main Astronomical Observatory in Kiev delivering network adjustments for Ukraine and surrounding areas; the Geodetický a Kartografický Ústav, Slovakia (GKU); Glowny Urzad Geodezji i Kartografii, Poland (ASG); Serbian Geodetic Institute- Republički geodetski zavod (RGZ) and FOMI Satellite Geodetic Observatory, Hungary (SGO). The EPN Guidelines are adopted by every Analysis Center, ensuring consistency and homogeneity of methods and results. The multiyear normal equations stackings of each network computed by the AC’s are eventually combined into one position and velocity solution for the entire area covered by CEGRN. For each subnetwork involved in the combination four Helmert parameters (one scale and three translations) are solved for, ensuring consistency in origin and scale relatively to those of the combined solution. At this stage the entire network is aligned in position and velocity to the EPN network (C1980 release), and hence to the IGS14/ITRF2014 global network using Minimum Constraints. The resulting velocities in an European fixed frame (ETRF2000) form an invaluable sensor of deformation at a regional scale. We summarize in Fig. 2a a number of profiles where the velocities have been interpolated to the profile and projected along the profile. These profiles are useful to visualize regions undergoing compression or extension. One additional profile across the South Eastern Alps (Fig. 2b) shows the velocities projected orthogonally to the profile, to highlight the shear deformation. The profiles have been chosen so as to balance the requirement of geological significance and that of coverage of GNSS velocities, to minimize interpolation errors. Profile A0-A1: shows the extension associated to the convergence of the Hellenic arc to the boundary with the Nubia plate. The southward increase of the velocity suggests a stretching of the crust driven by the counterclockwise rotation of the Anatolian block, which is considered the consequence of the collision of Arabia with eastern Anatolia, coupled with subduction in