We present new splitting measurements for SKS and SKKS phases recorded at four broadband seismic stations located within the Carpathian Arc, in the eastern part of the Transylvanian Basin. We made 75 measurements, of which 58 are nulls and 17 show clear splitting. We used two methods to estimate splitting parameters, ϕ (the fast polarization direction) and δt (the delay time between fast and slow split shear waves): minimization of energy on the horizontal component corresponding to the minimum eigenvalue of the polarization matrix, and simultaneous linearization of split waves from a suite of earthquakes recorded at a given station. We consistently observed no splitting at station IACB, in the northern part of the study area, for a range of well-recorded earthquakes at different backazimuths; we consider the station splitting to be null. Null splitting here is consistent with several possible tectonic scenarios, including downwelling of asthenospheric material as a result of the sinking Vrancea body, asthenospheric upwelling beneath Ciomadul volcano, or lack of anisotropic fabric development in the large Ciomadul magma chamber beneath the station. Fast polarized shear waves trend NW–SE at CHDM, in the Transylvanian Basin, and at PMAR, which lies at the boundary between the Transylvanian Basin and the E–W-striking Southern Carpathians; these fast directions are consistent with a regional trend in splitting ϕ azimuths over much of the Eastern Carpathians, the East European and Moesian Platforms, and the northwestern Transylvanian Basin away from the Vrancea Bend Zone. The regional NW–SE splitting trend parallels the NW-to-SE temporal migration of Neogene volcanism along the Eastern Carpathians, and is likely due to a combination of southeastward rollback of the Vrancea high velocity body and simultaneous southwestwards absolute plate motion of the thick leading edge of the East European Platform (Tornquist-Teisseyre Suture Zone), which together constrain regional mantle flow. Fast shear wave polarization, ϕ , at station COMD, at the juncture between the Vrancea Bend Zone and the Transylvanian Basin, trends NE–SW, consistent with splitting at stations VOIR and MLR in the Southern Carpathians. Splitting at these stations reflects perturbed mantle flow in the vicinity of the Vrancea body, which entails an abrupt change from NE–SW at VOIR to NNW–SSE at PMAR, consistent with redirection of mantle flow along the northwestern side of the Vrancea body. Delay times at the three stations with clear splitting vary from 0.96 to 2.73 s, indicating either the presence of an anisotropic layer ranging between ∼100 km thick beneath CHDM and 300 km beneath PMAR, or an unusually strong anisotropic fabric beneath the latter.

The Vrancea region in the South-East Carpathians of Romania is a seismically active area. The tectonic processes responsible for the earthquakes in this region are not uniquely identified yet. GPS campaigns of 3-D crustal displacements in this region, performed by ISES (Netherlands) in collaboration with SFB-461 (Germany) and the National Institute for Earth Physics and the University of Bucharest (Romania), are expected to assist in solving this issue. Measurements have been performed in this area since 1995 and the current GPS network consists of more than 50 campaign points and six permanent GPS stations.

Results are presented from GPS measurements performed in the Southeastern Carpathians between 1997 and 2004. Data from 25 stations observed during 13 campaigns were analyzed by the Department of Earth Observation and Space Systems (DEOS) of Delft University of Technology. The repeatabilities of the solutions are on the order of 1–4 mm for the horizontal, and 4–8 mm for the vertical component. The resulting velocity estimates have an uncertainty of <1 mm/yr and <3 mm/yr, respectively. The region southeast of the Carpathian bend zone shows a horizontal movement towards SSE of ∼2.5 mm/yr, while the Transylvanian Basin shows very small motions with respect to Eurasia. The vertical velocity field indicates the existence of uplift and subsidence domains in the SE Carpathians, in good agreement with Pliocene–Quaternary orogen and basin studies. Another 29 GPS stations installed in the last 3 yr will generate a denser velocity field in the coming years for this region.

PLATE TECTONICS KINEMATICS & GEODYNAMICS O9 - 04 TEN YEARS OF GPS OBSERVATIONS IN ROMANIA 1 1 2 3 4 5 B.A.C. Ambrosius A.G.A. van der Hoeven V. Mocanu L. Munteanu W. Spakman G. Schmitt 1 Delft University of Technology The Netherlands 2 University of Bucharest Faculty of Geology and Geophysics Bucharest Romania 3 National Institute of Earth Physics Bucharest Romania 4 Utrecht University The Netherlands 5 Geodetic Institute Karlsruhe Germany Summary This paper presents an overview of the GPS network development and the measurements performed in eastern Romania between 1995 and 2004. In this time span the GPS network