Surface motions in Romania derived from 15 years of continuous GNSS measurements

In this paper we present the results of an analysis of 15 years of continuous GNSS/GPS measurements in Romania. The purpose of this work is threefold: monitoring of crustal changes occurring in the Romanian territory in correlation with tectonic processes in South-East Europe (Africa-Europe plate interaction), observation of crustal movements in order to establish the surface-to-depth relationship of deep earthquakes in the area of the Eastern Carpathians bend zone (Vrancea region) and improving the accuracy of the coordinates of the national seismic network stations. The development of the Romanian GNSS/GPS (Global Navigation Satellite System/Global Positioning System) network started in 2001 when the first permanent station was installed at Lacauti in the Vrancea seismogenic area. Since then the network has grown to 27 stations covering the entire Romanian territory and is still expanding. In our analysis we also include 18 GNSS stations of the GeoPontica network, developed and maintained by the National Research and Development Institute for Marine Geology and Geoecology, with the purpose to monitor geodynamic processes in the Western Black Sea coastal area. It consists of 13 stations in Romania and 5 in Bulgaria. In this paper we provide a comprehensive description of the GPS networks, reflecting the actual capabilities, justifying implementation choices and presenting the results. The GPS data were processed in daily batches with the “precise point positioning” (PPP) strategy using the GIPSY-OASIS software. From the resulting position timeseries the horizontal and vertical motion vectors were calculated relative to a stable Eurasian reference frame. The results of our study show that internally the Romanian territory appears to be quite stable, but the whole country trends to move slightly southward relative to Eurasia at velocity rates of about 2.5 – 3.0 mm/yr, especially the southern part. We speculate that this is a far-field effect of slab roll-back due to the subduction of the African plate under the Eurasian plate at the Aegean trench, way to the south. Until now, we did not observe any long-term signals due to the deep Vrancea earthquakes because the effects on surface motion are probably small, but we will do more detailed studies of some recent (2016) Mw 5.3 seismic events in the Vrancea area.