Analysis of long-term GPS observations in Greece (1993-2009) and geodynamic implications for the Eastern Mediterranean

The Global Positioning System (GPS) is used to determine rates of crustal motion in the Hellenic plate boundary zone since the late eighties. The zone comprises Greece and its adjacent regions. It is characterized by the interaction of the Eurasian, Anatolian and Nubian plates moving relative to each other. Tectonic processes associated with this motion cause the highest seismic activity of Europe. Nine earthquakes with magnitude six or higher occurred within this region in the period 2006 to 2011. International efforts are being made to achieve a better understanding of the origin and characteristics of ongoing seismotectonic processes. The ETH Zurich has taken active part in these efforts. Results achieved so far provide valuable boundary conditions for geodynamic modeling. In close collaboration with other institutes an extensive GPS network has been established and periodically remeasured. The network consists of campaign-type and continuous GPS sites. The corresponding data record has been significantly extended in the course of the thesis. A 16 year record of GPS data (1993–2009) is available now for Greece. In its first part the thesis deals with the evaluation of the data record. The strategy of GPS analysis was enhanced compared to proceeding projects by applying recent GPS processing models and improving postprocessing procedures. The concept of including data of numerous IGS and EUREF sites located in Europe, North Africa and the Middle East was continued. This allowed for a precise alignment of the GPS network to a global reference frame (ITRF2005), the reduction of processing induced systematic signals and the determination of a pole of rotation for the Eurasian plate. An improved kinematic field was derived which was analyzed in terms of slowly deforming regions by using a block model. The modeling provides several insights. For instance, northwestern Greece rotates clockwise (cw) and the region south of the North Aegean trough (NAT) rotates counterclockwise (ccw) relative to Eurasia. Both areas form blocks with small internal deformation. Moreover, they describe the western termination of the North Aegean trough. A large part of ongoing deformation is located in confined regions. The GPS derived deformation field provides information about ongoing tectonic processes. These include N-S extension between Northern Greece and the Gulf of Corinth and dextral shear strain in the North Aegean sea as well as along the Kefalonia fault zone in the Ionian sea. New results concerning ongoing deformation processes were achieved in the Southeast Aegean sea and in Northern Greece. Pronounced N-S extension (100 nstrain/yr) was determined across the Mygdonian graben. NNW-SSE extension amounting to 150 nstrain/yr was found between the islands of Kalymnos and Tilos in the SE Aegean sea. The NAT and the Kefalonia fault zone are among the most pronounced transform faults in the Hellenic plate boundary zone. Rates of closely located GPS sites were used for the estimation of the slip rates and locking depths of the structures. Four profiles across the NAT show a decrease of slip rates from the Ganos fault (23 mm/yr) towards the southwestern edge of the Sporades basin (<5 mm/yr). The geodetic results provide further evidence that the NAT forms the westward continuation of the North Anatolian fault zone commencing at the Saros basin. The decrease of slip rates west of the Chalkidiki peninsula is related to a different orientation of the NAT and to NNE-SSW extension in the Sporades basin. Finite element models were used to relate GPS rates to basic geodynamic models. The first model I focused at the analysis of the subduction rate near the central Hellenic trench. The results point to a largely uncoupled interface between the Nubian and Aegean plates. The second model quantifies the slip rates along the three major NE-SW to ENE-WSW trending transform faults in the North Aegean sea. Rates amounting to 21 mm/yr were derived for the NAT, 10 mm/yr for the SkyrosEdremit fault and 4 mm/yr for the Psara-Lesvos fault. Moreover, the model reveals additional deformation zones such as NNE-SSW extension in the Sporades basin. The derived GPS rates and the conducted analyses improve the current understanding of seismotectonic processes in Greece. The investigations also highlight remaining problems and bring forward new ideas which will ultimately be valuable for further analysis and assessment of natural hazard in Greece.