Tectonic motions and earthquake deformation in Greece from GPS measurements

Sites in a 66-station geodetic network in central Greece have been occupied up to six times since 1989 using GPS surveying, and accurate positions have been computed using fiducially-improved or precise orbits. Site velocities are calculated under the assumption that they are constant with time, after correcting for co-seismic effects, and that the position of the fixed base station (and hence the entire network) may be subject to small errors. Low-order polynomial expressions do not fit the velocity field well. The pattern of observed strain closely resembles that derived from independent geodetic observations made over a hundred-year time-scale. Significant geodetic strain is observed across the Gulf of Korinthos, even after the co-seismic displacement field of the Ms=6.2 1995 Egion earthquake has been removed by forward modelling. Geodetic strain is higher in the western than eastern Gulf, in contrast to the seismic strain which is similar throughout. Seismic strain matches geodetic strain in the east, but a significant deficit of seismic moment exists in the west which may represent a high earthquake hazard in the medium term. The Ms=6.6 1995 Grevena earthquake struck a previously seismically quiet region well covered by a recent triangulation / trilateration survey. Ninety-one points from this network were reoccupied with GPS immediately after the earthquake, and site displacements computed. To invert for the earthquake source parameters from the geodetic displacement field, a novel inversion scheme is used which combines the Monte-Carlo and simplex approaches. A priori parameters are not required, even though the inverse problem is strongly nonlinear. The resulting focal mechanism agrees well with the global CMT solution and locally observed aftershocks, but implies a significantly higher scalar moment than do seismological studies. A network for observing post-seismic deformation has been established, which in view of the low background seismicity seems likely to provide significant results