GEODETIC STRAIN OF GREECE IN THE INTERVAL 1892-1992

A first-order triangulation of Greece was carried out in the 1890s, Re occupation, using Global Positioning System receivers, of 46 of the 93 original markers yielded estimates of the deformation of the region o ver the intervening interval. Broad regions have similar geodetic stra in over the 100-year time span. Strain north of the Gulf of Korinthos is predominantly north-south extension, though with a significant east -west component. The central Peloponnisos is relatively stable, wherea s the gulfs of the southern Peloponnisos are all characterized by unia xial east-west extension. The seismic expression of strain for the ent ire region, calculated from the seismic moment tensors of earthquakes of M-s greater than or equal to 5.8 during the past 100 years, account s for only 20-50% of the geodetically determined strain. At a scale of 50-100 km, the fraction of the strain that is expressed seismically v aries much more than this range. In particular, whereas seismic strain in the eastern Gulf of Korinthos over the past 100 years is commensur ate with the geodetic strain, there is rapid extension across the west ern Gulf of Korinthos (similar to 0.3 mu strain yr(-1)), with negligib le seismic strain for the 100 year period prior to 1992. The Egion ear thquake of June 1995 in the western Gulf of Korinthos released only a small proportion (less than or equal to 20%) of the elastic strain tha t had accumulated in that region. The observed distribution of displac ements can be explained by the relative rotation of two plates with a broad accommodation zone between them, but it is equally consistent wi th the deformation that would be expected of a sheet of fluid moving t oward a low-pressure boundary at the Hellenic Trench. A simple calcula tion implies that if the region does behave as a fluid, then its effec tive viscosity is similar to 10(22)-10(23) Pa s. Such viscosities are consistent with the deformation of a lithosphere obeying a rheological law similar to that obtained for olivine in the laboratory.