CRUSTAL STRAIN IN CENTRAL GREECE FROM REPEATED GPS MEASUREMENTS IN THE INTERVAL 1989-1997

A 66-station GPS network spanning central Greece, first observed in 19 89, has been occupied fully on three occasions: June 1989, October 199 1 and May 1993, Subsets of this network bounding the Gulf of Korinthos have also been occupied in June 1995, October 1995, May 1996 and Sept ember/October 1997, The first three occupations were processed using a fiducial GPS methodology, whereas later surveys were processed using CODE precise orbits. Combination of data from different surveys to yie ld smooth site velocities requires global network translations at each epoch to compensate for errors in the realization of the reference fr ame. This method provides a posteriori estimates of the relative coord inate errors and reference frame noise. Only one earthquake, the 1995 June 15 Egion event, has caused significant local coseismic displaceme nt, and its effects on the interseismic velocity field are removed usi ng an elastic dislocation model. We constrain the orientation of the 1 00 yr triangulation-GPS velocity estimates of Davies ct al. (1997) usi ng 14 sites common to the two networks. The goodness of fit of this tr ansformation indicates that the short-term and 100 yr geodetic estimat es of deformation are highly compatible. We infer that short-term geod etic studies are capable of determining longer-term deformation rates provided that transient, local effects can be modelled. From the combi ned velocity field, we estimate principal strains and rigid-body rotat ion rates at points on a regular grid using data from neighbouring sit es. Strain rates are high within the Gulf of Korinthos and much lower elsewhere. The extension rate across the Gulf of Korinthos increases f rom east to west. Comparison of the extension rate with historical and recent rates of seismic release of strain reveals significant medium- term seismic hazard in the western Gulf of Korinthos, and may also ind icate long-term aseismic strain.