REPEATED GPS SURVEYS ACROSS THE IONIAN SEA - EVIDENCE OF CRUSTAL DEFORMATIONS

Geodetic evidence of crustal deformation in the Ionian area detected b y GPS surveys is given in this paper. A network consisting of nine geo detic sites crossing the Ionian sea from Calabria (southern Italy) to northwestern Greece was repeatedly surveyed, starting in 1991, within the framework of the TYRGEONET project. The results, obtained from pro cessing data from three GPS campaigns performed-on the same network in 1991, 1994 and recently in 1995, show significant changes in the posi tions of five sites. The deformations in terms of displacements (coord inate differences), are obtained by applying a rigorous statistical ap proach that analyses their significance by an original iterative proce dure based on the classical F (Fisher) test. Although more substantial geophysical conclusions require more observations, the estimated rela tive displacement pattern is generally coherent with the features of t he main tectonic structures identified for this area. The detected def ormations for the Greek sites confirm the activity of the Kephalonia r ight-lateral transform fault, and a right-lateral motion of the Mattin ata fault (northern Apulia) seems to have been detected by the Italian sites. Moreover, within the time span analysed, the site of Matera sh ows a different behaviour from the three Adriatic sites (Tremiti, M.S. Angelo and Specchia Cristi), since relative displacements among Mater a and these sites were detected. This fact may indicate the weakness o f the assumption, reported in some papers, that Matera could be a repr esentative site of the motion of the whole Adriatic plate. Furthermore , the site of Specchia Cristi shows the maximum relative displacement in the network, with a vector magnitude of about 5 cm with a 3 cm conf idence interval at the 95 pet cent level. Therefore, even if some addi tional GPS observations are needed to achieve a clearer picture of the tectonic behaviour of the Ionian Sea area, the detected relative disp lacement pattern of the Italian sites agrees with a possible rotationa l behaviour of the Apulian platform with respect to the Adriatic plate , as already hypothesized by Finetti (1982).