Repeated GPS surveys across the Ionian Sea: Evidence of crustal deformations

Geodetic evidence of crustal deformation in the Ionian area detected by GPS surveys is given in this paper. A network consisting of nine geodetic 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 processing data from three GPS campaigns performed-on the same network in 1991, 1994 and recently in 1995, show significant changes in the positions of five sites. The deformations in terms of displacements (coordinate differences), are obtained by applying a rigorous statistical approach that analyses their significance by an original iterative procedure based on the classical F (Fisher) test. Although more substantial geophysical conclusions require more observations, the estimated relative displacement pattern is generally coherent with the features of the main tectonic structures identified for this area. The detected deformations for the Greek sites confirm the activity of the Kephalonia right-lateral transform fault, and a right-lateral motion of the Mattinata fault (northern Apulia) seems to have been detected by the Italian sites. Moreover, within the time span analysed, the site of Matera shows a different behaviour from the three Adriatic sites (Tremiti, M.S. Angelo and Specchia Cristi), since relative displacements among Matera and these sites were detected. This fact may indicate the weakness of the assumption, reported in some papers, that Matera could be a representative 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 confidence interval at the 95 pet cent level. Therefore, even if some additional GPS observations are needed to achieve a clearer picture of the tectonic behaviour of the Ionian Sea area, the detected relative displacement pattern of the Italian sites agrees with a possible rotational behaviour of the Apulian platform with respect to the Adriatic plate, as already hypothesized by Finetti (1982).