ACTIVE CRUSTAL DEFORMATION IN THE MARMARA SEA REGION, NW ANATOLIA, INFERRED FROM GPS MEASUREMENTS

Deformation in the boundary zone between the Anatolian and Eurasian pl ates has been determined by means of repeated Global Positioning Syste m (GPS) measurements. Three campaigns (1990, 1992, 1994) were carried out around the Marmara Sea. The network lies within the active earthqu ake belt of NW Anatolia, extending from the western part of the North Anatolian Fault Zone (NAFZ) to the Aegean extensional province. Result s of the dense network consisting of 52 sites clearly reveal the westw ard motion of Anatolia. The rates amount to 17 mm/a +/- 3 mm/a on aver age relative to Istanbul, located at the northeastern edge of the Sea of Marmara. Based on the trajectories for the displacements of the obs ervation sites it is concluded that the Marmara Sea forms a transition zone between the Europe fixed region of Thrace/Black Sea (Eurasia) an d NW Anatolia. At the western extremity of NW Anatolia the motion bend s from W to WSW(247 degrees). The strain field calculated yields an E- W oriented right-lateral strike-slip movement (average 0.12 ppm/a, max imum 0.41 ppm/a) accompanied by NE-SW extension (average -0.12 ppm/a, maximum -0.52 ppm/a) at the eastern end of the Marmara Sea and N-S ext ension at the Aegean shore line. Comparisons with the directions of T axes derived from 65 fault plane solutions of earthquakes show remarka ble agreement.