The North Anatolian Fault (NAF) is a major continental transform system that extends E-W across Turkey for over 1600 km, separating the Anatolian and Eurasian plates. A portion of its northern branch runs below the Sea of Marmara. This portion constitutes a "seismic gap" because the last destructive earthquakes occurred at the western (1912 Ganos earthquake, M 7.4) and eastern (1999 Izmit and Duzce earthquakes, M-w 7.4, 7.2) edges of the Marmara basin. It is likely that fault ruptures will fill this gap in the next decades. This region of the North Anatolian Fault is critical to our understanding of fault interactions, stress buildup during seismic cycle and seismic hazard in the Istanbul area. We obtained high-resolution acoustic images of the NAF in the floor of the eastern Marmara Sea (Gulf of Izmit), and measured fault-related offsets of C-14-dated subseafloor channels and paleoshorelines. The resulting average slip rate on the fault is similar to10 mm/year for the last 10 kyr. This is less than half the total Anatolia-Eurasia relative motion, estimated at 24 mm/year for the last similar to10 years from satellite geodetic measurements. We conclude that either much of the strike-slip motion along this branch of the NAF did not occur on the main fault segment or the slip rate increased recently, or both. These results affect kinematic models of the NAF and assessments of seismic hazard for the city of Istanbul and the surrounding region. (C) 2004 Elsevier B.V. All rights reserved.