Evidence of right-lateral offsets associated with the 1912 earthquake (M-w 7.4) along the North Anatolian Fault (Gazikoy-Saros segment) allow us to survey (using DGPS) the co-seismic and cumulative slip distribution. The damage distribution and surface breaks related with the earthquake show an elongated zone of maximum intensity (X MSK) parallel to the fault rupture on land but this may extend offshore to the north-east and south-west. Detailed mapping of the fault using topographic maps and aerial photographs indicates the existence of pull-apart basins and pressure ridges. At several localities, the average 1912 offset along strike is 3.5-4 m and cumulative slip is 2-6 times that of individual movement. The fault rupture geometry and slip distribution suggest the existence of three subsegments with a combined total length of 110-120 km, a fault length and maximum slip similar to those of the 1999 Izmit earthquake. The amount of slip at the north-easternmost section and in the coastal region of the Sea of Marmara reaches an average 4 m, thereby implying the offshore extension of the 1912 rupture. The results suggest that the 1912 event generated up to 150 km of surface faulting, which would imply a M-w 7.2-7.4 earthquake and which, added with rupture lengths of the 1999 earthquakes, help to constrain the remaining seismic gap in the Sea of Marmara.