The catastrophic rupture of the North Anatolian Fault east of the Marmara Sea on 17 August 1999 highlighted a need for mapping the underwater extension of that continental transform. A new bathymetric map of Izmit Gulf indicates that the fault follows the axis of the gulf with a few minor bends. Submerged shorelines and shelf breaks that formed during the Last Glacial Maximum provide markers to quantify vertical deformation. Variable tilting of these horizons reveals that vertical deformation is highest just south of the fault. A correlation between vertical deformation of the southern fault block and distance to fault bends can be accounted for by a fault dipping steeply to the south. Hence subsidence (uplift) of the southern, hanging wall block would be expected where the fault strikes at a slightly transtensional (transpressional) orientation to relative plate motion. Subsidence reaches about 8 mm/yr west of the town of Golcuk and might be accommodated in 1--2 m subsidence events during large earthquakes. That scenario is compatible with the tsunami runups and the coseismic subsidence of the southern shore that occurred in 1999. Seafloor morphology also suggests that earthquakes are accompanied by widespread gas and fluid release. The periphery of the deepest basin displays a hummocky texture diagnostic of sediment fluidization, and mud volcanoes occur west of Hersek peninsula that might be activated by earthquakes. Finally, the backscatter imagery reveals a series of lineaments midway through the gulf that are interpreted as products of the 1999 surface rupture. The seafloor is undisturbed farther west, suggesting that surface slip decreased to an insignificant level beyond Hersek. Possibly, the stress shadow from the 10 July 1894 earthquake, which was felt strongly along the western Izmit Gulf, contributed to arrest the 1999 surface rupture.