Several active faults were determined in the Gulf of Izmit, using high-resolution shallow seismic and multi-beam bathymetry data. One of them is a buried fault (Fault 2), following central axis of the gulf which forms the current morphology within its releasing and restraining bends. Releasing bends control the current morphology of the sub-basins. Also restraining bends form folds in the central basin and a thrust in the western basin. A left step over between two strike-slip faults (Faults 2 and II) creates a transpressional area. Therefore, Hersek and Catal deltas are uplifted.