The Iznik Basin is an active depression created by a series of faults developed in relation to the Middle Strand of the North Anatolian Fault (NAFMS). The most important of these faults is the oblique Soloz fault, which runs parallel to the NAFMS. It is interpreted as a releasing bend, and plays a crucial role in the evolution of the 75-m-deep ellipsoid-shaped depression in the southern part of the lake. The faults that limit the coastal alluvial plain on land, some of which developed during the early evolution of the NAFMS, are equally important in the development of the Iznik Basin, and correspond to P-shear or secondary synthetic shear faults. These faults are cut by the main fault zone and are right-laterally displaced by approximately 7-8 km. Considering an annual slip rate of 2 mm on the fault, such a displacement is only possible in a time span of 3.5 to 4 million years. An earthquake periodicity of similar to 2000-2500 years is estimated on the basis of recent GPS data, and a limited number of historical earthquakes. Linear terraces observed on the northern and western parts of the Lake Iznik, which have been affected by tectonic activity, show occasional rises of the lake level. The seismic data show that the lake level was approximately 40 m below its present level during the last glacial period.