Active faults in the Gulf of İzmit on the North Anatolian Fault, NW Turkey: a high-resolution shallow seismic study

The northern strand of the North Anatolian Fault enters the Sea of Marmara through the Gulf of İzmit which was affected by the Mw=7.4 İzmit earthquake on August 17, 1999. A significant segment of the fault ruptured in between the area west of Gölcük and east of Lake Sapanca during the earthquake. In the southeastern corner of the gulf and from Gölcük westward the active trace plunges into the sea and is lost to view. We investigated the Gulf of İzmit by means of high-resolution shallow seismic data acquired during a post-August 17, 1999 earthquake to locate and map the active faults. In the area, considering the neotectonic features around the gulf, two different sets of faults were differentiated: an earlier and now mostly inactive set of faults that are responsible for the formation of the large pull-apart depression in which the Gulf of İzmit is located and a younger, second set of active faults in the gulf that cut through the former set. The second set was further divided into two groups: the main fault and the secondary faults. The main fault, which extends roughly in an E–W direction as rather throughgoing and longer segments, was traced between the southeast end of the gulf in the east and south of Darıca in the west where it joins the Sea of Marmara Fault. The secondary faults, on the other hand, lie along the main fault; they are shorter and their strikes are controlled by the bathymetry. Two main seismic sequences were identified in the gulf according to the seismic profiles and existing borehole data: Holocene marine sediments and pre-Holocene fluvio-lacustrine sediments.