The 17 August 1999 Izmit earthquake occurred on the northern strand of the North Anatolian fault zone. The earthquake is associated with a 145-km-long surface rupture that extends from southwest of Duzce in the east to west of Hersek delta in the west. Detailed mapping of the surface rupture shows that it consists of five segments separated by releasing step-overs; herein named the Hersek, Karamursel-Golcuk, Izmit-Sapanca Lake, Sapanca-Akyazi, and Karadere segments from west to east, respectively. The Hersek segment, which cuts the tip of a large delta plain in the western end of the rupture zone, has an orientation of N80degrees. The N70degrees-80degreesE-trending Karamursel-Golcuk segment extends along the linear southern coasts of the Izmit Gulf between Karamursel and Golcuk and produced the 470-cm maximum displacement in Golcuk. The northwest-southeast-striking Golcuk normal fault between the Karamursel-Golcuk and Izmit-Sapanca segments has 2.3-m maximum vertical displacement. The maximum dextral offset along the Izmit-Sapanea Lake segment was measured to be about 3.5 m, and its trend varies between N80degreesE and east-west. The Sapanca-Akyazi segment trends N75degrees-85degreesW and expresses a maximum displacement of 5.2 m. The Karadere segment trends N65degreesE and produced up to 1.5-m maximum displacement. The Karadere and Sapanca-Akyazi segments form fan-shape or splaying ruptures near their eastern ends where the displacement also diminished.