The 17 August 1999 Izmit earthquake broke four distinct structural segments of the North Anatolian fault, from east to west: the Karadere, Sakarya, Sapanca, and Golcuk fault segments. These segments are separated from one another and the adjoining Yalova and Duzce fault segments to the west and east, respectively, by distinct right-releasing step-over and/or gaps in the fault trace. These include, from east to west, the Eften Lake step-over (2-4 km wide), Akyazi gap (6-13 kin long), Sapanca step-over (1-2 km wide), Golcuk step-over (1-2 kin wide), and Karamursel step-over (4-5 kin wide). The earthquake nucleated within or near the Golcuk step-over with bilateral rupture propagation to the west and east. To the west, about 4-5.5 m of surface rupture (up to 7 in of subsurface rupture) on the Golcuk segment was mostly, if not entirely, arrested by the 4- to 5-km-wide Karamursel step-over. To the east, about 3-4 in of surface rupture (about 4 m of subsurface rupture) propagated through the 1- to 2-km-wide Sapanca step-over, triggering up to 4.5-5.5 in of surface rupture (up to 5-6 m of subsurface rupture) on the Sakarya segment. Rupture on the Sakarya segment stepped across the Akyazi gap triggering 1-1.5 in of rupture on the Karadere segment. Rupture on the Karadere segment was arrested by the 2- to 4-km-wide Eften Lake step-over.