We use Synthetic Aperture Radar interferometry (InSAR) to map the displacement field of the 17 August 1999 Izmit earthquake, which largely conforms to that predicted for an elastic upper crust. We determine the earthquake source parameters and show that slip continues farther west than the mapped fault ruptures. We also show that additional sub-surface displacements occurred on parallel strands of the North Anatolian Fault Zone. We argue that this was caused by changes in static stress accompanying the mainshock, or by the dynamic release of regional stresses. ¿ 2001 American Geophysical Union