A field experiment carried out from 17 July to 2 November 1999 with a dense seismic network surrounding the Marmara Sea region and including the rupture zone of the İzmit (Kocaeli) destructive earthquake of 17 August 1999 (MW 7.6, Harvard centroid moment tensor); (Delouis et al., 2002) permitted the collection of precise seismic data and the quantitative evaluation of the associated set of surface ruptures. A previous experiment conducted in 1995 had already given reliable information about the distribution of seismicity in space, focal mechanisms, and the regional stress regime. Aftershock distribution and clustering, together with the variations of slip amplitude along the fault, allow the identification of at least five segments. A maximum horizontal displacement of about 5 m is observed in Gölcük and east of Sapanca Lake. The rupture is extremely linear, but its complexity increases toward the western end, including the bifurcation. The stress field associated with the aftershocks is compared with the one obtained in 1995, showing the invariance of the smallest principal axis. Lack of definition of the other principal axes suggests a transition from shear to extension. A clear quiescence period of 18 days preceded the İzmit earthquake. A precursor, which occurred 100 sec before, had the same epicenter as the mainshock.

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