The İzmit/Düzce earthquake sequence is the largest sequence recorded by modern digital networks in the history of Turkish earthquakes. The (Mw 7.4) İzmit mainshock occurred at 29.967° E and 40.729° N, and 13 km depth at 00 hr 01 min 38.7 sec UTC, on 17 August 1999 and was preceded by at least eight immediate foreshocks of magnitude ML < 3.0. Aftershock locations show that a large region has become seismically very energetic following the mainshock and that most of the aftershock zone developed quickly in the first 3 days. The distribution of the aftershocks illuminates numerous seismotectonic structures associated with the various branches of the North Anatolian Fault (NAF) in the aftershock zone. The seismicity pattern defines a rupture plane that extends about 80 km east and 75 km west of the mainshock epicenter, confirming bilateral rupture during the mainshock. The western aftershock activity is almost entirely located offshore, extending along the axis of the İzmit Bay. We show that the westernmost extent of the mainshock rupture reached the offshore region, just 10 km south of the Princes Islands. The dominance of strike-slip motion is one of the main characteristics of the İzmit sequence. A number of significant aftershock clusters occur off the inferred mainshock rupture plane. The spatial and temporal occurrence of these clusters show that they are triggered by the complex redistribution of stresses and dynamic strains imposed by the dislocation of the mainshock. The focal mechanisms of the events located in the western end of the rupture zone show both strike-slip and normal-faulting mechanisms. We infer that the Yalova cluster was triggered by the mainshock. The dominance of the normal faulting mechanisms supports the notion that dynamic extensional strains played a major role in triggering this cluster. Four events located off the Princes Islands indicate similar source mechanisms characterized by right-lateral strike-slip motion parallel to the strike of the Northern Boundary Fault (NBF) and local trend of aftershock distribution. This observation leads to a first order seismotectonic evidence that the nature of the NBF, which is known to be a transtensional feature at the surface, is dominated by strike-slip motion at depth. The coexistence of strike slip and extensional seismotectonic features in the Sea of Marmara highlights the importance of strain partitioning amongst shallow transtentional structures accommodating north-south extension and deeper structures associated with the NAF accommodating east-west right-lateral shear.