We used seismic stations that we deployed after the İzmit earthquake along the shores and islands of the İzmit Bay-Çinarcik basin to study the seismic activity that took place after the earthquake in the eastern Marmara Sea. The aftershock distribution indicates the existence of three clusters of activity there in the days that followed the earthquake. One of the clusters shows the extension of the E-W-trending İzmit rupture into the eastern Marmara Sea. The seismic activity there clearly outlines the fault plane of the main rupture and shows its termination 35 km beyond the Hersek peninsula. Two other clusters of activity are present in the region. One is in the Armutlu peninsula and the other one is located a few kilometers southwest of the Tuzla peninsula beneath the northern slope of the Çinarcik basin. The focal mechanism solutions indicate strike-slip faulting along the main branch of the İzmit rupture and normal faulting within the two swarms. The presence of the different mechanisms suggests the existence of slip partitioning in the region. A remarkable feature of the aftershock data is the absence of seismicity above 4 km in the Marmara Sea.