We present a 2-D tomographic seismic velocity image in eastern Marmara region along a N-S trending 120-km long seismic refraction profile, which traverses the northern branches of North Anatolian Fault (NAF) and tectonically active ¿inarcik Basin, in the Sea of Marmara. The 2-D velocity model, which is constrained down to upper crustal depth of 7 km, shows significant heterogeneities in the upper crust displayed by well-constrained velocity anomalies. The seismicity observed following the 17 August, 1999 Izmit earthquake concentrates in three distinct zones and takes place near the inferred low velocity zones and below high velocity anomalies obtained from tomographic inversion. This correlation is interpreted as a result of intensely sheared zones of deformation imposed by strike-slip motion of the northern branches of the North Anatolian Fault.