Active faults aligning in NW-SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avcilar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW-SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the Cinarcik Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the Cinarcik Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through Buyukcekmece and Kucukcekmece lagoons. These data suggest that the rupture of the 17 August 1999 earth-quake in the NAFZ may have extended through Avcilar. Indeed, Avcilar and Izmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avcilar and Izmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avcilar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies.