The Ganos fault is the westernmost segment of the North Anatolian fault that generated the 9 August 1912 Mürefte (Ganos) earthquake in western Turkey (Mw = 7.4). We study the 1912 earthquake characteristics using coseismic fault slip and fault segmentation coupled with an analysis of historical seismic records. Surface ruptures with small releasing and restraining structures and 1.5–5.5 m right-lateral offsets have been measured at 45 sites of the on-land ∼45-km-long fault section. Similar structures are delineated by fresh fault scarps and prominent pull-apart basins in the Sea of Marmara and Saros Bay. A second shock with Mw = 6.8 occurred on 13 September 1912, implying a 20–40-km-long rupture; the damage distribution and analysis of seismic records suggest an epicenter located farther west near Saros Bay, near the western termination of the 9 August rupture. Modeling of seismic records reveals a relative source time function between the two events and indicates 40 s rupture duration, in agreement with a 120 ± 30 km-long fault rupture for the 9 August shock. An estimated 150 ± 30 km-long rupture for the two earthquakes, combined with onshore and offshore fault segmentation, allow us to better constrain the western limit of the Marmara Sea seismic gap and the related potential for a large earthquake (sharply increased by the devastating 1999 Izmit [Sea of Marmara] seismic event).