During the past century, a series of predominantly westward migrating M>7 earthquakes broke an similar to 1000km section of the North Anatolian Fault (NAF). The only major remaining seismic gap along the fault is under the Sea of Marmara (Main Marmara Fault (MMF)). We use 20years of GPS observations to estimate strain accumulation on fault segments in the Marmara Sea seismic gap. We report the first direct observations of strain accumulation on the Princes' Islands segment of the MMF, constraining the slip deficit rate to 10-15mm/yr. In contrast, the central segment of the MMF that was thought to be the most likely location for the anticipated gap-filling earthquakes shows no evidence of strain accumulation, suggesting that fault motion is accommodated by fault creep. We conclude that the Princes' Islands segment is most likely to generate the next M>7 earthquake along the Sea of Marmara segment of the NAF.