On 17 August 1999, a destructive magnitude 7.4 earthquake occurred 100 km east of Istanbul, near the city of Izmit, on the North Anatolian fault. This 1,600-km-long plate boundary(1,2) slips at an average rate of 2-3 cm yr(-1) (refs 3-5), and historically has been the site of many devastating earthquakes(6,7). This century alone it has ruptured over 900 km of its length(6). Models of earthquake-induced stress change(8) combined with active fault maps(9) had been used to forecast that the epicentral area of the 1999 Izmit event was indeed a likely location for the occurrence of a large earthquake(9,10). Here we show that the 1999 event itself significantly modifies the stress distribution resulting from previous fault interactions(9,10). Our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 (ref. 7) as well as secular interseismic stress change, constrained by GPS data(11). These models provide a consistent picture of the long term spatio-temporal behaviour of the North Anatolian fault and indicate that two events of magnitude equal to, or greater than, the Izmit earthquake are likely to occur within the next decades beneath the Marmara Sea, south of Istanbul.