The time-dependent stress changes within the seismic gap of the Eastern Marmara Sea (NW Türkiye) through multiple earthquake cycles since 715 AD.

The M7.4 1999 İzmit earthquake apparently advanced the occurrence of possible future event or events along the segments of North Anatolian Fault Zone (NAFZ) beneath the Eastern Marmara Sea due to the positive stress load. This part of the NAFZ did not produce any large earthquake since the May 1766 earthquake, constituting a seismic gap close to the city of Istanbul. In the present study we constructed a Coulomb stress evolution model for the seismic gap that includes the effect of coseismic, time-dependent postseismic viscoelastic relaxation of the substrate beneath the elastic crust and secular stress loadings through the multiple earthquake cycles since 715 AD. The snapshots of stress changes before and after the large and destructive earthquakes of 740, 989, 1343, 1509, May 1766 and 1999 İzmit have been carefully examined. It has been estimated that the total stress changes before 989, 1343, 1509 and May 1766 earthquakes were in the range from 26 to131 bars. Present stress values at the eastern, middle and western sampling points on the faults within the gap are computed as 115, 131 and 85 bars respectively. Considering that the global mean of stress drops for continental strike-slip faults is about 35 bars, it is suggested that the earthquake hazard for the seismic gap critically high.