Coulomb Stress Interactions and the 1999 Marmara Earthquakes

The effects of previous earthquakes on the 1999 Izmit and Duzce earthquakes, the influence of the Izmit earthquake on the Duzce earthquake, and the seismic hazard in the Marmara region are investigated using Coulomb failure stress. Calculation of the Coulomb stress changes using the fault parameters deduced from the modelling of the coseismic Synthetic Aperture Radar Interferometry (InSAR) and GPS data shows that the Izmit earthquake occurred where the Coulomb stress was increased by the previous events. Despite the stress decrease on the Duzce Fault due to the events before 1999, the Duzce earthquake appears to have been triggered by the high increase in the static Coulomb stress transferred by the Izmit earthquake. The Duzce and the previous earthquakes increased the static stress in western and eastern Marmara by over 5 bars. Calculation of secular stress loading based on the modelling of interseismic GPS measurements shows that stress accumulation along the northern branch of the North Anatolian Fault Zone is 0.37 bars per year. Thus, a stress increase of 5 bars corresponds to an increase normally accumulated in 12-13 years by secular loading due to continuous plate motion. In other words, the previous earthquakes brought forward the next earthquake in the Sea of Marmara by 12 years. The faults in this region therefore pose a serious seismic hazard, particularly for Istanbul.