The Sea of Marmara Fault is the submerged western continuation of the North Anatolian Fault, running east to west along the floor of the sea that separates European and Asian Turkey. It is a right-lateral strike-slip system where the Anatolian block slides westward past Eurasia at roughly 2 to 2.5 centimetres per year, carrying its accumulated strain directly beneath one of the world's largest cities.
The fault's Main Marmara segment includes a locked stretch off the coast of Istanbul that has not ruptured since 1766, forming a recognised seismic gap. Twentieth-century earthquakes marched steadily westward along the North Anatolian Fault, from Erzincan in 1939 toward the Marmara region, loading this final unbroken section with stress.
The 1999 İzmit earthquake (magnitude 7.6) struck the eastern edge of the sea, killing more than 17,000 people and rupturing right up to the gap's edge. Seismologists estimate a high probability of a magnitude 7-plus earthquake beneath the Marmara within decades, making it one of the most closely monitored and feared faults on Earth.