The North Anatolian Fault

Between 1939 and 1999 the North Anatolian Fault did something seismologists had rarely seen: it broke in a sequence of large earthquakes that marched westward across northern Turkey, each one loading the next segment along. Twelve damaging shocks of roughly magnitude 6.7 to 7.8 killed more than 60,000 people over those six decades. The last of the big ones, at İzmit in 1999, stopped just short of the Sea of Marmara. The segment beneath that sea, about 20 kilometers south of Istanbul and its roughly 16 million residents, has not broken in a large earthquake since 1766. That gap makes this fault a focus of earthquake planning in Turkey and among scientists who study the region.

The fault runs about 1,500 kilometers in a gentle arc from the Karlıova triple junction in eastern Anatolia, where it meets the East Anatolian Fault, to the northern Aegean Sea. It is a right-lateral strike-slip fault, the boundary along which the Anatolian block is squeezed westward between the Eurasian Plate to the north and the Arabian Plate pushing from the south. Slip is about 20 to 25 millimeters a year, comparable to the San Andreas. Along much of its length it follows river valleys and the southern edge of the Pontic Mountains. Near İzmit it splits into strands, and the northern strand dives beneath the Sea of Marmara, a pull-apart basin opened by the fault, before emerging at the Gulf of Saros.

The sequence began with the magnitude 7.8 Erzincan earthquake of December 1939, which killed more than 32,000 people, and ended, for now, with İzmit, magnitude 7.6, on August 17, 1999, which killed about 18,000, followed by Düzce in November. On April 23, 2025, a magnitude 6.2 earthquake struck the Marmara segment off Silivri, southwest of Istanbul, causing injuries and widespread alarm but rupturing only about 20 kilometers. Published forecasts put the chance of a magnitude 7 or larger quake affecting Istanbul at roughly 35 to 47 percent over 30 years, according to a 2016 study co-authored by USGS scientists. The February 2023 earthquakes that devastated southern Turkey and Syria were on a different system, the East Anatolian Fault, which meets this one at Karlıova.

• Anatolia
  Asia Minor, the peninsula that has always carried the traffic of continents bounds
• The San Andreas Fault
  California's 1,200-kilometer seam between the Pacific and North American plates comparable strike-slip fault
• Aegean Sea
  The island-strewn sea where Greek civilization was born extends into
• Turkey
  A transcontinental bridge between Europe and Asia located in
• Anatolian Plateau
  The high heartland of Asia Minor northern edge of
• The Tectonic Plates
  The seven great slabs and dozens of smaller ones that make up Earth's moving shell plate boundary
• The 2023 Turkey–Syria Earthquakes
  Twin magnitude 7.8 and 7.5 shocks on the East Anatolian Fault that killed nearly 60,000 people in southern Turkey and northern Syria related fault system
• The 2023 Turkey–Syria Earthquakes
  Twin magnitude 7.8 and 7.5 shocks on the East Anatolian Fault that killed nearly 60,000 people in southern Turkey and northern Syria related great earthquake
• Istanbul
  The city straddling Europe and Asia across the Bosphorus runs about 20 km south of