TEHRAN - A strong earthquake with a magnitude of 6.2 in the Sea of Marmara, 40 km from Istanbul, Turkiye, shook at 12:49 pm local time on April 23, 2025. All thirty-three stations of the IIEES Iranian broadband seismic network of the International Institute of Seismology and Earthquake Engineering throughout Iran recorded this earthquake. The mechanism of the earthquake is strike-slip, and the epicenter of the earthquake is located at a depth of approximately 10 km and about 40 km southwest of Istanbul within the Sea of Marmara.

The Istanbul region is seismically active due to its proximity to the North Anatolian Fault (NAF) and has been the site of several major earthquakes throughout history. This fault system, with a dominant dextral strike-slip mechanism, runs through northern Turkey and is known for its potential to produce large earthquakes. Istanbul is at risk of a major seismic event, and forecasts indicate that an earthquake could occur at any moment. These events often follow a westward "migration" pattern, with stress transferring along the fault after each rupture. Seismic migration has marked the westward progression of major ruptures since 1939, although the Marmara Sea fault segment has remained the most critical segment without rupture since 1766. The 10 September 1509 Marmara Sea (near Istanbul) earthquake with an estimated magnitude of 7.4, known as the "Little Apocalypse", destroyed much of Constantinople (Istanbul) and killed an estimated 1,000 to 13,000 people.

The 17 August 1668 earthquake near Erzincan (with an estimated magnitude of M7.8–8.0) is one of the largest recorded earthquakes in Anatolian history, causing widespread destruction from Erzincan to the Black Sea. 400 km of the fault system was ruptured. The 22 May 1766 earthquake in the Marmara Sea (near Istanbul) section of the North Anatolian Fault was estimated to have a magnitude of about 7.4, destroying parts of Istanbul and killing about 4,000–5,000 people. This last major rupture in the Marmara Sea section was the creation of the current seismic rift near Istanbul.

In the south and east of the Marmara Sea, the 17 August 1999 and 12 November 1999 earthquakes with magnitudes of 7.4 and 7.1, respectively, are the last major ruptures of this fault in southeast Istanbul and the Marmara Sea.

The Marmara section of the North Anatolian Fault System has been accumulating stress for 259 years since 1766 and has not ruptured.

Historically, Istanbul has experienced at least 34 major earthquakes in the past 2,000 years. The last major earthquake in this region occurred in 1999 in Izmit, about 80 km southeast of Istanbul, which had devastating effects and highlighted the vulnerability of many of the city's buildings. Current assessments indicate that only about a third of buildings in Istanbul are structurally resilient to seismic activity.

The accumulation of stress along the North Anatolian Fault System (Northern Branch) near Istanbul and the presence of a seismic gap in the fault lock near Istanbul have led to estimates of a 65%–70% chance of an earthquake of magnitude 7 or greater occurring in this region by 2040. Furthermore, there are concerns that such an event could cause catastrophic damage, potentially destroying up to 600,000 homes and affecting millions of people.

Tsunami warnings have also been issued following the April 23, 2025 earthquake. If a significant rupture occurs on the North Anatolian Fault in the Sea of ​​Marmara, it could generate a tsunami with waves up to 10 meters high.

The North Anatolian Fault is a rounded right-lateral strike-slip fault where the Anatolian Plate is moving westward relative to the Eurasian Plate at a rate of 2–3 cm per year. A full rupture of the Marmara section of the fault, which is about 150–200 km long, could produce an earthquake of M7.0–7.8. The population density of 16 million in Istanbul and the presence of dilapidated and vulnerable structures in about two-thirds of Istanbul's buildings indicate that most of Istanbul's population will be in weak buildings exposed to possible damage from the next major earthquake. In this coastal region, soft sediments along the Marmara coast intensify earthquake tremors. Liquefaction in coastal areas, along with the possible occurrence of tsunamis with possible wave heights of 5 to 10 meters, are other hazards from an earthquake in Istanbul. Vital facilities (hospitals, bridges) may fail, of course, in this city since 2013, an earthquake early warning system has been operational, especially for the safe shutdown of the gas network and the Istanbul metro, and can be effective in reducing secondary risks.

The upcoming earthquake in the North Anatolian Fault Zone (NAFZ) near Istanbul is very important for geological, social and economic reasons. The northern branch of the North Anatolian Fault in the north of the Sea of ​​Marmara (near Istanbul) has not ruptured since 1766 and has accumulated about 259 years of tectonic stress. This "locked" section is ready for a delayed major earthquake. With a probability of 62-75%, an M7.0+ event could be the site of a severe event in the vicinity of Istanbul by 2040. With the April 23, 2025 earthquake now being considered, the likelihood of another earthquake can be reassessed, and a 6.2 magnitude earthquake could have a domino effect on the locked fault segment near Istanbul. Istanbul is a metropolis of around 16 million people, many of whom live in cramped, overcrowded buildings, informal settlements, or buildings built before 2000 that do not meet modern seismic codes. Construction quality: Despite stricter regulations following the 1999 Izmit earthquake, poor construction and illegal construction continue. 40 percent of Istanbul’s buildings are at risk of collapse. Soft sediments along Istanbul’s coastal area – the Golden Horn – and the Marmara coast can amplify tremors by a factor of 2 to 3. Istanbul is Turkey’s financial, cultural, and logistical center, accounting for around 40 percent of its GDP. A large earthquake could disrupt global supply chains (such as shipping through the Bosphorus Strait).

A large earthquake could also cause tsunamis and liquefaction in Istanbul. Coastal areas and flatlands are at high risk. Bridges, airports, hospitals and energy networks (e.g., natural gas pipelines) could be inoperable. Estimates suggest hundreds of thousands of deaths and millions of displaced people in the worst-case scenario. Emergency response would be difficult due to Istanbul’s dense urban layout.

A full rupture 150–200 km long on the northern branch of the North Anatolian Fault in the Sea of ​​Marmara could generate an M7.4–7.8 earthquake. Smaller partial ruptures (e.g., 50–100 km long) could generate M7.0–7.4.

Since 1999, only 10% of vulnerable buildings have been retrofitted. Financing and implementing seismic regulations remain a significant challenge in Turkey. Thus, an earthquake with a magnitude greater than 7 near Istanbul could be one of the natural disasters of the 21st century, with the potential for a M7.0-7.8 earthquake, catastrophic loss of life, and significant economic consequences for Türkiye and the region.