Something ominous is happening deep below the Sea of Marmara in Turkey.
A fault line beneath this inland body of water, which links the Black Sea with the Aegean Sea, has caught the attention of scientists. Over the last two decades, earthquakes of rising magnitude have occurred, steadily moving to the east.
The latest in this sequence, in April, was a magnitude 6.2 temblor. And if the pattern were to continue, eventually a large quake could reach the waters just south of Turkey’s largest city.
“Istanbul’s in the cross hairs,” said Stephen Hicks, a seismologist at University College London who was not involved with the new study.
The new research, published Thursday in the journal Science, suggests that powerful quakes are marching toward a nine- to 13-mile-long locked-up part of what scientists call the Main Marmara Fault. That has the potential to unleash a tumult with a magnitude 7.0 or greater on the city of 16 million.
The precise nature and the timing of this seismic shock are uncertain.
“Earthquakes cannot be predicted,” said Patricia Martínez-Garzón, a seismologist at the GFZ Helmholtz Centre for Geosciences in Germany and an author of the new study. But understanding how this event might be initiated is of vital importance. “We need to focus on the mitigation and the early detection of any signals that tell us something out of the ordinary is happening.”
Turkey is no stranger to catastrophic earthquakes. In February 2023, a pair of earthquakes (a magnitude 7.8, followed just hours later by a magnitude 7.5) obliterated parts of southern Turkey and Syria, killing at least 55,000 people.
And the notion of a future quake pummeling Istanbul isn’t novel; experts agree on a grim inevitability. “A very large earthquake striking near Istanbul will likely cause one of the worst humanitarian disasters in recent history,” said Judith Hubbard, an earthquake scientist at Cornell University who was not involved with the study.
The peril highlighted in the new study involves the North Anatolian Fault Zone, a geologic boundary where the Anatolian tectonic plate slides sideways against the Eurasian plate.
A swath of this 750-mile-long fault zone has been active in modern times. But a section of it — named the Main Marmara Fault, which is under the sea southwest of Istanbul — has been suspiciously calm. A magnitude 7.1 quake struck the city in 1766, but ever since, a roughly 100-mile-long section has not ruptured.
The new study, which examined the past 20 years of quakes on the fault, noticed something odd. In 2011, a magnitude 5.2 quake hit a western part of the Main Marmara Fault — followed by a magnitude 5.1 quake to the east in 2012. In September 2019, a magnitude 5.8 temblor rocked the central part of the Main Marmara Fault. Then, in April 2025, just to its east, there was a magnitude 6.2.
Will the next quake be stronger than the last? And if so, will it appear on the fault right below Istanbul? The study’s authors think that it’s a possibility.
There is no guarantee that the next quake will be the magnitude-7-or-above cataclysm everyone dreads. But an additional fairly strong quake on the fault may load more stress onto its eastern section, which could trigger that feared rupture. And as small increases in magnitude correspond to giant leaps in released energy, even a quake in the mid-to-high 6s could be significantly damaging for the city.
It isn’t clear why quakes occasionally propagate along a fault, but they have been known to happen in the past — including on the North Anatolian Fault Zone itself. Still, the researchers include only four modest quakes in their analysis, a number that may be insufficient to identify a pattern. “I think there is a good chance that these are just typical earthquakes that happen to look like they are migrating,” Dr. Hubbard said.
But even if skepticism is warranted over this set of data, dangerous stress is clearly building on this fault. A giant destructive quake is coming. “I think we still can’t say whether that will be preceded by a sequence of migrating earthquakes, as implied by this paper, or will come as a bolt from the blue,” Dr. Hubbard said.
The deadliness of this destined quake depends on many factors, including the direction of the rupture. A recent study (also coauthored by Dr. Martínez-Garzón), warns it’s somewhat likelier to rip toward the east, unfortunately in Istanbul’s direction.
But as was the case with Turkey’s 2023 double-quake disaster, it is the human factor that may prime Istanbul for tragedy.
“Decades of unregulated growth and densification of the city, combined with a lack of enforced building standards and development on unstable soils, have set up this especially dangerous situation,” Dr. Hubbard said.
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