During the twentieth century, a remarkable sequence of large earthquakes unfolded across northern Türkiye. Beginning with the destructive 1939 Erzincan earthquake in the east, major ruptures repeatedly occurred farther west along the North Anatolian Fault. By 1999, devastating earthquakes had reached İzmit and Düzce, close to the Sea of Marmara.
That history gave rise to an unsettling description: earthquakes appeared to be walking toward Istanbul.
The fault does not literally move toward the city, and the sequence is not a reliable timetable for the next earthquake. But stress transfer between ruptured fault segments has helped produce a genuine geographic progression, leaving the fault beneath the Sea of Marmara as one of the most closely watched seismic gaps in the world.
A Fault More Than a Thousand Kilometers Long
The North Anatolian Fault is a major right-lateral strike-slip fault extending across northern Türkiye toward the Aegean region. It accommodates motion between the Anatolian block to the south and Eurasia to the north.
Viewed from opposite sides of the fault, the ground moves mainly horizontally. The Anatolian block is being driven westward as the Arabian Plate converges with Eurasia farther east, while tectonic processes in the Aegean region also help accommodate its motion.
This deformation does not occur smoothly everywhere. Some portions creep, releasing part of their accumulated strain gradually. Others remain locked for decades or centuries and then move suddenly during large earthquakes.
A Chain of Earthquakes Moved Westward
The magnitude 7.8-class Erzincan earthquake of 1939 ruptured a large section of the North Anatolian Fault in eastern Türkiye. Other major earthquakes followed along neighboring sections during subsequent decades.
The overall progression was predominantly westward.
This pattern was not simply a coincidence. When an earthquake ruptures a fault, it changes the stress in the surrounding crust. Some nearby sections may experience reduced stress, while others can be pushed closer to failure. This process is known as Coulomb stress transfer.
Scientists have found evidence that several North Anatolian earthquakes increased stress on segments that later ruptured. The sequence became one of the clearest natural examples of how earthquakes on a connected fault system can influence where future ruptures occur.
The progression eventually reached northwestern Türkiye. In August 1999, the magnitude 7.4 İzmit earthquake ruptured east of the Sea of Marmara. A magnitude 7.2 earthquake followed near Düzce that November.
The remaining concern shifted westward, beneath the Marmara Sea and toward Istanbul.
A Seismic Gap Lies Offshore
The Main Marmara Fault is the submarine portion of the North Anatolian system beneath the Sea of Marmara. Roughly 150 kilometers of this region have long been regarded as a major seismic gap because they did not participate in the twentieth-century sequence of large ruptures.
Historical evidence indicates that parts of the system last experienced major earthquakes in 1766.
Yet the Marmara fault is not uniformly locked. Seafloor measurements, earthquake patterns, and geodetic observations show a mixture of behaviors. Some western and central sections creep. Other portions accumulate much more strain.
Of particular concern is the region approaching the Princes Islands south of Istanbul, where evidence indicates that part of the fault remains strongly locked.
A locked segment does not prove that a large earthquake is imminent. It does mean that tectonic motion is being stored as elastic strain rather than being completely released through slow fault movement.
The 2025 Earthquake Added a New Piece
On April 23, 2025, a magnitude 6.2 earthquake struck beneath the Sea of Marmara near Silivri, west of Istanbul. It was the largest earthquake along the Main Marmara Fault in more than six decades.
The event did not rupture the entire seismic gap. Instead, studies indicate that it broke roughly 10 to 20 kilometers of the central fault system.
Its importance became clearer when researchers compared it with earlier moderate earthquakes.
Since around 2011, earthquakes larger than magnitude 5 have progressively activated parts of the Main Marmara Fault from west to east. The 2019 Silivri earthquake occupied part of this sequence, and the 2025 rupture extended activity farther toward the transitional zone leading to the locked segment south of Istanbul.
In other words, the famous westward progression across Türkiye has been followed by a smaller-scale eastward progression within the Marmara region itself.
“Walking” Does Not Mean Predictable
It is tempting to draw an arrow along the fault and assume each earthquake identifies where the next one will occur. Real faults are not that orderly.
The North Anatolian system contains bends, branches, locked patches, creeping sections, sediment-filled basins, and variations in temperature and rock strength. Ruptures can stop at structural barriers, jump between fault strands, or propagate in unexpected directions.
Even when one earthquake increases stress on another segment, scientists cannot determine exactly when that segment will fail.
The 2025 event therefore should not be described as either triggering an inevitable Istanbul earthquake or safely releasing enough stress to eliminate the threat. Neither conclusion is supported by the physics of such a complex fault.
Istanbul Sits Beside the Unfinished Part of the Story
The phrase “walking toward Istanbul” captures an extraordinary tectonic history, but the most important feature is not the metaphor. It is the distribution of accumulated strain beneath the Sea of Marmara.
A succession of twentieth-century ruptures brought major North Anatolian earthquakes from eastern Türkiye toward the Marmara region. More recently, moderate earthquakes within the Main Marmara Fault have progressed toward its eastern locked sections.
The remaining gap does not reveal when another major earthquake will happen. What it does reveal is why Istanbul sits beside one of Earth’s most intensely monitored plate-boundary fault systems: much of the surrounding tectonic motion has already expressed itself through earthquakes, while important offshore sections remain capable of storing and releasing substantial strain.