Earthquakes Are Migrating Toward Istanbul Along the Marmara Fault

Researchers analyzing two decades of seismic data have uncovered a striking eastward migration of earthquakes along the Main Marmara Fault.

In April 2025, the Main Marmara Fault beneath the Sea of Marmara in northwestern Türkiye experienced its strongest earthquake in more than sixty years. Researchers have now examined the event in detail using nearly two decades of seismic observations.

Their findings, published in Science, come from a team led by Prof. Dr. Patricia Martínez-Garzón of the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany. By studying rupture behavior and aftershock activity across multiple time scales, the scientists identified a sequence of earthquakes greater than magnitude 5 that has steadily progressed eastward along the fault during roughly the past fifteen years. These earthquakes have affected both slowly moving creeping sections and tightly stuck locked sections of the fault.

The results help clarify how stress has been building across the region. Because the remaining locked portion of the fault near Istanbul could produce an even stronger earthquake, potentially threatening the megacity of about 18 million people, the researchers emphasize the need for continuous real time monitoring in the area.

Historical earthquake cycles along the Main Marmara Fault

The Main Marmara Fault (MMF) is considered the most hazardous fault zone in the broader European region. It is the only section of the plate boundary along the North Anatolian Fault Zone between the Eurasian and Anatolian plates that has not produced a large earthquake greater than magnitude 7 since 1766.

Historical records that extend back more than two thousand years show that major earthquakes in the region occur on average every 250 years. Based on this pattern, scientists believe the Main Marmara Fault is already late in its seismic cycle and may be approaching another major rupture.

Earlier research by GFZ scientists showed that the Main Marmara Fault is divided into several distinct segments. In the western portion, known as the creeping section, as much as about half of the tectonic energy is released gradually through slow fault movement that does not produce noticeable earthquakes. This process has been identified through the study of small earthquakes that repeatedly occur in the same locations, known as repeaters. Moving eastward along the fault, creeping activity decreases in what researchers call a transitional section.

Farther east, immediately south of Istanbul, the fault becomes fully locked. Stress accumulates in these locked segments due to ongoing tectonic plate motion as well as stress transferred from previous earthquakes. In addition, the slow release of energy in neighboring creeping sections can increase the stress concentration in adjacent transitional and locked segments.

Investigating the progression of Marmara earthquakes

The magnitude 6.2 earthquake that struck the Marmara region on April 23, 2025 occurred within the transitional section of the fault and prompted a closer examination of how earthquakes have been progressing along the Main Marmara Fault.

The research team was led by Prof. Dr. Patricia Martínez-Garzón, working group leader in the Geomechanics and Scientific Drilling section at the GFZ Helmholtz Centre for Geosciences and professor at RWTH Aachen University. The collaboration included researchers from Germany, Türkiye, and the United States. Among them were Prof. Dr. Marco Bohnhoff, head of GFZ Section 4.2 Geomechanics and Scientific Drilling, and Prof. Dr. Fabrice Cotton, head of Section 2.6 Seismic Hazard and Risk Dynamics.

To understand the broader pattern of seismic activity, the scientists examined rupture characteristics from the April 2025 earthquake together with regional earthquake records collected over the past twenty years. They paid particular attention to the relationship between the 2025 magnitude 6.2 earthquake and the magnitude 5.8 Siliviri earthquake that occurred in 2019, since the epicenters of the two events are close to one another and their rupture zones partially overlap.

Earthquake sequence advancing toward Istanbul

The analysis shows that the Main Marmara Fault has experienced a gradual eastward progression of moderate earthquakes moving toward the locked fault segment south of western Istanbul. This sequence began around 2011 with earthquakes larger than magnitude 5 and continued over the following years.

The pattern culminated in the April 2025 magnitude 6.2 earthquake, which represents the largest event recorded on the Main Marmara Fault in more than sixty years. The earthquake ruptured approximately 10 to 20 kilometers of the central portion of the fault and involved strike-slip motion. Researchers believe stress transferred from the 2019 magnitude 5.8 earthquake may have influenced the timing of the 2025 event.

The eastward movement of earthquakes began within the creeping section of the fault, including the Western High and the Central Basin. These areas experienced two earthquakes larger than magnitude 5 in 2011 and 2012. Those events left a quiet region roughly 10 to 15 kilometers long along the eastern edge of the creeping section. This previously quiet zone later became active during the 2019 magnitude 5.8 earthquake. The rupture from that event extended about 10 kilometers and partially overlapped with the rupture of the 2025 magnitude 6.2 earthquake.

Both earthquakes occurred near the boundary between the creeping Central Basin segment and the transitional Kumburgaz Basin segment. In both cases, aftershocks were distributed unevenly around the epicenter and were concentrated mainly toward the east along the Main Marmara Fault. Aftershocks from the April 2025 earthquake stopped near the eastern edge of another quiet zone about 15 kilometers long in the Avcılar segment, located between the Kumburgaz Basin and the locked Princes Islands segment south of Istanbul. Scientists also observed that the aftershock pattern differs from that seen after the 2019 event. The relative proportion of smaller and larger aftershocks indicates that stress levels in this section of the fault remain high, suggesting that significant tectonic energy is still accumulating.

Stress increasing near the Istanbul fault segment

The earthquake sequence leading up to the April 2025 event indicates that seismic energy release is gradually moving closer to the locked Princes Islands segment of the Main Marmara Fault. This segment lies directly south of Istanbul and is capable of generating an earthquake of magnitude 7 on its own.

“Our results show a long-term progression of partial earthquake ruptures moving towards the Istanbul-adjacent locked fault segment there,” says Prof. Dr. Patricia Martínez-Garzón, GFZ Scientist and lead author of the study. “This does not tell us when a major earthquake may happen, but it does indicate which parts of the fault become increasingly critically stressed.”

Implications for earthquake hazard and risk

Previous studies have shown that seismic waves can carry more energy in certain directions than others, which can strongly influence how much damage an earthquake causes in populated areas. The new research also demonstrates that the April 2025 magnitude 6.2 earthquake generated relatively short but energetic seismic waves that produced stronger ground motion at monitoring stations located east of the epicenter.

This phenomenon, known as eastward rupture directivity, matches observations from earlier earthquakes in the region. It suggests that if a future moderate or large earthquake begins west of Istanbul and spreads eastward toward the city, ground shaking could be amplified. If the next major earthquake begins directly south of Istanbul, the directional effect would be weaker, but strong ground motion could still occur because the city lies very close to the fault.

According to the researchers, the recent earthquake sequence has left only a roughly 15 to 20 kilometer long segment with low seismic activity in the Avcılar region between the Kumburgaz Basin and the locked Princes Islands segment. Because the 2019 magnitude 5.8 earthquake sequence also began in a similarly quiet zone, this silent segment may be a possible location for the next moderate or large earthquake.

“A next significant earthquake is likely to occur on the fault patch west of or directly on the fully locked Princes Islands segment just south of Istanbul,” says Prof. Marco Bohnhoff, co-author of the study. “This could be a magnitude 6 event, or it could be a precursor that then initiates an even larger earthquake, since the fault is certainly critically loaded and has accumulated substantial energy already.”

Real-time monitoring becomes critical

If a future large earthquake spreads from west to east along the fault, ground shaking in the Istanbul region could become even stronger because of the directivity effect. For this reason, the researchers emphasize the importance of continuous and dense monitoring of the submarine fault beneath the Sea of Marmara.

Improving monitoring capabilities may involve installing additional borehole stations as part of the operational GONAF observatory, which is jointly coordinated by GFZ and the Turkish Disaster and Emergency Management Presidency AFAD. Scientists are also working toward deploying permanent ocean bottom seismic and geodetic stations, along with fiber optic offshore sensing systems that are planned within the SAFATOR Helmholtz Infrastructure project.

More advanced monitoring systems could help scientists detect changes in seismic activity more quickly and provide faster responses during earthquakes. Even a warning of seconds or minutes could be crucial for shutting down critical infrastructure and implementing basic safety measures that protect lives.

Reference: “Progressive eastward rupture of the Main Marmara fault toward Istanbul” by Patricia Martínez-Garzón, Xiang Chen, Dirk Becker, Sebastián Núñez-Jara, Recai Feyiz Kartal, Elif Türker, Georg Dresen, Yehuda Ben-Zion, Jorge Jara, Fabrice Cotton, Filiz Tuba Kadirioglu, Tuğbay Kiliç and Marco Bohnhoff, 11 December 2025, Science.
DOI: 10.1126/science.adz0072

Prof. Martínez-Garzón and Sebastián Núnez-Jara are funded by the ERC Starting Grant QUAKEHUNTER (101076119). GONAF is part of the GFZ Plate Boundary Observatory initiative and was co-funded by German, Turkish and US agencies as well as from the International Continental scientific Drilling Programme ICDP that is based at GFZ Potsdam.

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