The earthquake rupture advanced eastward along the main Marmara fault toward Istanbul

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Analysis of earthquake sequences along the Marmara Fault in the Sea of ​​Marmara off Greater Istanbul: green stars indicate moderate earthquakes with magnitude M>5, and circles of different sizes indicate aftershocks. The chronology shows eastward movement. The different parts of the fault are labeled with colours: blue is the creeping part, orange is the transition zone, and red is the currently closed part where a major earthquake could occur.

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Credit: Patricia Martinez-Garzon

summary

In April 2025, the main Marmara Fault under the Sea of ​​Marmara in northwestern Türkiye experienced its largest earthquake in more than 60 years. In a new study now published in the journal Science, a team of researchers led by Prof. Dr. Patricia Martinez-Garzón from the GFZ Helmholtz Geoscience Center in Potsdam, Germany, analyzed nearly two decades of seismic data framing the April 2025 magnitude 6.2 earthquake. Analysis of rupture dynamics and aftershock patterns on multiple time scales reveals a remarkable series of M>5 events propagating eastward along the fault over the past ~15 years, affecting both slow-slip (“creeping”) and tighter (“locked”) fault sections. This completes the large seismic picture of the region. Since the remaining closed section near Istanbul could produce a stronger earthquake with potentially devastating consequences for the megacity with a population of about 18 million people, the authors highlight the need for continuous real-time monitoring in the region.

Background: History of earthquakes that occurred on the main Marmara Fault and their rupture mechanisms

The Main Marmaray Fault (MMF) is the most dangerous and risk-prone fault zone in wider Europe, and is the only part of the plate-bound North Anatolian Fault Zone between the Eurasian and Anatolian plates that has not produced a large earthquake (magnitude M > 7) since 1766. The average frequency of large earthquakes in the region is 250 years, as inferred from historical records dating back more than 2,000 years. Therefore, the main Marmaray fault is already late in its seismic cycle and therefore has the potential to explode into a large earthquake.

Previous studies by researchers at GFZ revealed that the main Marmaray Fault is divided into several distinct sections: In the creeping section in the western part, a fraction (up to ~50%) of tectonic energy is released by slow creep (and thus aseismicity) as determined from the analysis of repeated small earthquakes at the same local locations, so-called “repeaters”. Eastward, the creep fraction (and the presence of recurrences) becomes smaller in a “transitional section,” until the fault closes completely just south of Istanbul. Fault segments are stressed primarily from tectonic loading by plate movement and the transfer of co-seismic and post-seismic stress from previous earthquakes. Transitional and locked fault segments are additionally loaded by slow slip release along adjacent creep segments, resulting in increased stress concentration.

New research on the directional progression of the Marmara earthquakes

The recent magnitude 6.2 Marmara earthquake of 23 April 2025 was located in the transition section and has stimulated and enabled deeper research into the directional progression of earthquakes in the region. The research team is headed by Prof. Dr. Patricia Martínez-Garzón, Head of the Working Group of the Department “Geomechanics and Scientific Drilling” at the GFZ Helmholtz Center for Geosciences and Professor at RWTH Aachen University, and includes other colleagues from Germany, Turkey, the United States and from the GFZ, including Prof. Dr. Marco Bonhof, Head of the GFZ 4.2 Department “Geomechanics.” and Scientific Drilling”, and Prof. Dr. Fabrice Cotton, Head of Section 2.6 “Seismic Hazards and Hazard Dynamics”.

Scientists analyzed the rupture characteristics of the April 2025 magnitude 6.2 earthquake and framed the regional seismic pattern based on a catalog of seismic events covering the past 20 years. Particular focus is on comparing the 2025 M 6.2 earthquake with the 2019 M 5.8 Silivri earthquake, because both have adjacent epicenters and partially overlap.

Activation of the main Marmara fault eastward for a decade

The new study shows that the Marmaray Main Fault (MMF) undergoes a gradual, eastward-moving sequence of moderate earthquakes toward the locked section southwest of Istanbul with approximately increasing magnitude starting in 2011 and culminating in the 2025 M6.2 Marmara earthquake, the largest event on the Marmaray Main Fault in more than 60 years. The latter ruptured approximately 10 to 20 km of the main central Marmara fault with strike-slip movements. It may have been influenced by the 2019 M 5.8 event through pressure transfer.

The observed eastward migration of M>5 events began in the creeping section including the Western Ridge and the Central Basin, which experienced two M>5 earthquakes in 2011 and 2012. They left a quiescent zone of 10–15 km at the eastern edge of the creeping section, which was then activated by the 2019 M>5.8 earthquake. Its approximately 10-km-long rupture partially overlapped the 2025 AD 6.2-m quake. Both contiguous centers represent the change from the creeping central basin portion of the main Marmara fault to the transitional Kumburgaz basin portion. Both events showed asymmetric aftershock patterns with respect to the epicenter, with an eastward focus along the MMF. Aftershocks of the April 2025 earthquake ended near the eastern edge of an approximately 15 km seismically quiet zone in the Avcılar sector, between Kumburgaz and the enclosed Prince's Islands portion south of Istanbul. In addition, the aftershock characteristics in terms of the ratio between smaller and larger earthquakes and thus the stress level are different from the 2019 event, indicating that the shear stress in this part or in other words “loading state” remains high.

Implications for the Istanbul region: Increasing critical level of tension

The observed seismic pattern from the M 6.2 event in April 2025 – consistent with previously observed events – places the release of seismic energy closer to the locked Princes' Islands sector, part of the MMF south of Istanbul capable of producing a magnitude 7 earthquake on its own.

“Our results show a long-term progression of seismic partial faults moving towards the closed fault sector adjacent to Istanbul there,” says Prof. Dr. Patricia Martinez-Garzón, GFZ scientist and lead author of the study. “This doesn't tell us when a major earthquake might occur, but it does indicate which parts of the fault are becoming increasingly stressed.”

Implications of earthquake hazards and hazards

Studies conducted in recent years show that the energy transmitted by seismic waves can be stronger in certain directions and weaker in others, which has important consequences for potential damage in populated areas. In their study, Martinez-Garzón and co-authors also showed that the recent April 2025 Mw 6.2 earthquake in the Sea of ​​Marmara actually generated shorter, more energetic seismic waves and enhanced ground motion at measuring stations to the east of the epicenter. This so-called “eastward rupture pattern” is consistent with findings from previous earthquakes in the region and means that ground shaking could be stronger if future medium to large earthquakes also start west of Istanbul and propagate eastward toward the city. Alternatively, if the highly anticipated event starts directly south of Istanbul, this effect will be smaller, however, ground motions will also be high due to the small distance.

This study indicates that, together with previous earthquake ruptures in the MMF over the past decade, the M 6.2 earthquake left only a ~15-20 km long zone with low seismic activity in the Avcılar sector, between Kumburgaz and the locked Princes' Islands part south of Istanbul. Because the 2019 M 5.8 seismic sequence started in a similarly quiet area, the results suggest that this seismically silent section is a likely candidate for the next moderate or large earthquake.

“The next major earthquake is likely to occur on the fault patch west of or directly on the part of the Princes Islands that is completely closed south of Istanbul,” says Professor Marco Bonhoff, co-author of the study. “This could be a magnitude 6 Richter scale event, or it could be a precursor that then leads to a larger earthquake, since the fault is certainly critically loaded and has already accumulated significant energy.”

Outlook: The importance of intensive real-time monitoring of submarine fault

If a large earthquake in the future spreads from west to east – as recent events indicate – ground shaking in the Istanbul area could intensify due to directional effects. For this reason, the authors emphasize the importance of intensive monitoring of submarine error in real time. This includes the establishment of more well stations – as part of the operational GONAF observatory jointly coordinated by GFZ and the Turkish Disaster and Emergency Management Presidency AFAD – but also permanent seismic and geodetic stations on the ocean floor, as well as fibre-optic marine sensor systems, which will be used in the near future within the framework of the SAFATOR Helmholtz infrastructure.

Better monitoring helps you better anticipate what might happen and, if an event does occur, be able to respond more quickly at timescales of seconds or minutes – critical for shutting down critical infrastructure and other essential safety measures.

Background: GONAF and SAFator infrastructure

The GONAF Observatory currently consists of 10 well stations up to 300 meters deep, the most modern of which are the most advanced equipped stations of their kind in the world. It has been operating since 2015 and has allowed the extraction of a more detailed model of the ongoing seismic deformation and seismicity processes along the main Marmara Fault beneath the Sea of ​​Marmara. The recently launched SAFAtor project is a major initiative by the Helmholtz Association and coordinated by GFZ to promote the use of submarine fiber optic cables for scientific purposes.

Finance

Professors Martinez-Garzón and Sebastian Nunez-Jara are funded by an ERC QUAKEHUNTER start-up grant (101076119). GONAF is part of the GFZ Plate Boundary Observatory initiative and has been jointly funded by German, Turkish and US agencies as well as by the International Continental Scientific Drilling Program (ICDP) based at GFZ Potsdam.

Original study

for. Martinez-Garzón et al., Eastward progressive rupture of the main Marmara fault toward Istanbul, Science 10.1126/science.adz0072 (2025).

More information

Conjure

www.gonaf-network.org

https://research-infrastructure.gfz.de/en/infrastructure/37

press release

SAFator

Project website

press release

Previous posts:

D. Becker, P. Martínez-Garzón, C. Wollin, T. Kılıç, M. Bohnhoff, Variation of fault creep along the long-sought Istanbul-Marmara seismic gap in northwestern Turkey. Geophys. Accuracy. Lett. 50, e2022GL101471 (2023). doi:10.1029/2022GL101471

press release

x. Geophys. Accuracy. Lett. 52, e2024GL111460 (2025). doi:10.1029/2024GL111460

press release

Search method

Observational study

Research topic

Not applicable

Article title

Gradual rupture of the main Marmara fault eastward towards Istanbul

Date the article was published

11-Dec-2025

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