Eastward earthquake rupture progression along the Main Marmara Fault towards Istanbul

(Press-News.org) Summary

In April 2025, the Main Marmara Fault below the Sea of Marmara in north-western Türkiye has experienced its largest earthquake in over 60 years. In a new study now published in Science, a team of researchers led by Prof. Dr Patricia Martínez-Garzón from the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany, analyses nearly two decades of seismic data framing the 2025 April magnitude M 6.2 earthquake. The analysis of rupture dynamics and aftershock patterns on multiple temporal scales reveals a remarkable series of eastward propagating M>5 events along the fault over the last ~15 years, affecting both slowly-slipping (“creeping”) and more tightly stuck (“locked”) fault sections. This further completes the big seismic picture of the region. Because the remaining locked section near Istanbul could produce an even stronger quake with potentially devastating consequences for the mega city with ~18 million inhabitants, the authors highlight the need of continuous, real-time monitoring in the area.

Background: History of earthquakes on the Main Marmara Fault and their rupture mechanisms

The Main Marmara Fault (MMF) is the most hazardous and risk-prone fault zone in broader Europe and is the only part of the plate-bounding North Anatolian Fault Zone between the Eurasian and Anatolian plates that has not produced a large (magnitude M>7) earthquake since 1766. The average recurrence rate of large earthquakes in the region is 250 years, as deduced from historic records dating back more than 2000 years. Therefore, the Main Marmara Fault is already late in its seismic cycle and hence has potential to rupture into a large quake.

Earlier studies led by GFZ researchers revealed that the Main Marmara Fault is divided into several characteristic sections: In the creeping section in the western part, a fraction (up to ~50%) of the tectonic energy is released by slow (and thus aseismic) creep as identified from analysing small earthquakes recurring on the same local spots, so-called “repeaters”. Towards the East the fraction of creep (and the occurrence of repeaters) becomes smaller in a “transitional section”, until the fault is completely locked immediately south of Istanbul. Fault segments are stressed primarily from tectonic loading by plate motion and co-seismic and post-seismic stress transfer from previous earthquakes. Transitional and locked fault segments become additionally loaded by the slow slip release along neighboring creeping sections, resulting in higher stress concentration.

New research on the directional progression of Marmara earthquakes

The recent M 6.2 Marmara earthquake on 23 April 2025 was localised in the transition section and it triggered and enabled deeper research on the directional progression of earthquakes in the region. 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, comprising further colleagues from Germany, Türkiye, the US, and from GFZ, among them 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”.

The scientists analysed the rupture characteristics of the April 2025 M 6.2 earthquake and the framing regional seismicity pattern based on a catalogue of seismic events covering the last 20 years. A special focus lies on comparing the 2025 M 6.2 with the 2019 M 5.8 Siliviri earthquake, because both have neighbouring epicentres and partially overlap.

A decade-long eastward activation of the Main Marmara Fault

The new study shows that the Main Marmara Fault (MMF) has been undergoing a progressive, eastward-moving sequence of moderate earthquakes towards the locked segment south of western Istanbul with roughly increasing magnitude starting 2011 and culminating in the 2025 M 6.2 Marmara earthquake, which is the largest event on the MMF in more than 60 years. The latter ruptured ~10 to 20 kilometres of the Central Main Marmara Fault with strike-slip kinematics. It might have been influenced by the 2019 M 5.8 event through stress transfer.

The observed eastward migration of M>5 events started on the creeping section including the Western High and the Central Basin, which experienced two M>5 earthquakes in 2011 and 2012. They left a ~10-15 kilometres quiet region at the eastern edge of the creeping section, which was then activated through the 2019 M 5.8 quake. Its ~10 km rupture was partially overlapped by the 2025 M 6.2 quake. Both neighbouring epicentres mark the change from the creeping Central Basin segment of the Main Marmara Fault to the transitional Kumburgaz Basin segment. And both events have shown asymmetric aftershock patterns with respect to the epicentre, with a focus on an eastward direction along the MMF. The aftershocks of the April 2025 quake terminated near the eastern edge of a ~15 km long seismically quiet zone in the Avcılar segment, between the Kumburgaz and the locked Princes Islands segment south of Istanbul. In addition, the aftershock characteristics in terms of the ratio between smaller and larger quakes and thus the stress level is different from the 2019 event, suggesting that on this segment shear stress or in other words the ‘loading status’ remains high.

Implications for the Istanbul region: increasingly critical stress level

The observed earthquake pattern from the April 2025 M 6.2 event – in line with the events observed before – brings seismic energy release closer to the locked Princes Islands segment, the part of the MMF south of Istanbul which is capable of producing a magnitude ~7 earthquake by itself.

“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

Studies from the recent years show, that the energy transported by seismic waves can be stronger in certain directions and weaker in others which has important consequences for the damage potential in populated regions. In their study, Martínez-Garzón and co-authors have now also shown that the recent April 2025 MW 6.2 quake in the Sea of Marmara in fact generated shorter and more energetic seismic waves and enhanced ground motion at measuring stations to the East of the epicentre. This so-called “eastward rupture directivity pattern”’ is consistent with findings from previous earthquakes in the region and means that ground shaking could be stronger if future moderate to large earthquakes also start west of Istanbul and propagate eastwards towards the city. Alternatively, if the pending large event would start immediately south of Istanbul this effect is smaller, however, the ground motions would be likewise high due to the small distance.

This study reports that, together with the previous earthquake ruptures on the MMF over the last decade, the M 6.2 earthquake leaves only a ~15–20 km-long region with low seismic activity in the Avcılar segment, between the Kumburgaz and the locked Princes Islands segment south of Istanbul. As the 2019 M 5.8 seismic sequence began in a similarly quiet area, the results suggest that this seismically silent segment is a possible candidate 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.”

Outlook: Importance of dense real-time monitoring of the submarine fault

If a future large earthquake propagates from west to east—as recent events suggest—ground shaking in the Istanbul region could be intensified by directivity effects. For this reason, the authors emphasize the importance of densely monitoring the submarine fault in real-time. This includes the establishment of further borehole stations – as part of the operational GONAF observatory that is coordinated jointly between GFZ and the Turkish Disaster and Emergency Management Presidency AFAD –, but likewise also permanent ocean-bottom seismic and geodetic stations, as well as fiber-optic offshore sensing systems, which will be used in the near-future in the frame of the SAFATOR Helmholtz Infrastructure.

Better monitoring helps to better anticipate what might happen and in case of an event to be able to react faster on time scales of seconds or minutes – which is critical for the shut-down of critical infrastructure and other very basic safety measures.

Background: GONAF and SAFAtor infrastructure

The GONAF observatory currently consists of 10 up to 300m-deep borehole stations, the latest of which are the most advanced instrumented stations of its kind in the world. It is in operation since 2015 and has allowed to derive at a much more detailed model of the ongoing seismic and aseismic deformation processes along the Main Marmara Fault below the Sea of Marmara. The recently launched SAFAtor project is a major initiative by the Helmholtz Association and coordinated by the GFZ to advance the use of offshore fibre-optic cables for scientific purposes.


Funding

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.

Original study

P. Martínez-Garzón et al., Progressive eastward rupture of the Main Marmara Fault towards Istanbul, Science 10.1126/science.adz0072 (2025).

 

Further information

GONAF

www.gonaf-network.org

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

Press release

SAFAtor

Project website

Press release


Previous publications:

D. Becker, P. Martínez-Garzón, C. Wollin, T. Kılıç, M. Bohnhoff, Variation of Fault Creep Along the Overdue Istanbul-Marmara Seismic Gap in NW Türkiye. Geophys. Res. Lett. 50, e2022GL101471 (2023). doi:10.1029/2022GL101471

Press release

X. Chen, P. Martínez-Garzón, G. Kwiatek, Y. Ben-Zion, M. Bohnhoff, F. Cotton, Rupture Directivity of Moderate Earthquakes Along the Main Marmara Fault Suggests Larger Ground Motion Towards Istanbul. Geophys. Res. Lett. 52, e2024GL111460 (2025). doi:10.1029/2024GL111460

Press release

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