Analysed earthquake sequence along the Marmara fault in the Sea of Marmara off the megacity of Istanbul: Green stars mark moderate earthquakes with a magnitude of M>5, circles of various sizes mark the aftershocks. The chronological sequence shows the eastward movement. The different sections of the fault are marked in colour: blue is the creeping segment, orange is the transition zone and red is the currently locked segment where a major earthquake could occur. Credit: Patricia Martínez-Garzón.

A new study has looked into the eastward earthquake rupture progression along the Main Marmara Fault towards Istanbul.

The preliminary climax of the series was an M 6.2 earthquake, which critically reached the locked area of the fault near Istanbul.

In April 2025, the Main Marmara Fault (MMF) below the Sea of Marmara in north-western Türkiye experienced its largest earthquake in over 60 years.

In a new study, a team of researchers led by Professor Patricia Martínez-Garzón from the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany, analysed 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 or so 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 megacity with around 18 million inhabitants, the authors highlighted the need for continuous, real-time monitoring in the area.

Late in the cycle

The MMF is already late in its seismic cycle and earlier studies led by GFZ researchers revealed that the 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’; and 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 neighbouring creeping sections, resulting in higher stress concentration.

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 scientists analysed the rupture characteristics of the 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

The new study shows that the 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 in 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 from 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 MMF 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

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 M ~7 earthquake by itself.

Martínez-Garzón, GFZ scientist and lead author, said: “Our results show a long-term progression of partial earthquake ruptures moving towards the Istanbul-adjacent locked fault segment there.

“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 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 this 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.

GFZ’s Professor Marco Bohnhoff said: “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.

“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.”

The study is published in Science.