Risk of major earthquake rising for Istanbul, geoscientists warn

Geological stresses in the subsurface south of the Turkish city of Istanbul are continuing to rise, increasing the risk of a strong earthquake in the coming years, according to a study led by the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany.

At Izmit, east of Istanbul, a magnitude 7.4 quake in 1999 killed more than 17,000 people. In 2023, Turkey’s level of earthquake readiness was questioned again when more than 50,000 people were killed in a series of quakes in the centre and south of the country.

The geoscientists are now warning that a comparable event could have devastating effects in the Istanbul metropolitan region with more than 15 million residents and millions of tourists passing through every year.

The authors suspect the next stronger quake could occur south-west or south of Istanbul. “This could be a magnitude 6 event or a foreshock that then triggers an even larger earthquake,” said Marco Bohnhoff of the GFZ, a co-author of the study

The study, by a team led by Patricia Martínez-Garzón of the GFZ and published in the journal Science, examined the frequency of earthquakes – or seismicity – since 2007.

Researchers are calling for improved monitoring of seismic activity beneath the Sea of Marmara and the provision of real-time data.

“Enhancing seismic monitoring systems along this submarine fault is of paramount importance, and may require further borehole stations,” the authors write.

The analysis took as its starting point the magnitude 6.2 earthquake on April 23, 2025 in the Sea of Marmara, south and south-west of Istanbul.

Beneath the Sea of Marmara, which lies between the Black Sea and the Mediterranean, the Marmara Fault forms the western section of the North Anatolian Fault zone between two tectonic plates. In this zone the small Anatolian Plate slides westwards past the larger Eurasian Plate.

Depending on the rock type, the plates lock to varying degrees along different sections. In areas with lower seismic activity, weaker locks release through small, barely perceptible quakes. Where rupture surfaces are very rough, locking is stronger and larger stresses can accumulate, including beneath the Princes’ Islands south of Istanbul.

The authors analysed fault-zone dynamics and aftershock patterns on multiple timescales between 2007 and 2025. Notable is the sequence of earthquakes of magnitude 5.0 or higher: a 5.2 in 2011 and a 5.1 in 2012 in the western Sea of Marmara, a 5.8 in the central Marmara on September 26, 2019, and this year’s 6.2 nearby.

“Our results show a long-term development of partial activation of the Marmara Fault moving eastwards towards the locked Princes’ Islands segment south of Istanbul,” Martínez-Garzón said.

In the western and central Sea of Marmara, instruments recorded numerous weaker quakes, a sign that seismic stress is released after relatively short periods. In the past five years there have been many smaller events slightly east of the 6.2 epicentre.

Further east beneath the Princes’ Islands there have been hardly any quakes. For the scientists this indicates large tectonic stresses are building up in the subsurface south of Istanbul. “The Marmara Fault must be regarded as critically loaded,” said Bohnhoff. “The April quake brought only negligible relief.”

The Marmara Fault is the only section of the North Anatolian Fault where there has been no earthquake of magnitude 7.0 or more since 1766.

The GFZ has coordinated the GONAF observatory with borehole seismometer stations together with Turkey’s disaster agency Afad for 10 years.