How hidden factors under Istanbul pose the risk of earthquakes

The fault beneath Istanbul is not behaving the way scientists previously thought.

New research from the University of Southern California shows that differences in underground temperature and sediment thickness divide the main Marmara Fault in ways that control where earthquakes start, how far they spread, and where they stop — findings that could reshape risk assessments for one of the world's most vulnerable megacities.

The study, published in the journal Nature Communications Earth & Environment, focuses on the North Anatolian Fault beneath the Sea of ​​Marmara that has not produced a major earthquake since 1894. Using physical simulations representing more than 10,000 years of seismic activity, the researchers found that the fault is unlikely to burst in a single catastrophic event. Instead, it will likely break into pieces, with the maximum earthquake magnitude reaching around 7.3.

“Fault geometry tells us where earthquakes are likely to occur, but rheology — how rocks deform under pressure — tells us how they actually unfold,” said Sylvain Barbot, the study's lead researcher and associate professor of geosciences in the USC Dornsife College of Letters, Arts and Sciences. “Differences in temperature and rock type along the main Marmara Fault act as barriers that can stop cracking or cause the fault to creep rather than trigger a major earthquake.”

How scientists have simulated thousands of years of earthquakes

The Marmara Main Fault is part of the North Anatolia Fault System, which has produced devastating earthquakes throughout Turkish history. While previous studies have mapped fault geometry and slip rates, researchers have not fully understood why earthquakes stop where they do, a question critical to estimating the maximum possible magnitude.

The answer lies beneath the seabed. In the central Sea of ​​Marmara, thick sedimentary basins lie above the warmer crust, creating what researchers call a strong rheological barrier. The frictional properties of sedimentary rocks under specific conditions of temperature and pressure show that they deform slowly and steadily at shallow depths rather than breaking suddenly. At the same time, higher temperatures at greater depths weaken rocks in ways that prevent large cracks from developing.

“The main takeaway is that temperature and sediment thickness fundamentally change how a fault behaves,” said Cesem Guvercin, a postdoctoral researcher at USC Dornsife and first author of the study. “These variations create rupture-resistant zones, especially beneath sedimentary basins in the central Sea of ​​Marmara.”

To test this, the research team built a 3D model of the earthquake cycle that combines realistic fault geometry, frictional properties of rocks and thermal structure based on regional heat flow measurements. The simulations used Unicycle, an open source code that can model thousands of years of seismic cycles.

Different sectors, different earthquake patterns

When the model incorporated both sedimentary stratigraphy and temperature changes, it reproduced key features of the historical record, including the large earthquakes of 1766 and 1912. During the simulation period, no earthquake exceeded magnitude 7.3.

Different parts of the fault showed distinct patterns. The western Janus and Tekirdağ sectors, which are cooler and geometrically simpler, produce more regular earthquakes — including magnitude 7.2 events that occur about every 150 years. The eastern parts, Kumbourgaz and the Princes' Islands, generate smaller and more frequent double earthquakes, typically a magnitude 6.2–6.8 earthquake approximately every 100 years and a magnitude 7.0 earthquake approximately every 500 years.

Models also predict surface creep — slow, continuous sliding that releases stress without breaking — in parts of the fault near the central basin. This behavior matches geodetic observations and clusters of frequent small earthquakes recorded over the past two decades.

“Earthquakes tend to form near fault bends, where stresses are highest,” Barbeau said. “But whether rupture continues or stops is largely controlled by rheology.”

Models that ignored sedimentary basins or thermal structure consistently overestimated earthquake magnitudes and ignored behaviors, such as creep segments. Only by incorporating the physical complexity of underground geology were the simulations able to match the observed patterns.

What does this mean for Istanbul?

The results do not reduce the risk of earthquakes in Istanbul. Moderate to large earthquakes that occur near a city, or in quick succession, can cause catastrophic damage. Instead, the research provides a more accurate picture of how the fault actually behaves, information that is essential for building codes, emergency planning, and infrastructure decisions.

“Our work shows that what's underground — heat, rock and structure — matters a lot in earthquake behavior,” Guversen said. “Integrating these factors is essential to improve seismic hazard predictions in regions like Istanbul.”

Closed portions of the main Marmaray fault on both sides have now gone more than 100 years without major rupture. If these parts follow the patterns observed in simulations, the region could experience major earthquakes in the coming decades. A precise understanding of how rifting occurs when it does can help determine which parts of Istanbul face the greatest risks.

###

The study is funded by the National Science Foundation under award number EAR-1848192.

magazine

Earth and Environment Communications

Search method

Computational simulation/modelling

Research topic

Not applicable

Article title

Fragmentation of the ongoing rupture of the main Marmaray fault

Date the article was published

4-December-2025

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