How hidden factors beneath Istanbul shape earthquake risk
Underground heat and sediment patterns control how earthquakes behave along one of the most dangerous faults in the eastern Mediterranean
University of Southern California
The fault beneath Istanbul doesn’t behave the way scientists once thought.
New research from USC shows that variations in underground temperature and sediment thickness segment 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 Nature Communications Earth & Environment, focuses on the North Anatolian Fault beneath the Sea of Marmara that hasn’t produced a major earthquake since 1894. Using physics-based simulations that model more than 10,000 years of seismic activity, researchers found that the fault is unlikely to rupture in a single, catastrophic event. Instead, it will likely break in segments, with maximum earthquake magnitudes reaching about 7.3.
“Fault geometry tells us where earthquakes are possible, but rheology — how rocks deform under stress — tells us how they actually unfold,” said Sylvain Barbot, the study’s principal investigator and associate professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences. “Variations in temperature and rock type along the Main Marmara Fault act as barriers that can stop ruptures or cause the fault to creep instead of breaking in a large earthquake.”
How scientists simulated thousands of years of earthquakes
The Main Marmara Fault is part of the North Anatolian Fault system, which has produced devastating earthquakes throughout Turkish history. While previous studies mapped the fault’s geometry and slip rates, researchers hadn’t fully understood why earthquakes stop where they do — a critical question for estimating maximum possible magnitudes.
The answer lies beneath the seafloor. In the central Sea of Marmara, thick sedimentary basins sit above the warmer crust, creating what the researchers call a strong rheological barrier. Frictional properties of sedimentary rocks under specific temperature and pressure conditions show that they deform slowly and stably at shallow depths rather than breaking suddenly. Meanwhile, elevated temperatures at greater depths weaken rocks in ways that prevent large ruptures from growing.
“The main takeaway is that temperature and sediment thickness fundamentally change how the fault behaves,” said Sezim E. Guvercin, a postdoctoral researcher at USC Dornsife and first author of the study. “These variations create zones that resist rupture, particularly beneath sedimentary basins in the central Sea of Marmara.”
To test this, the research team built a three-dimensional earthquake-cycle model combining 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 segments, different earthquake patterns
When the model incorporated both sedimentary layers and temperature variations, it reproduced key features of the historical record, including the large 1766 and 1912 earthquakes. Over the simulated period, no earthquake exceeded magnitude 7.3.
Different parts of the fault showed distinct patterns. The western Ganos and Tekirdağ segments, which are cooler and geometrically simpler, produce more regular earthquakes — including magnitude 7.2 events recurring roughly every 150 years. The eastern segments, Kumburgaz and Princes’ Islands, generate smaller, more frequent earthquake doublets, typically between magnitude 6.2 and 6.8 roughly every 100 years and a magnitude 7.0 earthquake roughly every 500 years.
The models also predict shallow creep — slow, continuous slip that releases stress without breaking — in parts of the fault near the Central Basin. That behavior matches geodetic observations and clusters of small, repeating earthquakes recorded over the past two decades.
“Earthquakes tend to nucleate near bends in the fault, where stresses are highest,” Barbot said. “But whether a rupture keeps going or stops is largely controlled by rheology.”
Models that ignored sedimentary basins or thermal structure consistently overestimated earthquake sizes and missed behaviors such as creeping segments. Only by incorporating the physical complexity of underground geology did the simulations match observed patterns.
What this means for Istanbul
The findings don’t reduce Istanbul’s earthquake risk. Moderate to large earthquakes occurring closer to the city, or in rapid succession, could still cause catastrophic damage. Instead, the research provides a more accurate picture of how the fault actually behaves — information essential for building codes, emergency planning and infrastructure decisions.
“Our work shows that what’s underground — heat, rocks and structure — matters enormously for earthquake behavior,” Guvercin said. “Integrating these factors is essential for improving seismic hazard forecasts in regions like Istanbul.”
The locked segments of the Main Marmara Fault on both sides have now gone more than 100 years without a major rupture. If these segments follow the patterns observed in simulations, the region may experience major earthquakes in the coming decades. Understanding exactly how the fault will break when it does could help identify which parts of Istanbul face the greatest risk.
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The study is funded by the National Science Foundation under award number EAR-1848192.
Journal
Communications Earth & Environment
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Persistent rupture segmentation of the Main Marmara Fault
Lisa Wald honored by SSA for creating USGS Earthquake Information “Hub”
Seismological Society of America
image:
Lisa Wald
view moreCredit: Courtesy of Lisa Wald
For her groundbreaking work in creating the earthquake information “hub” of the Earthquake Hazards Program, of the U.S. Geological Survey, the Seismological Society of America honors Lisa Wald with the 2026 Frank Press Public Service Award.
Wald began work at the USGS in 1987 in Pasadena, California, as a geophysicist and outreach director before becoming science communications and web content manager at the National Earthquake Information Center in Golden, Colorado, in 2002. She retired from the USGS in 2024.
“In the United States, the U.S. Geological Survey makes a wealth of earthquake information available, both educational resources and real-time products, in a variety of forms and through a variety of channels—including internet, text, and email—serving everyone from the general public to seismologists to governments to the media. Today we take that kind of information access for granted,” said USGS research geophysicist Sarah Minson, who nominated Wald for the award.
Minson noted that Wald worked with various USGS earthquake offices to pull together all the disparate USGS earthquake information into a single internet location, creating or facilitating the creation of most of the subsequent content on the USGS Earthquake Hazards Program website.
Wald taught herself HTML in the 1990s to build earthquake.usgs.gov, ushering in the internet era for the Survey. She was a key architect of the Earthquake Notification Service (ENS), which today allows users to receive custom automated earthquake event alerts, PAGER notifications about earthquake damage and fatality estimates, aftershock forecasts and more.
In 2015, Wald helped to produce the first U.S. Federal Crowdsourcing and Citizen Science Tool Kit to aid federal agencies in carrying out citizen science and crowdsourcing projects. She was invited by the White House Office of Science and Technology Policy to be the web designer on the tool kit team.
During her time at USGS, Wald created a wide range of digital products for students, educators and the public, including Earthquakes for Kids, The Science of Earthquakes and the Science for Everyone series. Wald was also on the receiving end of all email inquiries sent to eq_questions@usgs.gov for almost two decades. She answered every one of what amounted to around 1000 emails every year. In their commendations of Wald, her colleagues noted her service in providing communications tutorials and advice for generations of scientists in and out of the federal government.
The Frank Press Public Service Award honors outstanding contributions to the advancement of public safety or public information relating to seismology. This award may be given to any individual, combination of individuals, or organization.
