The research was conducted primarily in the laboratory of Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology and Mechanical and Civil Engineering and director of the Center for Geomechanics and the Mitigation of Geohazards. Postdoctoral scholar Solène Antoine is the study's first author. The study is described in a paper appearing in the journalProceedings of the National Academy of Sciences on August 11.

The Sagaing fault runs in a relatively straight north-to-south line throughout Myanmar. As its two sides slowly move against one another in opposite directions, stress accumulates along the fault. When the stress buildup reaches a breaking point, the fault slips rapidly, causing an earthquake. The Sagaing and San Andreas faults are very similar -- both relatively straight strike-slip faults running hundreds of kilometers -- and the 2025 Myanmar earthquake, therefore, sheds light on possible future earthquakes on the San Andreas fault.

"This earthquake turned out to be an ideal case to apply image correlation methods [techniques to compare images before and after a geological event] that were developed by our research group," Antoine says. "They allow us to measure ground displacements at the fault, where the alternative method, radar interferometry, is blind due to phenomenon like decorrelation [a process to decouple signals] and limited sensitivity to north-south displacements."

Based on studies of historic earthquakes along the Sagaing fault, researchers anticipated that a large earthquake would occur on a 300-kilometer section where no large earthquakes had occurred since 1839. This theory is known as the seismic gap hypothesis: Stuck sections of a fault where there has not been movement are expected to slip to "catch up." While this section did indeed rupture during the 2025 quake, the fault actually slipped along a total of more than 500 kilometers, indicating that the fault did indeed make up the deficit of slip and more.

In the new study, the team used correlation of satellite optical and radar imagery of the fault -- a technique originally developed in the Avouac laboratory and now widely used in seismology -- and its surroundings to determine that the 500-kilometer section shifted a net of 3 meters after the quake, that is, the eastern side moved south by 3 meters relative to the western side.

Current models used for seismic hazard assessment are mostly based on earthquake statistics and are time independent, meaning they can only give probabilities of an earthquake during a chosen timespan. For example, these models might estimate, forany given 30-year period and particular area, the probability that an earthquake would exceed a chosen magnitude. However, in order to make truly informed estimates of potential seismic hazards for specific time periods -- say, the next 30 years -- it is crucial for models to take into account how recently a fault has slipped, where the slip occurred, and by how much.

"The study shows that future earthquakes might not simply repeat past known earthquakes," Avouac says. "Successive ruptures of a given fault, even as simple as the Sagaing or the San Andreas faults, can be very different and can release even more than the deficit of slip since the last event. In addition, historical records are generally far too short for statistical models to represent the full range of possible earthquakes and eventual patterns in earthquake recurrence. Physics-based models provide an alternative approach with the advantage that they could, in principle, be tuned to observations and used for time-dependent forecast."

The paper is titled "The 2025 Mw7.7 Mandalay, Myanmar, earthquake reveals complex earthquake cycle with clustering and variable segmentation on Sagaing fault." In addition to Antoine and Avouac, Caltech co-authors are graduate student Rajani Shrestha, postdoctoral scholar Chris Milliner, and senior research scientist Kyungjae Im. Additional co-authors are Chris Rollins (PhD '18) of GNS Science Te Pu Ao in New Zealand, Kang Wang of EarthScope Consortium Inc., and Kejie Chen of Southern University of Science and Technology in China. Funding was provided by the Center for Geomechanics and Mitigation of Geohazards, the Statewide California Earthquake Center, the National Science Foundation, and the US Geological Survey.

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