Engineering as the leading earthquake prediction

Researchers at Brown University have discovered that the geometry of fault networks, not just the friction at fault lines, significantly affects the occurrence and intensity of earthquakes. Credit: SciTechDaily.com

Researchers from Brown University found that fault geometry, including dislocations and complex structures within fault zones, plays a critical role in determining the probability and strength of an earthquake. This finding, based on studies of fault lines in California, challenges traditional views that focus primarily on friction.

By taking a closer look at the geometric composition of the rocks from which earthquakes originate, researchers at Brown University are adding a new wrinkle to the long-held belief about what causes earthquakes in the first place.

Earthquake dynamics revisited

The research, described in a recently published article in the journal Nature, reveals that the way fault networks are aligned plays a crucial role in determining where an earthquake occurs and how strong it is. These findings challenge the traditional idea that it is the type of friction that occurs at these faults that primarily governs whether earthquakes occur or not, and could improve the current understanding of how earthquakes work.

“Our paper paints this very different picture of why earthquakes happen,” said Victor Tsai, a geophysicist at Brown University and one of the paper's lead authors. “This has very important implications for where earthquakes can be expected to occur versus where earthquakes cannot be expected, and also for predicting where earthquakes will be most damaging.”

Traditional views on earthquake mechanics

Fault lines are the visible boundaries on the planet's surface where the solid plates that make up Earth's lithosphere collide with each other. For decades, geophysicists have interpreted earthquakes to occur when stress builds up on faults to the point where the faults rapidly slide or break over each other, releasing pent-up stress in an action known as slip behavior, Tsai says.

The researchers hypothesized that the rapid slip and the intense ground movements that follow are the result of unstable friction that can occur at faults. In contrast, the idea is that when friction is stable, plates slide against each other slowly without an earthquake occurring. This steady, smooth movement is also known as crawling.

New perspectives on fault line behavior

“People try to measure these friction properties, like whether a fault zone has unstable friction or stable friction, and then, based on laboratory measurements of that, they try to predict whether or not you'll have an earthquake there,” Cai said. He said. “Our findings suggest that it may be more important to look at the geometry of faults in these fault networks, because it may be the complex geometry of the structures around those boundaries that creates this unstable versus stable behavior.”

The geometry to consider includes complexities in the underlying rock structures such as bends, gaps, and steps. The study is based on mathematical modeling and study of fault zones in California using data from the US Geological Survey's Quaternary Faults Database and from the California Geological Survey.

Detailed examples and previous research

The research team, which also includes Brown University graduate student Jaesuk Lee and geophysicist Greg Hirth, provides a more detailed example to illustrate how earthquakes occur. They say to imagine defects colliding with each other as having serrated teeth like the edge of a saw.

When there are fewer teeth or blunt teeth, rocks slide over each other more smoothly, allowing crawling. But when the rock structures in these faults are more complex and rough, these structures stick together and stick. When this happens, they increase the pressure, and eventually as they pull and push harder, they break, pulling apart and causing earthquakes.

Effects of geometric complexity

The new study builds on previous work investigating why some earthquakes generate more ground motion than other earthquakes in different parts of the world, and sometimes even those of similar strength. The study showed that the collision of blocks within a fault zone during an earthquake contributes significantly to the generation of high-frequency vibrations and raised the idea that subsurface geometric complexity may also play a role in where and why earthquakes occur.

Earthquake imbalance and intensity

Analyzing data from faults in California – which includes the well-known San Andreas Fault – the researchers found that fault zones that had complex geometry underneath, meaning the structures there were not parallel, turned out to have stronger ground motions than motions that were less geometrically complex. Error zones. This also means that some of these areas will have stronger earthquakes, others will have weaker earthquakes, and some will have no earthquakes.

The researchers determined this based on the average imbalance of the errors they analyzed. This misalignment ratio measures how close the faults are in a given area and all go in the same direction versus going in different directions. The analysis revealed that fault zones where faults are more oblique cause episodes of slip in the form of earthquakes. Fault zones where the fault geometry was more aligned facilitated smooth creep of the fault without earthquakes.

“Understanding how faults behave as a system is essential to understanding why and how earthquakes occur,” said Lee, the graduate student who led the work. “Our research suggests that the complexity of the error network architecture is the key factor and creates meaningful connections between sets of independent observations and integrates them into a new framework.”

Future directions in earthquake research

The researchers say more work needs to be done to fully validate the model, but this preliminary work suggests the idea is promising, especially since misalignment or misalignment is easier to measure than misalignment properties. If this work is valid, it could one day be incorporated into earthquake prediction models.

This remains a long way off at the moment, as researchers begin to determine how to build on the study.

“The most obvious thing that comes next is to try to go beyond California and see how this model holds up,” Tsai said. “This is potentially a new way to understand how earthquakes occur.”

Reference: “Fault network geometry affects the frictional behavior of earthquakes” by Jaeseok Lee, Victor C. Tsai, Greg Hirth, Avigyan Chatterjee and Daniel T. Trugman, 5 June 2024, Nature.DOI: 10.1038/s41586-024-07518-6

The research was supported by the National Science Foundation. Besides Li, Tsai and Hirth, the team also included Avighyan Chatterjee and Daniel Trugman of the University of Nevada, Reno.

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