Researchers at Brown University have found that the geometry of fault networks, rather than just friction at fault lines, significantly influences the occurrence and intensity of earthquakes. Credit: SciTechDaily.com
Brown University researchers found that fault geometry, including misalignments and complex structures within fault zones, plays a crucial role in determining the likelihood and strength of earthquakes. This finding, based on studies of fault lines in California, challenges traditional views that focus primarily on friction.
By closely examining the geometric composition of the rocks that cause earthquakes, Brown University researchers are adding a new wrinkle to a long-held belief about the initial cause of earthquakes.
Rethinking earthquake dynamics
The research, described in an article recently published in the journal Nature, reveals that the way fault networks are aligned plays a critical role in determining where an earthquake will occur and how strong it will be. The results challenge the more traditional notion that it is primarily the type of friction occurring at these faults that determines whether earthquakes occur or not, and they could improve current understanding of how earthquakes work. earth.
“Our paper paints a very different picture of why earthquakes happen,” said Brown geophysicist Victor Tsai, one of the paper’s lead authors. “And this has very important implications for knowing where to expect earthquakes versus where not to expect earthquakes, as well as predicting where the most devastating earthquakes will occur.”
Traditional views on earthquake mechanics
Fault lines are the visible boundaries on the planet’s surface where the rigid plates that make up Earth’s lithosphere brush against each other. Tsai says that for decades, geophysicists have explained earthquakes as occurring when stresses at faults build up to the point where faults slide or break rapidly on top of each other, releasing pent-up pressure in an action known as stick-slip behavior.
The researchers hypothesize that the rapid sliding and subsequent intense ground movements are the result of unstable friction that can occur at faults. In contrast, the idea is that when friction is stable, the plates slide slowly against each other without earthquakes. This steady, fluid movement is also known as creep.
New Perspectives on Fault Line Behavior
“People have tried to measure these friction properties, for example whether the fault zone has unstable or stable friction, and then based on laboratory measurements they try to predict whether you’re going to have an earthquake there or no,” Tsai said. said. “Our results suggest that it may be more relevant to examine the geometry of the faults in these fault networks, as it may be the complex geometry of the structures around these boundaries that creates this unstable versus stable behavior.”
The geometry to be considered includes the complexities of the underlying rock structures such as bends, gaps and spans. The study is based on mathematical modeling and investigation of fault zones in California using data from the U.S. Geological Survey and California Geological Survey Quaternary Fault Database.
Detailed examples and previous research
The research team, which also includes Brown graduate student Jaeseok Lee and Brown geophysicist Greg Hirth, offers a more detailed example to illustrate how earthquakes occur. It is said to imagine defects that brush against each other as having jagged teeth like the edge of a saw.
When there are fewer teeth or less sharp teeth, rocks slide past each other more easily, allowing creep. But when the rock structures on these faults are more complex and irregular, these structures cling to each other and get stuck. When this happens, they build up pressure and eventually pull and push harder and harder, they break, moving away from each other and causing earthquakes.
Implications of geometric complexity
The new study builds on previous work examining why some earthquakes generate more ground motion compared to other earthquakes in different parts of the world, sometimes even those of similar magnitude. The study showed that colliding blocks inside a fault zone during an earthquake contribute significantly to the generation of high-frequency vibrations and sparked the idea that geometric complexity beneath the surface also played a role in where and why earthquakes occurred.
Misalignment and earthquake intensity
By analyzing data from California faults, including the famous San Andreas Fault, researchers found that fault zones with complex geometry underneath, meaning structures there were not as aligned, appeared have stronger ground motions than less geometrically complex fault zones. fault zones. This also means that some of these areas would experience stronger earthquakes, others would experience weaker earthquakes, and still others would have no earthquakes at all.
The researchers determined this based on the average misalignment of the analyzed faults. This misalignment rate measures how closely the faults in a certain region are aligned and all going in the same direction rather than in many different directions. The analysis revealed that fault zones where faults are more misaligned cause episodes of stick-slip in the form of earthquakes. Fault zones where fault geometry was more aligned facilitated smooth fault creep 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 indicates that the complexity of the fault network geometry is the key factor and establishes 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 early work suggests the idea is promising, particularly because the alignment or misalignment of faults is easier to measure than the friction properties of faults. flaws. If valid, the work could one day be integrated into earthquake prediction models.
That’s still a ways off for now, as researchers begin to map out how to take advantage of the study.
“The most obvious thing going forward is to try to go beyond California and see how that model holds up,” Tsai said. “This is potentially a new way to understand how earthquakes occur.”
Reference: “Fault network geometry influences the frictional behavior of earthquakes” by Jaeseok Lee, Victor C. Tsai, Greg Hirth, Avigyan Chatterjee and Daniel T. Trugman, June 5, 2024, Nature.
DOI: 10.1038/s41586-024-07518-6
The research was supported by the National Science Foundation. Besides Lee, Tsai and Hirth, the team also included Avigyan Chatterjee and Daniel T. Trugman of the University of Nevada, Reno.