Researchers at Brown University have found that the geometry of fault networks, and not just the friction on fault lines, significantly influences the occurrence and intensity of earthquakes. Credit: SciTechDaily.com
Researchers at Brown University found that the geometry of faults, including misalignments and complex structures within fault zones, plays a crucial role in determining the likelihood and strength of earthquakes. This finding, based on research on California’s fault lines, challenges traditional views that focus primarily on friction.
By scrutinizing the geometric composition of rocks where earthquakes originate, researchers at Brown University add a new wrinkle to a long-held belief about what causes seismic earthquakes in the first place.
Rethinking earthquake dynamics
The research, described in a newly published article in the journal Nature, reveals that the way fault networks are aligned plays a crucial role in determining where an earthquake will occur and its strength. The findings challenge the more traditional idea that it is primarily the type of friction that occurs on these faults that determines whether earthquakes occur or not, and they could improve current understanding of how earthquakes work.
“Our paper paints a very different kind of picture of why earthquakes happen,” said Brown geophysicist Victor Tsai, one of the paper’s lead authors. “And this has very important implications for where we can expect earthquakes and where we can’t expect earthquakes, and also for predicting where the most damaging 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 collide. Tsai says geophysicists have explained for decades that earthquakes happen when stress on faults builds up to the point where the faults quickly slide past each other or break, releasing pent-up pressure in an action known as stick-slip behavior.
Researchers theorized that the rapid slip and intense ground movements that follow are the result of unstable friction that can occur at the faults. The idea, on the other hand, is that when the friction is stable, the plates slowly slide against each other without an earthquake occurring. This steady and smooth movement is also called creep.
New perspectives on faultline behavior
“People have been trying to measure these friction properties, like whether the fault zone has unstable friction or stable friction, and then, based on laboratory measurements of that, they try to predict whether you’re going to have an earthquake there or not,” Tsai says. said. “Our findings suggest that it may be more relevant to look at the geometry of the 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 transitions. The study is based on mathematical models and studying fault zones in California using data from the US Geological Survey’s Quaternary Fault Database and from the California Geological Survey.
Detailed examples and previous research
The research team, which also includes Brown student Jaeseok Lee and Brown geophysicist Greg Hirth, offers a more detailed example to illustrate how earthquakes happen. They say to imagine the faults colliding with each other as serrated teeth, like the edge of a saw.
If there are fewer teeth or teeth that are not as sharp, the rocks slide past each other more smoothly, allowing crawling. But when the rock structures in these faults are more complex and erratic, these structures get stuck together and become stuck. When that happens, they build up pressure and eventually, as they pull and push harder and harder, they break, causing them to slide away from each other and leading to earthquakes.
Implications of geometric complexity
The new study builds on previous research into why some earthquakes produce more ground motion compared to other earthquakes in different parts of the world, sometimes even of similar magnitude. The study showed that blocks colliding in a fault zone during an earthquake contribute significantly to the generation of high-frequency tremors and led to the idea that subsurface geometric complexity might also play a role in where and why earthquakes occur.
Misalignment and earthquake intensity
Analyzing data from faults in California – including the well-known San Andreas Fault – the researchers found that fault zones with complex geometry underneath, meaning the structures there were not as aligned, appeared to have stronger ground movements than less geometrically complex ones. fault zones. This also means that some of these zones will have stronger earthquakes, some will have weaker earthquakes, and some will have no earthquakes.
The researchers determined this based on the average misalignment of the errors they analyzed. This misalignment ratio measures how closely the faults in a given area align and all go in the same direction versus in many different directions. The analysis showed that fault zones where the faults are more aligned produce stick-slip episodes in the form of earthquakes. Fault zones where fault geometry was better 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 fracture network geometry is the key factor, establishing meaningful connections between sets of independent observations and integrating 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 initial work suggests the idea is promising, especially because the alignment or misalignment of faults is easier to measure than frictional properties of faults. If valid, the work could one day be woven into earthquake prediction models.
That remains a long way off for now, as researchers begin to outline how to build on the study.
“The most obvious thing next is to try to go beyond California and see how this model holds up,” Tsai said. “This may be a new way to understand how earthquakes happen.”
Reference: “Fault Network Geometry Influences the Friction 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. In addition to Lee, Tsai and Hirth, the team also included Avigyan Chatterjee and Daniel T. Trugman of the University of Nevada Reno.