Brown University researchers have found that the geometry of fault networks, not just friction along fault lines, significantly affects the occurrence and intensity of earthquakes. Credit: SciTechDaily.com
Brown University researchers have found that fault geometry, including the misalignment and complex structures in fault zones, plays a key role in determining the likelihood and strength of an earthquake. This finding, based on studies of California fault lines, challenges traditional views that focus primarily on friction.
By looking closely at the geometric makeup of rocks where earthquakes originate, Brown University researchers are adding a new wrinkle to long-held beliefs about what causes seismic tremors in the first place.
Earthquake dynamics reconsidered
Research described in a newly published journal article Nature, reveals that the way fault networks are aligned plays a critical role in determining where an earthquake occurs and how strong it is. The findings challenge the more traditional notion that it is primarily the type of friction that occurs on these faults that determines whether or not an earthquake occurs, and could improve the current understanding of how earthquakes work.
“Our paper paints this very different picture of why earthquakes happen,” said Brown geophysicist Victor Tsai, one of the paper’s lead authors. “And that has very important implications for where to expect earthquakes and where not to expect earthquakes, as well as for predicting where the most destructive earthquakes will be.”
Traditional views of earthquake mechanics
Fault lines are visible boundaries on the planet’s surface where the solid plates that make up the Earth’s lithosphere touch each other. For decades, Tsai says, geophysicists have explained earthquakes as occurring when stress on faults builds to the point where the faults quickly slide or break over each other, releasing pent-up pressure in an action known as strike-slip behavior.
Scientists believed that the rapid slippage and the intense ground movements that follow are the result of unstable friction that can occur on faults. In contrast, the idea is that when friction is stable, the plates then slide slowly over each other without earthquakes. This smooth and fluid movement is also known as creep.
New insights into fault line behavior
“People have been trying to measure these frictional properties, like whether a fault zone has unstable friction or stable friction, and then based on laboratory measurements, they’re trying to predict whether you’re going to get an earthquake there or not,” Tsai said. “Our findings suggest that it might be more relevant to look at the geometry of the fractures in these fracture networks, because 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 complexities in the underlying rock structures such as bends, gaps, and elevations. The study is based on mathematical modeling and study of fault zones in California using data from the Quaternary fault database of the US Geological Survey and the California Geological Survey.
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 that illustrates how earthquakes occur. They say imagine bugs rubbing against each other like saw teeth like a saw blade.
When there are fewer teeth or teeth that are not as sharp, the stones slide more smoothly over each other, allowing creep. But when the rock structures in these faults become more complex and jagged, these structures get stuck on each other. When this happens, they exert pressure and eventually, as they pull and push harder and harder, they break, tear apart, and lead to earthquakes.
Consequences of geometric complexity
The new study builds on previous work that looked at why some earthquakes generate more ground motion compared to other earthquakes in different parts of the world, sometimes even of similar magnitude. The study showed that blocks colliding inside a fault zone during an earthquake contribute significantly to the generation of high-frequency vibrations, and raised the idea that perhaps geometric complexity below the surface also plays a role in where and why earthquakes occur.
Misalignment and earthquake intensity
Analyzing data from faults in California – which include the famous San Andreas fault – the researchers found that fault zones that have complex underlying geometries, meaning the structures there were not as aligned, turned out to have stronger ground motions than less geometrically complex. fault zones. This also means that some of these zones would have stronger earthquakes, others would have weaker ones, and some would have no earthquakes.
The researchers determined this based on the average misalignment of the errors they analyzed. This misalignment ratio measures how closely faults in an area are aligned and all going in the same direction versus going in many different directions. The analysis revealed that fault zones, where the faults are more misaligned, cause “stick-slip” episodes in the form of earthquakes. Fault zones, where the 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 shows that the complexity of the fault network geometry is a key factor in creating 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 since the alignment or misalignment of faults is easier to measure than the frictional properties of the fault. If valid, the work may one day be woven into earthquake prediction models.
That’s a long way off yet, as scientists begin to outline how to build on the study.
“The most obvious thing that will come next is to try to get outside of California and see how this model holds up,” Tsai said. “This is potentially a new way to understand how earthquakes happen.”
Reference: “Fault Network Geometry Affects Earthquake Frictional Behavior” 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. Along with Lee, Tsai and Hirth, the team also included Avigyan Chatterjee and Daniel T. Trugman from the University of Nevada Reno.