Creep faults may have simpler geometries

The San Andreas Fault in California is known for its large and destructive earthquakes. But some fault extensions slide in slow motion, with little or no seismic activity. These “creeping” fault segments slide steadily, releasing stresses that can build up and trigger large earthquakes.

“This has very important implications regarding where earthquakes can be expected to occur versus where earthquakes cannot be expected.”

Geologists generally believe that creep occurs on faults that penetrate particularly slippery rock. But a recent study suggests that the overall structure of the fault network may also determine why some faults creep rather than slide more intermittently.

“This has very important implications for where earthquakes can be expected to occur versus where earthquakes are not expected, as well as for predicting where earthquakes will be most damaging,” Victor Tsai, a geophysicist at Brown University and co-author of the study, said in a statement.

Although the study focused on faults in California, which includes the San Andreas, the findings could apply to other faults around the world, according to the researchers. Their findings were published in the journal Nature.

Complex errors

Mistakes are rarely simple and flat. They contain vibrations, gaps and cross-links, complexities known to affect seismic risk.

For example, the curvature of a fault can stop an earthquake in its tracks or cause it to jump to another branch and grow larger. “There has been a lot of work on fault engineering in the past,” said Julian Lozos, a seismologist at California State University, Northridge, who was not involved in the study. “Fault complexity is one factor that goes into risk assessments.”

Cai and his colleagues investigated how geometry affects creep. They measured the direction of the surface fault traces on US Geological Survey maps. They divided the maps into zones 30 kilometers (19 miles) wide, and assigned each zone a value between 0 and 1, to indicate whether it had parallel fault branches or random fault directions, respectively. They then compared this data set to previously measured creep rates, which were determined using instruments that record displacement across the fault and other techniques, such as monitoring roads and fences.

The researchers found that complex fault zones have slower creep rates and experience more earthquakes than faults that occur in parallel, suggesting that these complexities may play a directional role in creep.

So far, seismologists are studying how creep is related to some rock properties, Lozos said. For decades, geologists have discovered a green metamorphic rock, called serpentinite, in creeping parts of the San Andreas. Serpentinite is inherently slippery, but under certain conditions it can turn into talc, which is weaker. These low-friction rocks allow the sides of the fault to slide slowly and steadily. These rock properties have been demonstrated in the laboratory on samples pulled from the depths of San Andreas.

“The amount of misaligned faults seems to correlate well with where faults creep or where earthquakes occur,” Cai said. He and his colleagues believe that the jagged, interlocking interfaces between misaligned faults may cause rocks to collide with each other. They suggest that rock properties may play less of a role in creep than previously thought. “Our findings contradict previous understanding that creep occurs along faults with high roughness, heterogeneous structure, and certain compositions,” they wrote in the study.

Combined factors

“I don't think it's just engineering. The creep must be caused by a combination of factors.

“I think they have an interesting and useful result here,” Lozos said. But the researchers' explanation is very vague, he said, “I don't think it's just engineering. The creep must be caused by a combination of these things.”

Tsai agreed. “Our work shows that error engineering is important, but not necessarily the most important factor,” he wrote in an email. He added that the lack of data on rock friction at depth makes determining the relative roles of geometry and rock properties difficult.

One way to test whether one factor is dominant is to use a model to simulate a complex fault with the added rock properties of a creeping fault, Lozos said. He explained that the new study comes to explain why some parts of San Andreas are closed, when in fact they contain rocks suitable for crawling.

“There is a highway in the Bay Area that follows the path of San Andreas,” Lozos explained. “You can see the green rocks in the cut. But that part of the fault is closed. It was involved in the 1906 San Francisco earthquake. In contrast, the nearby Hayward and Calaveras fault, which branch off from the San Andreas River and run almost parallel to it, have a serpentine but They crawl, as expected.

The San Andreas is slightly off from the plate boundary compared to the other two faults in this region, Lozos said. “They've stopped long enough for this argument [about orientation controlling creep] “It's still confusing.”

—Erin Martin Jones, science writer

Citation: Martin-Jones, E. (2024), Creep faults may have simpler geometries, Eos, 105, https://doi.org/10.1029/2024EO240524. Published November 20, 2024. Text © 2024. The Authors. CC BY-NC-ND 3.0 Unless otherwise noted, images are subject to copyright. Any reuse without express permission of the copyright owner is prohibited.

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