Earthquake science illuminates landslide behavior

How scientists model slow-slip earthquakes can also help them understand slow-moving landslides, a common hazard along the West Coast.
 

By Rebecca Owen, Science Writer (@beccapox)
 

Citation:Owen, R., 2025, Earthquake science illuminates landslide behavior, Temblor, http://doi.org/10.32858/temblor.365
 

The iconic ribbon of road that hugs the California coastline — Highway 1 — has long been plagued by landslides. For instance, as of this writing, a section between San Luis Obispo and Big Sur remains closed after the Regent’s Slide knocked out a section of the road in February 2024, just north of ongoing repair work related to an earlier landslide. Keeping a close eye on steep slopes that pose hazards to roadways and residences alike is crucial to maintaining safety in a region where natural hazards like earthquakes, atmospheric rivers, and wildfire can all contribute to unstable ground.
 

 
There are several different types of landslides that may befall California slopes, explains University of Oregon geomorphologist Josh Roering. For instance, rapid, dangerous landslides involve large volumes flowing quickly downhill. These slides pick up rocks, trees, houses, and roads as they go. But there are slow-moving landslides to worry about too.

Slow-moving landslides “are really mysterious because they get saturated and they flow. But they don’t typically fail catastrophically,” Roering says. “These slow-movers are really interesting because rather than evacuating all that unstable material, they just kind of progressively creep.”

Creeping, slow-moving landslides may have a lot in common with another ubiquitous California hazard, suggests a new study published in Science Advances. Earthquakes and landslides share some physical and geological principles, and using models designed for studying faults that move slowly, researchers were able to examine how quickly and easily a slope can slide as water satu-rates the ground. These findings are a step toward understanding why some landslides creep and some fail catastrophically.
 

 

Connecting the rocks above to the rocks below

The California Coast Ranges, where landslides occur frequently, stretch hundreds of miles from northern California to Santa Barbara, spliced by parts of the San Andreas fault system. The Franciscan Complex, which underlies much of the Coast Ranges, includes significant swaths of melange — a jumbled mix of strong blocks of rock wrapped in weak, clay-rich mudstone. In the melange, erosion gnaws at the fragile matrix, leaving behind boulders of mostly metamorphic rocks. The Franciscan Complex tells a story of what happened before the San Andreas existed, when California’s coast sat atop an ancient subduction zone. These rocks are the remnants of the old accretionary wedge, where sediments entering the trench stuck to the non-subducting North American Plate as the Farallon Plate (today, remnants include the Juan de Fuca Plate to the north and the Cocos Plate to the south) slid beneath the West Coast.

Those deformed rocks of the Franciscan — including the mudstone-wrapped blocks — have been exhumed over the past few million years. The Franciscan hosts “a whole lot of this interesting slow-slip behavior that we also know is happening in modern subduction zones,” says study author Noah Finnegan, a geomorphologist at the University of California, Santa Cruz.
 

 

Finnegan connected with Demian Saffer, a geophysicist at the University of Texas, Austin, to further examine the parallels between the dual hazards of earthquakes and landslides. “We realized they’re not that different,” says Saffer. “Earth flows, which are coherent blocks of material that are sliding — they’re basically a very shallow fault.”

For both earthquakes and landslides, friction — the forces that resist the movement of one thing against another — depends on the types of rocks present for each hazard. Friction governs whether materials will stay stable or if they will slip. Soft, clay-like rocks move slowly but easily. Harder rocks are brittle, less likely to move. But, they can build up stress that can lead to a faster, more catastrophic release when an earthquake or landslide does occur.

It’s difficult to observe faults as closely as landslides. To understand what’s happening on a fault requires drilling deep underground, working in a lab setting to replicate what’s occurring in the rocks below the surface, or using computers to model fault behavior. On the other hand, landslides, especially slow ones, can be monitored and studied in real time.

This means that the limits that exist when studying faults in an artificial, human-controlled environment are reversed when looking at landslides. “It’s like doing a lab experiment that’s controlled by nature — but you’re going to be able to measure all the key parameters and everything that comes out, including the motion of the slide.”

We can use the detailed measurements that we actually make in the field, and then use the framework that’s been developed for understanding the friction on faults to explain the sorts of observations that we see through models,” Finnegan says.
 

Creeping rocks, compiling data

Finnegan and Saffer used field observations from two creeping California landslides, one in the East Bay part of the Bay Area and the other in Humboldt County, to the north. In the East Bay site, Finnegan has been measuring the motion of the slide as well as the water pressure inside for eight years. The second dataset came from measurements taken in the 1980s by a different scientist.

How easily rocks can slide along weak surfaces, like a fault or landslide failure surface, depends on two factors — one is the coefficient of friction, which measures how much the material resists sliding. The other is effective normal stress, which is the balance between the pressure from the weight of overlying rocks and the pressure of water in the rocks’ pores. If water pressure increases, it reduces the effective normal stress, making it easier for rocks to slide — especially in the wet winter months. “Seasonal increases in water pressure from rainfall infiltrating into the ground tend to trigger landslide motion,” Finnegan says.

Although the above mathematical (and physical) terms may account for a landslide’s initial movement, the coefficient of friction also depends on the speed that the material is sliding. This necessitates a more complex framework known as a rate-and-state model — common in earthquake science — which describes the relationship between sliding speed and the coefficient of friction.

“Materials can be either ‘velocity strengthening,’ where they tend to resist acceleration because the coefficient of friction increases with sliding speed,” says Finnegan. “Or they can be ‘velocity weakening,’ in which case they tend to accelerate catastrophically because friction drops as speed increases.”

The relationship between friction and sliding speed is usually determined through controlled lab experiments, Finnegan explains. These simplified simulations lack the nuance of real-world situations — like an active fault that is dozens of kilometers below ground. But in the case of these two California landslides, Finnegan and Saffer were able to constrain all the parameters required to link friction to sliding velocity measurements using real-world data. GPS data tracked strain changes — how much the slide moved — while measurements of the landslides’ water pressure fluctuated on a daily basis.

Their field-derived estimates of friction matched laboratory experiments, suggesting that the friction of clay-rich materials may govern slow slip in landslides, as it is thought to do on tectonic faults, says Finnegan. “Landslides provide natural laboratories where models for fault friction can be tested.”

“This has been an idea that’s been in the literature theoretically,” says Roering. “But no one’s really demonstrated it with good field data. That’s what makes this study so compelling.”
 

Keeping a watchful eye on future slides

The potential for both earthquakes and landslides put a wide swath of Californians at risk for danger and destruction from shifting rock and soil underneath their feet. Currently, the slow-moving slide occurring in Rancho Palos Verde is displacing residents and causing local authorities to shut off utilities. Governor Gavin Newsom has declared a state of emergency.
 

 

Both Finnegan and Saffer think that this study can be a first step toward better predicting landslide behavior by understanding the material properties of rocks involved in the slide — information that is readily available for many regions in which landslides occur. “We think that we could now look at what kinds of rocks landslides are in, and start to make generalized predictions about which landslides might be more likely to fail catastrophically, and which ones should have this kind of modulated creep,” Saffer says.

This work provides an example of transferring lessons from one type of hazard to another, allowing scientists to work across disciplines to solve problems. And the study opens doors to learning more about the parallels between California’s dual hazards. “We started this by saying we could take the tectonic theories and bring them to landslides,” Saffer says. “But we think we could actually learn something in the other direction too.”

 

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