“`html
In coastal California, the phrase “not if, but when” often comes to mind regarding earthquakes and landslides. This challenging truth has gained a measure of predictability, thanks to researchers at UC Santa Cruz and The University of Texas at Austin who discovered that conditions prompting shifts along underground fault lines can also trigger landslides above ground.
The recent research, led by geologist Noah Finnegan from UC Santa Cruz, relied on extensive data collected from two landslide locations in Northern California that have been closely monitored over several years. Finnegan and his co-author utilized a model that was originally created to explain slow slips along faults, and they found an impressive result: This model applies equally well to landslides.
This discovery marks a significant advancement, indicating that a model intended for faults can also help predict landslide behaviors. In California, where slow-moving landslides are a constant issue costing hundreds of millions of dollars each year, this development represents a crucial advancement in predicting landslide activity—especially as it relates to environmental changes like fluctuations in groundwater levels.
Finnegan describes landslides as a “plumbing problem.” During rain, when the ground becomes saturated, the water pressure in rocks rises, reducing friction that typically opposes slipping, he explained.
“On a practical level, this study gives us a framework to understand the expected movement linked to changes in rainfall, which then alters the water pressure in the ground and leads to motion,” remarked Finnegan, who is a professor of earth and planetary sciences. “We have a limited number of predictive tools, so this represents a progressive step toward that goal. While it doesn’t completely resolve the larger issue, it provides us with useful information we can apply now.”
In earthquake dynamics, particularly in areas like California, a main challenge is comprehending the varying behaviors of fault lines. Some faults remain “locked” and fail only at intervals, causing major earthquakes, while others slip continuously at a consistent rate. Seismologists have spent years trying to decipher the reasons behind the diverse behaviors of faults to improve earthquake predictions and assess hazards.
In the last twenty years, researchers have acknowledged that faults can demonstrate various slip patterns. Certain behaviors might not lead to noticeable earthquakes but still have an impact on fault mechanics. These quiet, “silent” slip events alter the hazard landscape and pose a challenge because they are challenging to detect and comprehend.
Similarities between slides and quakes
Like faults, landslides exhibit varying behaviors. Some result in catastrophic failures causing loss of life and widespread damage, while others move slowly, leading to ongoing infrastructural issues. A current example is the landslide in Rancho Palos Verdes, located in northern Los Angeles County. The Portuguese Bend Landslide Complex has experienced increased movement over the past two years, causing significant disruptions, including gas and power outages for hundreds of residents due to safety fears. Despite its gradual nature, the situation was severe enough for Governor Gavin Newsom to declare a state of emergency.
“A crucial question in landscape-hazards science is identifying what influences behavior styles. Why do some landslides creep while others fail suddenly and destructively?” stated co-author Demian Saffer, director of the Institute for Geophysics at the University of Texas and a professor at the UT Jackson School of Geosciences. “Landslide movement shares many analogies with tectonic faulting. Understanding why some systems slip slowly while others fail dramatically sheds light on the physical principles driving these behaviors.”
Just as in earthquake research, there’s a limited understanding of what governs landslide behaviors—why some move slowly while others collapse rapidly. In earthquake studies, the impact of friction is more clearly established, particularly how it alters as ground materials shift. Scientists commonly differentiate between “static friction,” which keeps objects in place, and “dynamic friction,” which occurs during movement. However, friction behaves differently under various conditions, and these variations are crucial for grasping how both earthquakes and landslides manifest.
For landslides, the exploration of friction is still in its nascent stages, but this study marks a significant leap forward. The team discovered that friction affects both faults and landslides similarly by utilizing stress measurements from equipment at the landslide sites and monitoring their movement speed. They compared this real-world data with friction experiments done in the lab, focusing on how friction within the landslides changed with movement. The findings showed a consistent relationship between field measurements and lab experiment results, offering a coherent understanding of how friction impacts landslide motion.
Finnegan highlights California’s famous Highway 1 as a key instance where the study’s insights could have practical benefits. “Caltrans faces an ongoing challenge to maintain access,” he noted. “The strength of this model lies in its potential to inform operational decisions. It doesn’t just isolate data points; it provides context that helps predict how changes in rainfall could affect ground movement.”
Importance of material properties
A vital aspect of the research assessed various rock types and their differing behaviors under stress. For instance, rocks rich in clay tend to deform slowly and steadily, while those containing more quartz might experience a rapid decline in friction leading to sudden failures. This knowledge could eventually empower scientists to forecast a landslide’s behavior based on the rock types present in a region.
Researchers collected field data from two Northern California sites. One is located just east of Fremont, which Finnegan has studied and monitored for eight years. The other site is in Humboldt County, much farther north, where scientists have made observations since the 1980s. Both locations are within the “Franciscan Melange,” a formation known for its susceptibility to slow-moving landslides, remnants of a prehistoric subduction zone where one tectonic plate slid beneath another, akin to ongoing activities in Northern California’s Cascadia region.
Saffer notes that a significant breakthrough occurred when they linked real-world observations from the two locations with data from lab-based rock deformation tests. They realized that viewing the landslide as a large-scale “experiment” revealed insights about the underlying physics of the materials involved.
“It’s essentially a massive rheology (rock deformation) experiment,” he explained. “This suggests that if we sample rocks in an area and conduct detailed rheological analysis in the lab, we could identify regions at higher risk for rapid, catastrophic landslides compared to areas likely to experience gradual land movements. That’s our next objective with this research.”
Unlocking tectonic insights
The study also has broader implications, particularly regarding plate tectonics and subduction zones. The rocks involved in the landslides studied were once located at the interface of an ancient subduction zone, typically associated with producing powerful magnitude-9 earthquakes—some of the most destructive natural disasters on the planet.
Researching slow landslides within these rock types could enhance our understanding of slip processes in subduction zones. Given the challenges of obtaining direct measurements from these deep-sea fault environments, investigations into landslides could shed light on the behavior of these plate interfaces under various conditions. Specifically, grasping slip behavior in seafloor fault zones may improve predictions related to earthquake-triggered tsunamis, assisting experts in understanding the timing and nature of these significant seismic events.
“Beyond the practical implications of this study, it exemplifies how interdisciplinary approaches can yield new insights into longstanding challenges,” Finnegan concluded. “In this instance, we demonstrate how studying landslides—where measurements are more feasible—can provide critical insights into processes occurring deep within faults, where direct observation is nearly impossible but crucial for understanding potential hazards.”
“`
