Residents of Peartree Lane in Rolling Hills Estates had just minutes to evacuate last summer before the canyon wall on which their homes sat collapsed.
The world He soon saw the red-clad houses slide rapidly down the hill, cracking, crumbling and falling to pieces.
The landslide came as a shock on a dry afternoon, but new research has shown the slope was actually a “slow, creeping slide” with unpredictable movement that began five months before the tragedy on July 8, 2023.
By analyzing satellite imagery and data to measure subtle changes in the ground before that fateful day, the team of researchers was able to measure the speed of the slope and then use this metric to calculate – with an obvious gift of feedback, but a high degree of accuracy – how steep the hill was due to destruction.
“Our results show the time at which catastrophic failure occurred (the Rolling Hills Estates hillside) could be calculated to be within a few days of the actual failure,” the researchers wrote in the paper. published this month in Geophysical Research Letters. “Our findings provide further evidence that satellite data can contribute to landslide early warning systems and can also be used to better understand the impact of climate change on landslide risk.”
An animation showing ground movement on Perry Lane in Rolling Hills Estates prior to the July 2023 landslide.
(Xian Li)
Experts say more research is needed to use this satellite monitoring to detect large-scale landslides, but that is the ultimate goal. With more testing and technology (and the necessary buy-in from government officials), researchers hope the method could provide early warning to prevent or at least reduce damage from another similar disaster.
“If we can capture these motions to predict the potential for catastrophic failure, we will greatly improve risk management and prediction,” said Xiang Li, a postdoctoral researcher at UCLA. Joint Institute for Regional Earth Systems Science and Engineering and lead author of the study.
Analysis of satellite images and data from January 2016 to July 2023, using a technique known as synthetic interferometric radar or InSAR – Researchers at UCLA Jet Laboratory and NASA have found measurable movement in Rolling Hills Estates since February 2023. In June, they found the area had moved about 1.6 inches, before dropping nearly 40 feet between July 8 and July 14. At least eight homes were destroyed and four others were left unsafe to enter.
Using movement data from the months leading up to the landslide, the team was able to model possible future movements. Their model predicted that the slope would fail on July 11, three days after the landslide erupted.
“Here we saw that there are these subtle motions that can provide information about expected or future collapse, which is really important,” said Alexander Handwerger, a research scientist at JPL and the Joint Earth System Science and Engineering Institute at UCLA and an author on the paper. The findings could help inform future early warning systems and landslide monitoring, he said, while working with a JPL team. database creation with satellite-ready data for the nation.
Hanwerger noted that the study found no significant movement where the Rolling Hills Estates slide occurred through 2023, separating the slide from other active slides on the Palos Verdes Peninsula, particularly the Portuguese Bend landslide complex, one of the largest and slowest-moving landslides in the country. That landslide zone has seen a rapid and devastating acceleration in recent years after two winters of excessive rainfall.
And while the composition of the Rolling Hills Estates slide is different than others in the area, its movement is also due to heavy winter rains that fell in the months leading up to the collapse, according to a city report. The UCLA and JPL study agreed that an unusually rainy season “played a major factor” in the landslide, but also noted that infrastructure, drainage changes and other human interventions could have affected the ground movement.
The study also explained the significant delay between rainfall, the initial movement of the landslide and its final failure: this is due to instability when water enters the soil and creates an internal surface over which the top layer of earth can slide.
“Formation of a slip surface causes some movement, whereas rupture occurs only when the slip surface is fully developed,” said UCLA’s Lee. “Progress can occur over hours, months or years.”
The study’s findings are worrying for residents of Pertree Lane, many of whom are unable to re-enter their homes after a year, even if they have not been destroyed, due to instability or sewage problems. But researchers hope the work could inform early warning and landslide prediction systems that could help limit similar disasters in the future.
Getting to this point requires further research, as satellite analysis is only reliable in a few scenarios – most recently in Rolling Hills Estates.
“It’s questionable whether it can be applied broadly,” Lee said. There are questions about how the technique will work in different environments around the world, on densely forested hillsides (which makes satellite images more accurate) and on slides that have no forward motion.
Geologist El Hachemy Bouali, who was not involved in the study, said the study only used ground motion to predict the timing of the rupture, so a more refined formula that takes into account factors such as geology, slope angle and recent climate may be needed. More reliable, something that could be introduced in the future by artificial intelligence.
Still, the study’s results are promising, Buali said. An assistant professor of geology at Nevada State University who has long studied landslides on the Palos Verdes Peninsula, he has no doubt that the science and technology for a reliable landslide early warning system will continue, but he is concerned about how feasible such a system might be.
“The biggest obstacle to an early warning system is the cost and installation of all the infrastructure required,” Buali said. “In Los Angeles County alone, you’ll likely have an inventory of tens of thousands of individual tracks.”
But the work is especially urgent because the UCLA and JPL researchers note that climate change is expected to cause more frequent and extreme “weather events” — severe droughts followed by heavy rains — that make landslides more severe. The study said dry stretches can open cracks to allow water to penetrate deeper, which can lead to greater changes in groundwater pressure during the rainy season.
After another winter of heavy rain, neighbors around the landslide in Rolling Hills Estates are feeling nervous and worried about increased traffic as local officials work to implement additional resiliency measures. Although the UCLA and JPL researchers only looked back in time, Li said he hopes they can expand their analysis in the future.
Two separate groups of Peartree Lane residents filed lawsuits in Los Angeles County Superior Court, alleging that their neighborhood associations failed to maintain the slope their homes were on before the landslide. One of the cases claims that residents noticed some cracks in early July 2023, but HOA representatives did not respond to their concerns.
Attorneys for the homeowners association denied the charges against them in one of the cases, court records show.
Both lawsuits are ongoing. All of the lawyers involved either declined or did not respond to The Times’ requests for comment.
