West LA Times

Landslides triggered by intense rainfall can sometimes be predicted along with incoming storms, but dry-season landslides often take people by surprise. The July 2023 Rolling Hills Estates landslide that destroyed 12 homes seemed to come out of nowhere, but new research shows it began as early as December 2022. Researchers are developing a database that will enable scientists to plug in new data to monitor potential landslides in real time and possibly predict them.

Californians are familiar with landslides that occur around storms when saturated soil and rock lose their grip and slip from their perch on the substrate. These types of landslides can be triggered by intense rainfall, and incoming storms can be a warning that neighborhoods need to evacuate. Landslides that happen during the hot, dry summers, though, tend to take people by surprise. In July 2023, for example, a landslide seemed to come out of nowhere to devastate a neighborhood in Rolling Hills Estates, located on the northern side of the Palos Verdes Peninsula in Los Angeles County.

Now, landslide researchers at UCLA and NASA’s Jet Propulsion Laboratory (JPL) have published a paper in Geophysical Research Letters showing that the 2023 Rolling Hills Estates event was a slow-moving progressive landslide that began the winter before when unusually heavy rainfall infiltrated into the slope and reduced its strength. The researchers used satellite data to measure minute shifts in the surface of the affected area before, during, and after the slide and concluded that this method could be used to detect future landslides before they become catastrophic.

“Movement on the Palos Verdes Peninsula’s Portuguese Bend Landslide has been recorded since the late 1950s,” said paper co-author Alexander Handwerger, a research scientist at UCLA’s Joint Institute for Regional Earth System Science & Engineering and JPL. “But there was no discernible movement in this region of the nearby Rolling Hills Estates before 2023. People began reporting movement, as indicated by cracks in houses, in April 2023, which matches our observations. There was initial slow movement that accelerated progressively, culminating in complete collapse several months later.”

The study led by UCLA postdoctoral researcher Xiang Li used satellite radar and optical data taken over Los Angeles every few weeks to measure ground motion over time. The satellite radar data for Rolling Hills Estates from 2016 to July 2023 revealed that after very slight movement during the 2019 rainy season, the ground remained stable until heavy winter rainfall starting in December 2022 kickstarted movement in February. By June, the area had moved 0.04 meters or about 1.6 inches; on July 8 — a sunny dry day preceded by 40 dry days — around ten meters or thirty-three feet of horizontal motion occurred destroying twelve homes.

The likely reason for the delay between initial movement in February and complete failure in July is that it took time for increased instability to develop. The researchers hypothesize that as water seeped through the ground, a sliding surface formed causing the landslide body including the ground surface to slide progressively until rapidly moving all at once.

“Formation of the sliding surface will induce some movement while collapse will only occur when fully developed,” Li said. “The progression can happen over hours months or years.”

The researchers then attempted determining if predicting such events were possible computing displacement over time arriving at predicted failure date three days post actual event note encouraging results needing further refinement lacking historical satellite areas not predictable via current methods.

Li said one challenge forecasting periods requires continuous historical ongoing measurements.

Handwerger core member project JPL building analysis-ready database U.S., territories Canada within border mainland countries southern border Panama project called Observational Products End-Users Remote Sensing Analysis OPERA containing near real-time monitoring prediction capabilities.

“These motions can be quite subtle before they begin fast,” Li said.“Cracks structures people notice first local residents reported April signs active require caution monitoring signal progressive future.”

###