On April 21, 2024, at around 5 in the morning, about 200 tons of rock and dirt crashed down from the cliffs in Del Mar, California, landing on the beach below. Nobody was hurt. That wasn't luck — it was science. Just days earlier, a tiny sensor buried in the ground had detected the cliff was starting to move, sending a real-time alert to researchers at Scripps Institution of Oceanography in San Diego.
Scientists there have spent four years studying whether sensors could give people enough warning to get out of the way before a landslide happens. Their new report, published in the Journal of Geophysical Research: Earth Surface, says the answer is yes — with hours to days of advance notice.
"Scientists have been trying to predict earthquakes for a very long time. This study provided a unique opportunity to leverage what we've learned with earthquakes and apply it to these coastal hazards," said Mark Zumberge, a geophysicist at Scripps who helped lead the research.
The scientists installed different types of sensors at three locations: Beacon's Beach in Encinitas, the Railway Corridor in Del Mar, and San Elijo State Beach. One key tool was a tiltmeter — an instrument usually used to measure movement along earthquake faults. These tiltmeters can detect ground tilting smaller than the width of a human hair, just 10 micrometers over one meter. That sounds tiny, but it's enough to show that a cliff is about to give way.
The research was partly inspired by tragedy. In August 2019, a 30-foot by 25-foot section of cliff collapsed at Grandview Beach in Encinitas, killing three women. That event prompted California Assemblymember Tasha Boerner to push for better cliff monitoring and funding for early warning systems.
"Bluff collapses are a constant threat to coastal neighborhoods in my district and across the California coast," said Boerner. "The science is clear: We can save lives, protect our coastal economy, and vital transportation networks in the face of sea-level rise with a bluff-collapse early warning notification system."
About 70 percent of California's coastline is made up of cliffs that are slowly eroding, or wearing away. These cliffs sit beneath important roads, train tracks, and neighborhoods — and as the ocean rises, the problem is only getting worse. The researchers say their work is a step toward a real early warning system that could protect people and property. More work is needed to turn the findings into something communities can actually act on, but the technology already exists.
For Zumberge, the study offered a chance to apply decades of earthquake research to a new kind of danger. "While others had reported signals that preceded landslides, we wondered what we would see in this environment," he said. Now they know: the ground tells you it's about to move — you just have to listen.