LOS ANGELES — Federal geologists blame the latest collapse of Highway 1 in Big Sur on a relatively common rockfall caused by weather, waves and gravity that eventually cracked the steep cliffside beneath the roadway, according to a new analysis by the U.S. Geological Survey.

The good news is that researchers didn't see a larger landslide at work, which would suggest greater instability in the surrounding area. The bad news is that it's an ongoing challenge to predict where and when another rockfall could happen along Highway 1 — the stretch of highway that the USGS considers most vulnerable to coastal erosion in California.

The March 30 slip-out that occurred just south of Rocky Creek Bridge — where a chunk of the southbound lane fell into the ocean during a rainy weekend — left much of the famous Big Sur coast cut off from the rest of the state, with only limited convoys allowed through the damaged stretch. A separate stretch of Highway 1 — about 40 miles to the south — has been closed since January 2023, pounded by a string of landslides, including one this winter.

"Could I have told you it was (going to be) Rocky Creek next? No," said Jonathan Warrick, a USGS research geologist at the Pacific Coastal and Marine Science Center in Santa Cruz. "We're seeing a lot of activity, there's just a lot of movement out there. ... In general, what we tend to see along the Big Sur coast, the wetter the winter, the more of these rockfalls and landslides occur."

California Department of Transportation officials recently announced the Rocky Creek Bridge section should reopen with unrestricted access by Memorial Day. CalTrans officials are working to reopen just the northbound lane, which was not damaged during the initial slip-out, for alternating one-lane traffic. Crews have been working since the collapse to improve stability, safety features and drainage in the area.

Memorial Day "will be a big milestone because it allows the 24/7 passage without restriction" on the single lane, said Kevin Drabinski, a CalTrans spokesperson. "We'll be able to open [the lane] up once the signal is installed."

Traffic across that section has remained severely limited, with twice-a-day convoys allowing locals and essential workers through — and the convoys have been repeatedly canceled due to weather concerns.

"We know how important Highway 1 is to the regional economy, especially during the summer, so we are working to reopen the roadway as quickly and safely as possible while at the same time making it more resilient to future extreme weather events," Caltrans Director Tony Tavares said in a statement.

Unlike what geologists found and continue to monitor in several other damaged sections of Highway 1, imaging from the Rocky Creek slip-out area did not reveal other nearby land movement, according to preliminary images and analysis published by the USGS this weekend. The new analysis compared aerial images taken April 6 — a few days after the recent damage — and Feb. 23.

"There wasn't any obvious trigger there," Warrick said. "It doesn't have characteristics of a deeper-seated landslide, as far as we can tell yet. ... That's kind of a sigh of relief."

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