A landslide that dumped about 6 million cubic yards of rock and debris across California Highway 1 near near Big Sur, California, in May 2017 was the result of drought followed by deluge, a team of scientists say.

The Mud Creek landslide began as a “stable landslide,” or a landslide that slowly creeps over decades, even over the course of a century. On May 20, 2017, the stable landslide transformed into a "catastrophic collapse," causing damage to the Pacific Coast Highway that would cost the state $54 million and a year to repair, according to a new study published this month in Scientific Reports.

The collapse occurred after several days of heavy rain during one of the wettest years in a century for the area. That came in the wake of a five-year drought. The researchers determined that water replaces air in the tiny spaces between soil particles, which greatly increased the pressure on those particles, speeding up the rate of collapse.

"This pressure change could have destabilized the sliding surfaces below ground and triggered the collapse," the researchers said.

Using an eight-year data set from the Uninhabited Airborne Vehicle Synthetic Aperature Radar (UAVSAR), the team of scientists led by Nasa's Jet Propulsion Laboratory determined that the landslide was originally moving only about 7 inches per year since 2009, according to a press release.

Using the European Space Agency’s Sentinel-1A/B satellite, they were also able to document how the rate of the slide began to change.

"After the 2017 rains, the sliding slowed in February but began to accelerate again in March and continued until the May collapse," the press release notes.

Lead author Alexander Handwerger of JPL said the data collected by satellites and aircraft can only measure changes at ground surface so they turned to computer models to simulate how water affects soil.

“From that, we tried to infer what may have happened to the landslide’s sliding surface, tens of meters underground, that allowed the Mud Creek slide to transition from stable to unstable,” Handwerger said.

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The scientists say they will continue to monitor California's 650 known stable slides, with the hope that a better understanding can be gleaned.

“Since we now know that stable landslides in this region can fail catastrophically and we have good data coverage here, our plan is to monitor this whole stretch of the Pacific Coast Highway and look for these unusual velocity changes. If we get enough examples, we can start to actually figure out the mechanisms that are controlling this behavior,” Handwerger said.