Why Are the California Storms Causing Sinkholes?

While they looked like craters caused by a celestial body, the fresh scars that pockmarked the waterlogged landscape of California this week instead sprung up from the earth.

One sinkhole, at least 50 feet wide and 30 feet deep, parted a street in the suburban Chatsworth neighborhood of Los Angeles and swallowed two cars along with concrete chunks on Monday; another one damaged 15 homes in Santa Barbara County; farther up the coast, a colossal chasm, caused by a collapsed culvert, forced the Oakland Zoo to close until next month.

Most sinkholes in the United States occur naturally, but they are not common in California because the state’s surface was not shaped by dissolving bedrock. Pictures of cars and homes falling into the ground across California are usually associated with earthquakes, not sinkholes. But the relentless parade of atmospheric rivers since December has changed the state’s geological imagery.

“With more heavy rain in the forecast, the erosion will continue and there are no doubt structures that are being impacted that will collapse,” said Randall C. Orndorff, a research geologist for the U.S. Geological Survey, in an interview on Friday.

The sinkholes were created when torrents of water rushed into man-made underground layers, like road drains, water mains, sewers and culverts that carry streams or open drains under roads. “When you get all that rain that fast, it doesn’t even have time to seep into the ground,” Mr. Orndorff said.

The overflow water churned and formed pockets of space, or found other voids to fill and widen. The collected water weakened the ground supporting the surface above, making it more prone to collapse. Above ground, the thoroughly drenched top layer also struggled under the extra water weight.

“We do believe that these events are becoming more frequent as the nation’s infrastructure is aging,” Mr. ​​Orndorff said.

Naturally occurring sinkholes, though, can appear just about anywhere. Unsurprisingly, they are most frequent in regions with rock that is soft and dissolves easily, like limestone. Sinkholes are also more common near salt beds, natural springs and caves.

Nearly 20 percent of the contiguous United States has this type of soluble bedrock, according to the Geological Survey. Sinkholes are most common in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. They can devour cars, homes, buildings and roads while causing millions of dollars in damage.

Sinkholes can also occur wherever there is heavy precipitation and are largely undetectable from the surface, but cracks in a home’s foundation or sagging parts of a yard may be warning signs, Mr. ​​Orndorff said.

“It’s very localized, he said, “and that’s what makes it difficult to predict.”