"We've had that kind of season," said Richard Pike, a research geologist with the U.S. Geological Survey in Menlo Park.
"This was simply a wet year."
A lot of rain soaking deep into bedrock that in California is relatively porous will mean mudslides when there isn't a cloud in the sky, said Pike
> Mudslides had closed six major roads in the state on Monday.
, who has published articles about California landslides, said there are two kinds in this state: those that slide suddenly during a pounding storm and the kind that are happening now.
A lot of rain over a longer period of weeks, not hours, soaks into the soil, he
As the rainy season expires, water is still sinking into the bedrock.
"Sometimes it takes a long time to get down there," he
Unlike other parts of the country, local bedrock is not as solid or rocky, which makes it more vulnerable to water and slipping, Pike
"It can be moving even when the sun comes out because it takes a long time to move," he
After an El Niño winter, a mile-long mudslide in 1998 slipped in an undeveloped part of Fremont.Called the Mission Peak Landslide, it was one of the Bay Area's biggest.
"Those are the kind to expect this year," Pike
They tend to move more slowly than the sudden, violent rush of mud off steep slopes where the top layer of soil is rain-soaked, he
The rain creates a slurry that runs downhill rapidly and can kill because you can't outrun those, Pike
After torrential rain in January 2005, the hillside over the Ventura County seaside village of La Conchita plunged into the settlement, killing 10 people caught at home.
California is particularly bedeviled by mudslides because of the climate, Pike
All the rain comes in one season.
But every state has landslide problems of some kind, such as in Ohio, where clay soil slides into rivers, he
Besides the rain, cutting into the land is a major factor in landslides, Pike
"Whether you cut into the landscape to build highways or roads, that's really destabilizing the terrain," he
"You change the slope and you're messing with Mother Nature and she
doesn't like it."
As the demand to build spreads into California hillsides, more people and property are living with risk, Pike
Northern California went through a similarly wet winter in 1906, he
When the San Francisco earthquake struck in April that year, dampened earth slid - just as it is now, he
"There just weren't as many people and structures," he
The 1989 Loma Prieta earthquake couldn't have hit at a better time, at least for mudslides, he
said: It was a dry year and it happened in the fall, the driest time of year.
"So if we have an earthquake this year," he
said, "that could be really bad."