Landslide danger in Bay Area high, new map reveals
Published 4:00 am, Thursday, May 19, 2011
Communities in the mountainous areas of the Bay Area, including much of Marin County, are in more danger than other places in the state of being leveled by a catastrophic landslide, the California Geological Survey revealed Thursday.
The map of California's most landslide-prone areas is an attempt by the Geological Survey to determine the most vulnerable spots in the event of an epic superstorm, known as an ARk storm, which scientists warned about earlier this year.
"The goal in this was to develop a scenario for a major storm," said Chris Wills, the Geological Survey's supervising geologist. "We felt it was important to get this basic information out so people can look at what are the most susceptible areas."
The map is the most detailed analysis of landslides that has ever been compiled in California. It shows that the North Coast between Sonoma County and Oregon and the Coast Range between San Francisco and Los Angeles are the most susceptible areas in the state. About two-thirds of Marin and Sonoma counties are categorized as high-hazard areas. The Santa Cruz Mountains and the East Bay hills are also highly susceptible to landslides, according to the map.
Mendocino, Humboldt, Santa Barbara and Ventura counties also have large swaths of land that could be termed high-hazard zones.
The map analyzed the rock, soil and steepness of the terrain in the locations of 57,000 historic landslides that had been compiled in a database. The steepest areas with the most crumbly rock and loosest soil got the worst ratings, Wills said.
"The hills all the way around the bay have significant landslide potential," Wills said. "There are a number of areas in the East Bay and Marin County and down on the Peninsula where there are communities dating back to the 1920s. These are fairly high-population areas with older homes built at a time when landslides weren't recognized and considered in development."
Landslides kill from 25 to 50 people and cause more than $2 billion in damage in the United States every year. More than 100 Californians have been killed by debris flows during the past 25 years.
The last fatal landslide in California occurred in Mill Valley in 2006 when a fast-moving wall of mud buried a 76-year-old landscape architect behind his home. Ten people were killed, 14 were injured and 31 homes were destroyed by a 30-foot wall of mud in Ventura County in 2005.
"We've seen the kind of tragedy that landslides can produce not just in Marin but throughout the state," said Assemblyman Jared Huffman, D-San Rafael. "We know that we are going to have a lot more weird weather in the years ahead, so the more information we have the better we will be in putting together emergency response plans and keeping people out of harm's way."
The idea of the map is to help prepare for the kind of catastrophic storm that last hit California 150 years ago, flooding the Central Valley. A scientific model released earlier this year estimated that such a storm now would cause $300 billion in property damage.
Geologists have dubbed these events ARk, or Atmospheric River storms, but to most people the name refers to the kind of gully washer that prompted Noah to build his ark. Property damage and death from landslides generally go way up in years when the state's rainfall totals are abnormally high.
"Recent events in Japan have made the public exceptionally aware of earthquakes and tsunamis, but landslides are a more common hazard in California," said Derek Chernow, acting director of the California Department of Conservation. "People associate landslides with rain - indeed, this map was created for a storm scenario - but they can happen anytime. Fortunately, they often give some warning to those who familiarize themselves with the signs."
Wills said future studies will analyze events that trigger landslides and how often they occur.
"Landslides are just a natural part of the landscape, but they get to be a problem when people build around them," Wills said. "Now that we have the susceptibility map, we can take any storm or any storm season and see what the likelihood of a major landslide is."