Roughly two-thirds of Marin County is highly susceptible to landslides, according to state officials who released a new statewide map Thursday showing areas where the earth might move in heavy rains or during a major earthquake.
"Areas of Marin were built on hillsides in the 1920s and 1930s before grading codes were enforced," said Chris Wills, California Geological Survey supervising geologist, who led work on the map project. "They were built in an era when landslides were not considered."
While much of the rock in Marin is moderate to strong, the steep slopes on which some homes are built in Sausalito, Mill Valley and other communities put them more at risk, Wills said.
"If you are buying or own a house in Marin, you should try to get more information about what the susceptibility to a landslide is," he said.
The new map is more general than specific, but it is intended to provide some idea to emergency planners, property owners and the public about potential threats of slides during heavy rainstorms or large earthquakes.
"Landslides are one of the bigger winter storm issues we face; they can cause a lot of damage," said Janelle Myhre, an emergency services coordinator with Marin County's Office of Emergency Services. "You also have to look at this from the perspective of a catastrophic earthquake. It's good to know where your vulnerabilities are."
On the new map, landslide susceptibility is ranked on a scale of zero to 10 based on a combination of strength of the rock and the degree of slope. The combination of greater slope and weaker rock produces the highest susceptibility. Much of Marin ranks high.
"There are other landslide susceptibility maps out there, but those are very general," said John Parrish, state geologist of California and head of the California Geological Survey. "This map has a very robust data set behind it. Although you cannot use it to determine the risk potential at a specific site, you can get a good sense of whether landslides are something you need to be aware of in a given area."
While Marin is among the most susceptible counties in the state, the new map shows risk just about everywhere in California.
"One glance at this map tells you the relative risk for the entire state," Wills said. "You don't need to be a scientist to understand that the brighter colors indicate a generally higher potential for landslides. Some areas with higher landslide potential are densely populated and many are crossed by roadways, pipelines, rail lines and the like."
About 57,000 deep landslides that already have occurred are shown on the map. Because they have slid once, the rock is loose and more susceptible to slide again.
"There are probably many thousands of additional landslides that are not yet mapped," Wills said. "They may not all impact population centers, but even in remote areas infrastructure is a concern."
A landslide is any mass of earth and rock that moves downhill by sliding, flowing or falling, according to the Department of Conservation. Large, slow-moving landslides composed of bedrock can cause extensive property damage but usually do not result in loss of life.
Debris flows, commonly called mudslides, are more dangerous types of landslides because they can move quickly before people have a chance to get out of harm's way. Mud, rock and debris caught by these rapid flows can travel from 10 mph to 100 mph.
The last fatal landslide in California occurred in Mill Valley. Walter Guthrie, 74, was killed when he went behind his house to clear rocks from a culvert at his home on 70 Bolsa Ave. in the early-morning hours of April 12, 2006. He was buried in an avalanche of mud unleashed by hours of torrential rain.
Sausalito city officials red-tagged a nine-unit complex at 545 and 555 Bridgeway after mud and earth seeped downhill toward the buildings and saturated earth gave way on a Bulkley Avenue hillside on March 29, 2006.
In February 1998, steady rains brought by an El Niño weather system caused several mudslides in Marin, resulting in $2.5 million in damage to property.
Those mudslides occurred in Mill Valley, Novato and Tomales. Two homes were "red-tagged," meaning the slides rendered them uninhabitable. Nine slides also damaged key roadways, such as Panoramic Highway and Dillon Beach Road.
The damage was far worse in January 1982 when Marin County was inundated by 12 inches of rain in 32 hours during one period in another El Niño event. Four people died, including residents in Tiburon, Sausalito and San Rafael whose homes collapsed in mudslides.
Mudslides blocked Highway 101 and other key thoroughfares, stranded thousands of commuters in San Francisco, snapped power and telephone lines and isolated Inverness and other neighborhoods for days. Damage totaled more than $80 million.
If the rains do come hard, areas of Mill Valley, Bolinas, Nicasio and San Geronimo Valley, the Ross Valley, Corte Madera, part of Novato and Lucas Valley could be at risk, officials said.