Sonoma, Marin Counties considered at risk for landslides

The locations include northern parts of Sonoma County, parts of Marin County, sections of the East Bay hills, and the Santa Cruz Mountains, according to the map.

The map shows the likelihood of landslides based on geologic conditions, such as rock strength and slope steepness, said Chris Wills, supervising engineering geologist for the California Geological Survey.

Rock strength is divided in three different categories: weak, moderate, and strong. Slope steepness is classified into eight different levels ranging from low to high.

Weak rocks on steep slopes are the most prone to landslides, Wills said.

High slopes that have moderate to strong rocks are also vulnerable, which is why more than two-thirds of Marin County is considered to be highly susceptible to landslides, he said.

The map was created using digital geologic maps that describe rock strength and digital maps that show areas previously affected by landslides, Wills said.

Locations previously affected by landslides have a greater chance to be affected again since the rocks have been destabilized, he said.

Source maps date back to the 1970s, but these maps haven't been digitized, making it time consuming to pull the information, Wills said.

Goals for the next edition of the landslide susceptibility map include adding information from these paper source maps and adding more details, he said.

"Only three categories of rock strength is pretty broad," Wills said.

The landslide susceptibility map is designed primarily for emergency planners and owners and operators of roads, pipelines, and utilities to assess the level of hazard for certain areas, he said.

"If there's a big storm next week with a number of inches of rainfall, you can say there might be a landslide in these areas," Wills said.

The map is broken down by county and provides details on a census-tracked level. Sometimes the focus covers several areas in a city, and sometimes the spot is smaller than a zip code, Wills said.

The map can also be useful for people looking to purchase a home, he said.

Although the information is not detailed enough to judge a specific property, it can hint to areas where contracting an independent assessment would be important, Wills said.

Ideally, the map will lead to more accurate loss estimates and will show a community's vulnerability, he said.

In the East Bay hills, Marin County and down the Peninsula, development dates back to the 1920s before landslide studies were considered, Wills said.

"We can start looking at the damage potential for each of these areas," he said.

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