California Geological Survey Releases New Landslide Inventory Maps for Alameda & Santa Clara Counties

ARCHIVE NR# 2011-09
June 28, 2011

SACRAMENTO – The California Geological Survey (CGS) today released its latest landslide inventory maps to enhance public safety and guide development in Alameda and Santa Clara counties.

“Most people in the greater Bay Area are aware of how dangerous landslides can be, but these maps are a reminder,” said State Geologist Dr. John Parrish, head of CGS. “They offer local experts an extra layer of information to use in the land development process, helping to ensure landslide hazards are taken into account.”

The five new maps cover 60 square mile areas known as “quadrangles” that include portions of the cities of Fremont and Newark in Alameda County, as well as parts of Milpitas, San Jose and Los Gatos in Santa Clara County.

Inventory maps indicate whether a landslide is considered active or dormant, the direction of movement, and the type of movement involved (some landslides are more destructive than others). CGS, a branch of the California Department of Conservation, produces the maps by incorporating previous mapping work with a detailed review of aerial photography and geologic fieldwork. Maps released today show 613 landslides in the Calaveras Reservoir Quadrangle, 85 in the Milpitas, 177 in the Niles, 277 in the San Jose East, and 15 in the San Jose West.

CGS has created several varieties of landslide maps over the years – the most recent one showing landslide susceptibility around the state -- but the inventory series is the most detailed to date. While the new landslide inventory maps are non-regulatory, they provide key details to geotechnical professionals and local government personnel, as well as property owners and developers dealing with properties in Zones of Required Investigations for earthquake-induced landslides.

“These new maps are comprehensive overviews of the areas they cover at a user-friendly scale – one inch equals 2,000 feet,” said CGS Supervising Geologist Chuck Real, who oversees the mapping program. “In making development plans and land-use decisions, communities need to know where the landslides are, how they move, and how recently they’ve moved. That’s what these maps show.

“While the potential for landslides sometimes can be mitigated, many times the best thing to do is to avoid building on or near them.”

The maps are available as a free download on the Web. Paper maps can be purchased at the CGS offices in Menlo Park (345 Middlefield Road) or Sacramento (801 K Street), or ordered by phone (650) 688-6327 or (916) 445-5716. The paper maps are $12; shipping and handling is $6.

A landslide is any mass of earth or rock that slides, flows and/or falls downhill. Landslides can affect land from a few square yards to hundreds of acres in area and can be a few feet to hundreds of feet thick. Many factors can contribute to the formation of landslides aside from rainfall, including improper construction or grading, earthquakes, weak or loose rock and soil, and steep slopes.

Large, slow-moving landslides composed of bedrock can cause extensive property damage but usually do not result in loss of life. A debris flow, commonly called a mudslide, is a more dangerous type of slope failure because it is fast moving and can cause not only property damage, but also injuries.

“Last winter, we saw several damaging landslides in the Bay Area and all around the state,” Parrish said. “That was a reminder of a saying that’s old but still very much valid: Civilizations exist by geologic consent.”

Property owners are advised to consult a licensed engineering geologist or geotechnical engineer before taking any steps intended to mitigate potential risks or harm associated with landslides.

CGS provides technical information and advice about landslides, erosion, sedimentation, and other geologic hazards to the public, local governments, and agencies and industries that make land-use decisions in California.

In addition to studying and mapping geologic phenomena, the California Department of Conservation categorizes mineral resources; administers agricultural and open-space land conservation programs, ensures the reclamation of land used for mining; and regulates oil, gas and geothermal wells.