Calif. Bay Area Faces Landslide Hazards

Although well aware of the region’s earthquake threat, many San Francisco Bay Area residents are uninformed about another dangerous geologic hazard: landslides triggered by heavy rainfall. According to the U.S. Geological Survey, the combination of steep slopes, weak rocks, and intense winter rainstorms make the Bay Area uplands an ideal setting for landslides.

USGS recently released a documentary film “Riding the Storm,” produced by former USGS Geologist Karen Adams, telling the dramatic stories of some of the region’s most significant landslide events and explores the science behind the hazard with USGS researchers Raymond Wilson and Ray Wells.

In January 1982 a single, catastrophic rainstorm triggered 18,000 landslides throughout the San Francisco Bay Area. The most destructive of these landslides was in the Love Creek area of the Santa Cruz Mountains, where a 1000-foot slab of heavily wooded hillslope crashed down without warning on sleeping residents of Love Creek Heights. Ten of the Love Creek residents were buried by the slide, USGS said.

During the drenching winter of 1997-1998, the strongest El Niño of the 20th century triggered a range of landslides in the Bay Area from deadly debris flows to destructive deep-seated slides. One of the El Niño-driven slides underlies an entire neighborhood in the La Honda area and destroyed 8 homes by the end of 1998. The slide reactivated in 2005 and is still on the move, displacing a county road and threatening two more homes, USGS indicated.

While earthquake prediction is still a long way off, rainfall conditions necessary to trigger landslides can be forecast and monitored with modern weather technology. In the Bay Area, landslides are triggered when hillslopes become saturated with rainwater. Therefore, anything a resident can do to keep a hillslope well drained – gutters, storm drains, and the like – will help protect against landslides. Maps of areas most susceptible to landslides in each of the Bay Area counties can be obtained from the USGS. If you live in a landslide-prone area, consider evacuating for the few hours that a storm is at its height.

For information on the film and landslide hazards in the San Francisco Bay Area, visit www.landslides.usgs.gov or call the National Landslide Information Center at 800-654-4966.

Source: USGS