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5.5 Landslides The CGS maps Earthquake Induced Landslide Study Zones. The map designates zones in which a landslide study is required before the land can be developed, similar to CGS’s Liquefaction Hazard Zone of Required Investigation. The CGS has only mapped portions of Alameda, San Francisco, and Santa Clara Counties. Portions of San Mateo and Contra Costa counties are currently being mapped, but may not be completed until 2016.17 This CGS map only depicts earthquake induced landslide zones, not areas at risk of landslide from storm events. Winter rain storms can impact hillsides by triggering fast-moving debris flows, or mudslides, and other slower-moving landslides. In general, landslides are most likely during periods of higher than average rainfall or El Nino winter storms. In addition, the ground must be saturated prior to the onset of a major storm for significant landsliding to occur. But there is currently no method to estimate the scale of individual landslides in terms of size or extent based on these maps, or to assign specific probabilities to these areas in terms of the likelihood of future landslides. The map shows areas where rainfall- induced landslides have occurred in the past, as landslides are most likely to occur in and around areas where they have previously occurred.18 5.5.1 Historic Bay Area Landslide Occurrences Flooding and landslides associated with severe storms have been among the most common disasters in the Bay Area during the period from 1950 to 2009. Extensive landslides have occurred in 24 times since 1950, approximately once every three years.19 Losses from landslides are typically lower than those from associated flooding. However, in the El Nino storms of early 1998, USGS documented approximately $150 million in losses due to approximately 300 landslides of varying sizes that occurred in the Bay Area and Santa Cruz County.20 The greatest number of landslides in the region since 1950 occurred in 1982, when a large storm event preceded by a wet winter triggered over 18,000 landslides in the region, which resulted in 33 deaths and 481 injuries.21 17 Tim McCrink, CGS, Personal communication, April 3, 2015 18 San Francisco Bay Landslide Mapping Team, (1997) 19 State of California Multi-Hazard Mitigation Plan, Appendix M, California Governor’s Office of Emergency Services, 20 Godt, J.W., ed., (1999) 21 Ellen, S.D., and Wieczorek, G.F., eds., (1998)