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5.5.2 Probability of Future Landslide – Climate Influenced As described above, landslides are typically triggered by earthquakes or prolonged severe wet seasons. Climate change is not expected to change the seismic risk, but climate change could change the behavior of winter storms. The regional models project fairly similar precipitation totals in the Bay Area, but the variability season to season may increase. If winters are compressed, with more rain falling in fewer months, or if individual years are more extreme the chance of rainfall-induced landslide will increase. Additionally, if fires burn greater portions of landslide- vulnerable hillsides, removing vegetation and increasing storm runoff, the landslide probability will increase. The increase in future fire risk in the more mountainous regions of the Bay Area is described in Section 0. Currently, there is not enough evidence to suggest with certainty that future landslide probabilities will increase across the region, however local studies that take local conditions into consideration may reveal the potential for greater landslide risks in the future. 5.5.3 Landslide Hazard in the Bay Area 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 2016.22 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 USGS developed a region-wide rainfall-induced landslide hazard map. 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.23 22 Tim McCrink, CGS, Personal communication, April 3, 2015 23 San Francisco Bay Landslide Mapping Team, (1997)