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others, and the geologic characteristics of a specific area will result in different responses to seismic activity. Two non-earthquake related geologic hazards exist in the Planning Area: landslides (not related to seismic shaking) and expansive soils. T ~nc)",11clp,::: Landsliding is a natural process of relatively rapid downslope movement of soil, rock and rock debris as a mass. The rate of landsliding is affected by the type and extent of vegetation, the slope angle, the degree of water saturation, the strength of the rocks, and the mass and thickness of the deposit. Some of the natural causes of this instability are earthquakes, weak materials, stream and coastal erosion, and heavy rainfall. In addition, certain human activities tend to make the earth materials less stable and increase the chances of ground failure. Activities contributing to instability include extensive irrigation, poor drainage or groundwater withdrawal, removal of stabilizing vegetation and over steepening of slopes by undercutting them or overloading them with artificial fill. Landslides can involve both the simple sloughing downslope of a small-sized surficial landmass for a distance of several inches or massive earth movements of many feet. Landslides may occur at any time but are more likely during the rainy season, when soils are saturated with water and their cohesive abilities are more likely to break down. In the Pleasanton Planning Area, about 8,000 acres are in areas of mosdy Iandslide.2 Figure V-1 shows areas with mosdy landslide, few landslide, or no landslide deposits. The areas of no landslide deposits are mainly level and generally do not contain past or current landslide debris. FYr~n~i1:TP Snl1.::: Expansive soils are surface deposits rich in clays that expand when wet and shrink when dried. Expansive soils are potentially present at or near the surface in areas in northern Pleasanton and along the northeastern flank of Pleasanton Ridge. A moderate potential exists for their presence throughout the remainder of the Planning Area, and site-specific studies are required to determine their actual presence at a given location. Seismic/Earthquake Setting As is the case for most of California, people and property in Pleasanton are subject to risks from seismic aCtiVIty. According to the U.S. Geological Survey (USGS) Workgroup on California Earthquake Probabilities, there is a 62 percent probability of at least one magnitude 6.7 or greater Green Valley, Green Valley, Greenville, Hayward, Mt. Diablo Thrust, and San Andreas Faults. Figure V -2 shows active earthquake faults in the Bay Area and earthquake magnitudes. 2 ABAG,2002 Public Safety 082906, dean V-2 City Council 8/29/2006