4.5 Geology and Soils

Environmental Impact Analysis Geology and Soils 4.5 Geology and Soils This section assesses potential impacts related to geologic and soil hazards. 4.5.1 Setting a. Topography and Geology San Leandro is located on the East Bay Plain (the Plain), a flat area that extends 50 miles from Richmond in the north to San Jose in the south. The Plain is about three miles wide in the San Leandro area. At its eastern edge, the plain transitions into low hills, rising to 526 feet at the highest point in the City’s Bay-O-Vista neighborhood. On its western edge, the Plain slopes down to San Francisco Bay, the largest estuary on the California Coast (City of San Leandro 2016i). San Leandro’s rich alluvial soils and temperate climate support a wide variety of plants and animals. Wetlands in the southwest part of San Leandro provide habitat for the salt marsh harvest mouse and other special status species. San Leandro Creek remains one of the few waterways in the urbanized East Bay that retains its natural character along most of its course (City of San Leandro 2016i). San Leandro is located within the United States Geological Survey’s (USGS) San Leandro and Hayward Quadrangle 7.5-minute topographic map areas. The area is typified by low topographic relief, with gentle slopes to the southwest in the direction of San Francisco Bay. By contrast, the San Leandro Hills that lie directly northeast of San Leandro have more pronounced topographic relief, with elevations that locally approach 1,000 feet above mean sea level (City of San Leandro 2016i). The geology in the vicinity of San Leandro has been mapped by a variety of organizations, including the USGS. In its 2000 geologic map and map database for the Oakland, California area, the shallow geology underlying much of San Leandro consists of Holocene alluvium with fluvial deposits frequently composed of medium dense to dense, gravelly sand or sandy gravel that often grade upward to sandy or silty clay. Close to the bay shoreline along the west edge of San Leandro, the shallow geology is dominated by artificial fill, and in places, Bay Mud. The bedrock geology beneath San Leandro is best expressed in the hills that flank the east side of San Leandro (i.e. the area between Interstate Highway 580 and Lake Chabot), that are directly underlain by highly altered Jurassic metamorphic and plutonic rocks. Some of these rocks include pillow basalts and keratophyres (a type of silica-rich volcanic rock) that have been mapped as members of the Coast Range Ophiolite complex, a rock assemblage that is widely believed to represent oceanic crustal material that was tectonically emplaced along the west margin of the North American (tectonic) Plate (City of San Leandro 2016i). Additionally, the Specific Plan Area is located in proximity to the San Andreas and Hayward fault zones, one of the most seismically active regions in the United States, however it is not located within the Alquist-Priolo fault zone. Specific Plan Area faults are discussed in greater detail below under part (d), Geologic Hazards, Figure 16, and Figure 17 show faults within the proximity of the Specific Plan Area to the Hayward Fault and the Alquist-Priolo fault zone, respectively. Draft Environmental Impact Report 149 City of San Leandro Bay Fair Transit Oriented Development (TOD) Specific Plan Figure 16 Regional Fault Map 150 Environmental Impact Analysis Geology and Soils Figure 17 Alquist-Priolo Fault Zone Map Draft Environmental Impact Report 151 City of San Leandro Bay Fair Transit Oriented Development (TOD) Specific Plan b. Soils The properties of soils city-wide are variable, ranging from poorly drained, plastic soils of the Clear Lake clay to the more permeable, well-drained soils of the Yolo silt loam. According to published test data, several soil types, notably the Clear Lake clay, are characterized by high shrink-swell potential, where alternating heavy and cracking in overlying foundations, utility lines, and/or flatwork (City of San Leandro 2016i). As mapped by the Natural Resource Conservation Service (NRCS), three soil types are located in the Specific Plan Area (USDA 2017). The Specific Plan Area is comprised primarily of drained Clear Lake clay 0 to 2 percent slopes, MLRA 14 (84.1 acres). The remainder of the Specific Plan Area is comprised of Botella loam 0 to 2 percent slopes, MLRA 14 (54.9 acres) and Danville silty clay loam 0 to 2 percent (15.8 acres). Specific Plan Area soils are shown in Figure 18 and soil characteristics for the Specific Plan Area soils related to water holding capacity, permeability, shrink-swell potential, rate of surface runoff, and erosion hazard are listed below in Table 9. Table 9 Specific Plan Area Soil Parameters Rate of Water Holding Permeability Shrink-Swell Surface Erosion Map Unit # Name Capacity (in.) (in/hr) Potential Runoff Hazard 106 Botella loam, 0 to 2 9-11 Moderately Moderate Slow Slight percent slopes slow 107 Clear Lake clay, 0 to 2 7-9.5 Slow High Slow None percent slopes, drained 111 Danville silty clay loam, 8.5-10.5 Slow Moderate Slow None 0 to 2 percent slopes to High Sources: U.S. Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS), Web Soil Survey of Alameda County, California, April 11, 2017; U.S. Department of Agriculture (USDA), Soil Conservation Service, Soil Survey of Alameda County, California, Western Part, March, 1981. c. Geologic Hazards Similar to much of California, the Specific Plan Area is located within a seismically active region. The seismic hazards relevant to the Specific Plan Area are described below. Faulting and Seismically Induced Ground Shaking The U.S. Geological Survey (USGS) defines active faults as those that have had surface displacement within Holocene time (about the last 11,000 years). Surface displacement can be recognized by the existence of cliffs in alluvium, terraces, offset stream courses, fault troughs and saddles, the alignment of depressions, sag ponds, and the existence of steep mountain fronts. Potentially active faults are faults that have had surface displacement during the last 1.6 million years. Inactive faults have not had surface displacement within the last 1.6 million years. Several faults are located in the vicinity of the Specific Plan Area (refer to Figure 17). These major faults and fault zones are described in the paragraphs below: San Andreas Fault The San Andreas Fault, which is the most likely source of a major earthquake in California, is located approximately 15 miles west of San Leandro. The San Andreas Fault is the primary surface boundary between the Pacific and the North American plates. There have been numerous historic 152 Environmental Impact Analysis Geology and Soils Figure 18 Specific Plan Area Soils Map Draft Environmental Impact Report 153 City of San Leandro Bay Fair Transit Oriented Development (TOD) Specific Plan earthquakes along the San Andreas Fault, and it generally poses the greatest earthquake risk to California. In general, the San Andreas Fault is likely capable of producing a Maximum Credible Earthquake (MCE) of 8.0. Hayward Fault The Hayward Fault, one of ten major faults that comprise the San Andreas Fault Zone, runs northeast of the Specific Plan Area and links with the Rodgers Creek Fault to the north. Although the last major earthquake generated by the Hayward Fault was in 1868, pressure is slowly building again and will begin to overcome the friction and other forces that are causing the fault zone to stick. According to a 2008 study of earthquake probabilities by the U.S. Geological Survey and other partners, the fault system that includes the Hayward and Rodgers Creek faults has a 31 percent probability of generating an earthquake with a magnitude greater than or equal to 6.7 on the Mercalli Richter Scale in the next 30 years (City of San Leandro 2016f). The Hayward Fault would likely cause extensive damage throughout the Specific Plan Area due to its close proximity to urban communities and infrastructure. The Hayward Fault and surrounding area is a designated Alquist- Priolo Zone, which lies just northeast of the Specific Plan Area (see Figure 17). Other active faults within the Specific Plan Area vicinity include the Chabot, Calaveras, Greenville, and Las Positas faults, as well as several potentially active faults and unnamed secondary faults adjacent to these faults. There are few or no studies pertaining to these additional secondary faults; therefore it is unknown if these faults may or may not experience secondary ground rupture during a large earthquake. In addition to the primary hazard of surface rupture, earthquakes often result in secondary hazards that may cause widespread damage. The three most likely secondary earthquake hazards in the Specific Plan Area are ground shaking, liquefaction and ground failure (City of San Leandro 2016f). Surface Rupture Faults generally produce damage in two ways: ground shaking and surface rupture. Surface rupture is limited to very near the fault. As discussed above, the Hayward Fault runs northeast of the Specific Plan Area. Since the fault zone is outside the Specific Plan Area, surface rupture in the Specific Plan Area is not expected to occur (see Figure 17). Ground Shaking Seismically induced ground shaking covers a wide area and is greatly influenced by the distance of the site to the seismic source, soil conditions, and depth to groundwater. The USGS and ABAG have worked together to map the likely intensity of ground-shaking throughout the Bay Area under various earthquake scenarios. The most intense ground-shaking scenario mapped in the Specific Plan Area assumes a 6.9 magnitude earthquake on the Hayward Fault system. The predicted ground-shaking from such an earthquake would be “very violent” or “violent” throughout the Specific Plan Area (ABAG 2016a).

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