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City of San Leandro Alvarado Commerce Center Project 34 Existing Setting Geology San Leandro is located in the United States Geological Survey’s (USGS) San Leandro and Hayward Quadrangle 7.5-minute topographic map areas (USGS 1993, 2012). 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 the city have more pronounced relief with elevations that approach 1,000 feet above mean sea level. The shallow geology underlying some of San Leandro consists of Holocene alluvium with fluvial deposits associated with distributary streams such as San Leandro and San Lorenzo Creeks (USGS 2000). These sediments are frequently composed of medium dense to dense, gravelly sand or sandy gravel that often grade upward to sandy or silty clay. Soils The soils in San Leandro are dominated by very deep, poorly drained, fine-grained soils such as clays and silty clay loams, with lesser areas of deep, well-drained silty loam in the northeast part of the city and very deep, very poorly drained clays in the tidelands that flank the west edge of San Leandro near San Francisco Bay. The soils beneath the project site are identified as Clear Lake clay (drained) with slopes ranging from 0 to 2 percent (San Leandro 2016f). Earthquakes Earthquakes are the most pervasive geologic safety hazard in San Leandro. The eastern portion of the city is crossed by the Hayward fault, which has created serious and widespread damage in the city in the past. The major earthquake hazards in San Leandro are ground shaking, ground failure, and liquefaction. These hazards tend to be amplified on artificial fill and deep alluvial soils (San Leandro 2016f). A 2008 study of earthquake probabilities by the USGS estimated that there is a 63 percent chance that a magnitude 6.7 of greater earthquake will strike the Bay Area in the next 30 years. A major earthquake could occur on the Hayward Fault, as well as the San Andreas Fault located 15 miles west of San Leandro. An earthquake of this magnitude could topple buildings, disrupt infrastructure, impact transportation systems, and trigger landslides throughout San Leandro Hills (San Leandro 2016f). Liquefaction Liquefaction is a phenomenon where loose, saturated, non-cohesive soils such as silts, sands, and gravels undergo a sudden loss of strength during earthquake shaking. Under certain circumstances, seismic ground shaking can temporarily transform an otherwise solid, granular material to a fluid state. Liquefaction is a serious hazard because buildings in areas that experience liquefaction may suddenly subside and suffer major structural damage. Liquefaction is most often triggered by seismic shaking, but it can also be caused by improper grading, landslides, or other factors. In dry soils, seismic shaking may cause soil to consolidate rather than flow, a process known as densification (San Leandro 2016f). 331