5A Public Hearing 2019 0204 - Laserfiche WebLink

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3.0 ENVIRONMENTAL CHECKLIST City of San Leandro 1388 Bancroft Avenue Project November 2018 Initial Study/Mitigated Negative Declaration 3.0-31 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 (San Leandro 2016b). 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 2016b). EARTHQUAKES The San Francisco Bay Area is one of the most seismically active in the country and contains numerous active faults. The eastern portion of San Leandro 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 2016b). 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 on the San Andreas fault that runs 15 miles west of San Leandro. An earthquake of this magnitude could topple buildings, disrupt infrastructure, impact transportation systems, and trigger landslides throughout the San Leandro Hills (San Leandro 2016b). 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 2016b). CHECKLIST DISCUSSION a) i) No Impact. The project site is not within an Earthquake Fault Zone, as defined by the Alquist-Priolo Earthquake Fault Zoning Act of 1972. According to the geotechnical investigation, no known surface expression of fault traces is thought to cross the site; therefore, fault rupture hazard is not a significant geologic hazard at the site (Cornerstone 2016). No impact would occur. ii) Less Than Significant Impact with Mitigation Incorporated. The San Francisco Bay Area is one of the most seismically active in the country and contains numerous active faults. As noted above, the project site is not located within a Alquist-Priolo Earthquake Fault Zone for known active faults. State-considered active faults proximate to the project site include the following: