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4.5 Geology, Soils, and Paleontological Resources This Section Describes the Environmental and Regulatory Setting for Geology, Soils, and Paleontological Resources

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4.5 Geology, Soils, and Paleontological Resources This Section Describes the Environmental and Regulatory Setting for Geology, Soils, and Paleontological Resources Environmental Impact Analysis City of Union City Geology, Soils, and Paleontological Resources 4.5 Geology, Soils, and Paleontological Resources This section describes the environmental and regulatory setting for geology, soils, and paleontological resources. It also describes impacts on geology, soils, and paleontological resources that would result from implementation of the project and mitigation for significant impacts where feasible and appropriate. No comments regarding geology, soils, and paleontological resources were received in response to the Notice of Preparation (NOP). 4.5.1 Existing Conditions 4.5.1.1 Environmental Setting Physiography Union City is located within the Coast Ranges Geomorphic Province, a relatively geologically young and seismically active region on the western margin of the North American plate. The ranges and valley trend northwest, sub-parallel to the San Andreas fault. The Coast Ranges are composed of thick Mesozoic and Cenozoic sedimentary strata. The northern and southern ranges are separated by a depression containing the San Francisco Bay. The project site is located west of the northwest- trending Hayward fault, which divides the low-lying, gently sloping, and nearly level alluvial and estuarine landforms that surround San Francisco Bay from the strongly sloping and steel upland forms of the northwest-trending East Bay hills. West of the Hayward fault, the land is urbanized, whereas east of the fault, development is sparser. Subsurface Conditions The approximately 26.5-acre project site is within the 105-acre Station East subarea of the Decoto Industrial Park Study Area Specific Plan (DIPSA Specific Plan). The project site is occupied by existing and vacant industrial uses, surface parking, and an agricultural field. The project site is bound by Decoto Road, 7th Street, Bradford Way, and the Niles subdivision Union Pacific Railroad (UPRR) tracks. The Natural Resources Conservation Service (NRCS) describes the underlying soils as consisting of Rincon clay loam. A Cone Penetration Test (CPT) performed within the vacant field portion of the project site encountered 4 to 5 feet of medium-dense to dense silty sand deposits underlain by a medium stiff to stiff layer of clay, increasing in stiffness down to 25 feet and interbedded with thin, medium-dense layers of silts and sands.1 Borings performed within a developed portion of the project site encountered less than 4 inches of asphalt to 8 inches of concrete (depending on the location of the boring), aggregate bases between 8 and 10 inches, with near surface soils consisting of low to moderate plasticity sandy clays ranging in consistency from stiff to hard to depths of 20 feet. Medium dense gravely sand with trace silt was encountered between 3.5 and 6.5 feet and 16.5 and 18.5 feet, depending on the boring. 2 1 ENGEO. 2013. Preliminary Geotechnical Exploration: Zwissig Way Development, Union City, California. (9399.001.000.) San Ramon, CA, November 18, 2013. 2 Berlogar Stevens & Associates. 2017. Due Diligence Level Geotechnical Investigation Airgas/Williams Mixed-Use Decoto Road and 7th Street, Union City, California. (9399.001.000.) Pleasanton, CA, October 11, 2017. Station East Residential/Mixed Use Project November 2020 4.5-1 Draft Environmental Impact Report ICF 00011.19 Environmental Impact Analysis City of Union City Geology, Soils, and Paleontological Resources Groundwater can fluctuate according to the season, as well as other factors such as temperature, irrigation, and precipitation. According to the California Geological Survey for the Newark quadrangle’s “depth to historically high groundwater” location map (within which the project site is located), historically high groundwater levels at the project site are between 30 to 40 feet below ground surface (bgs).3 Recent assessments have encountered groundwater at the project site between approximately 28 and 70 feet below the ground surface.4,5 A recent review of historic monitoring found the minimum depth to groundwater to be approximately 25 feet, with an average depth of 30 to 35 feet.6 Seismicity and Seismic Hazards The entire San Francisco Bay Area is located within the San Andreas fault system, a complex of active faults forming the boundary between the North American and Pacific lithospheric plates. Movement of the plates relative to one another results in the accumulation of strain along the faults, which is released during earthquakes. Numerous moderate to strong historic earthquakes have been generated in northern California by the San Andreas fault system. This level of active seismicity results in a relatively high seismic risk in the San Francisco Bay Area. The San Andreas fault system includes numerous faults in the Bay Area considered under the Alquist-Priolo Earthquake Fault Zoning Act to be active (i.e., to have evidence of surface fault rupture in the past 11,000 years). Active regional faults include the San Andreas, Hayward, Calaveras, Concord-Green Valley, and Greenville faults. In addition to the known active faults, recent research on the structural geology and tectonics of the region indicates that there is another potential source of large-magnitude earthquakes in the region. A structural trend of folds and thrust faults has been mapped in the hills north of the Livermore Valley. The largest of these features is the Mount Diablo anticline. Recent research has interpreted this feature to be a large fold developed above a blind (i.e., buried) thrust fault. The accumulation of strain on the blind Mount Diablo thrust fault presents the potential for an earthquake along this fault. The project site lies west of the three major northwest-trending fault zones within the City, the Hayward fault zone, the Mission fault zone, and the Silver Creek fault zone.7 No mapped active faults cross the project site; however, the Hayward fault is approximately 150 feet northeast of the project site. The largest earthquake on the Hayward fault occurred in 1868 with an epicenter south of San José, California,8 and surface fault rupture occurred along it from Fremont to San Leandro. Towns in the East Bay suffered the greatest damage from the earthquake: many buildings were destroyed and the nearby town of Hayward was nearly destroyed. Farther away in San Francisco, Oakland, and San José, walls, chimneys, and other heavy architectural elements of buildings fell. The magnitude of the earthquake is estimated at 6.8. 3 California Geological Survey, Seismic Hazard Zone Report for the Newark 7.5-Minute Quadrangle, Alameda County, California, Seismic Hazard Zones Report 090, 2003. 4 ENGEO. 2016. Phase I Environmental Site Assessment – Zwissig Way Parcels, Union City, California. April. 5 ENGEO. 2014. Phase I Environmental Site Assessment – 33945 7th Street, Union City, California. October. 6 ENGEO. 2016. Phase I Environmental Site Assessment – Zwissig Way Parcels, Union City, California. April. 7 City of Union City. 2020. Union City 2040 General Plan Update. Adopted: December 2019. Chapter 6: Safety Element. Available: http://www.uc2040.com/documents/. Accessed: March 25, 2020. 8 U.S. Geological Survey. 2018. The Hayward Fault—Is It Due for a Repeat of the Powerful 1868 Earthquake? August. (FS 2008-3019.) Available: https://pubs.usgs.gov/fs/2018/3052/fs20183052.pdf. Accessed: July 9, 2020. Station East Residential/Mixed Use Project November 2020 4.5-2 Draft Environmental Impact Report ICF 00011.19 Environmental Impact Analysis City of Union City Geology, Soils, and Paleontological Resources Primary Seismic Hazards Surface Fault Rupture Surface fault rupture occurs when the ground surface is broken due to fault movement during an earthquake. The location of surface fault rupture can be assumed to be along an active fault line. Because the Hayward fault is within 150 feet of the project site and it has a history of both surface fault rupture in the 1868 earthquake and fault creep,9,10 there is a risk of surface fault rupture within the project site. Seismic Ground Shaking Seismic ground shaking is a general term referring to all aspects of motion of the earth’s surface resulting from an earthquake. Ground shaking is normally the major cause of damage in seismic events. The extent of ground shaking is determined by the magnitude and intensity of the earthquake, distance from the rupture, and local geologic conditions. Intensity is a subjective measure of the perceptible effects of seismic energy at a given point and varies with distance from the epicenter and local geologic conditions. The Modified Mercalli Intensity Scale (MMI) is the most commonly used scale for measurement of the subjective effects of earthquake intensity. Earthquake size is generally quantitatively measured in terms of moment magnitude. A rupture of the Hayward fault is considered capable of generating a moment magnitude (MW) 7.4 earthquake.11 An earthquake matching this scenario is estimated to be capable of generating very strong (MM-VIII) to violent (MM–IX) seismic shaking at the project site.12 The 2014 Working Group for California Earthquake Probabilities at the U.S. Geological Survey (USGS) predicted a 72 percent chance of a magnitude 6.7 or greater earthquake occurring in the Bay Area in 30 years. The most extreme ground shaking within the City is expected to occur as a result of a seismic event along the Hayward fault, and intensity is expected to be greatest in the alluvial landforms west of the fault zone where the project site is located.13 This is due to the fine textured sediments and soils which tend to amplify seismic waves. Secondary Seismic Hazards Liquefaction Liquefaction occurs when saturated unconsolidated sediments lose strength and stiffness with applied stress and/or shaking, such as during an earthquake. The lack of cohesion causes solid soil to behave like a liquid, resulting in ground failure. Ground failure can take on many forms, including, but not limited to, loss of bearing, lateral spreading, lowering of the ground surface, ground settlement, ground 9 Fault creep is the slow movement of two sides of an earthquake fault moving past each other, in contrast to sudden and fast movement of an earthquake.
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