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4.6 GEOLOGY <br /> The following discussion is based,in part, on a Geotechnical Assessment prepared by PSI, Inc. in <br /> August 2011. A copy of the geotechnical report can be found in Appendix D. <br /> 4.6.1 Existing Setting <br /> 4.6.1.1 Geology and Soils <br /> Redwood City is located within California's Coast Ranges Geomorphic Province, a geologically <br /> young and seismically active region. Northwest-southeast trending ranges of low mountains and <br /> intervening valleys dominate this region. The project site is located in the central portion of the City <br /> which is a gently sloping plain draining northeast to the tidal marsh.9 The site is underlain by <br /> Holocene aged alluvial deposits.�o <br /> Soils beneath the site are classified as Urban Land Orthents which can be poorly drained to well- <br /> drained. This soils type is present on alluvial fans, flood plains, and stream terraces. Urban Land <br /> Orthents includes Novato and Reyes Series soils which are predominately clays and silty clays with <br /> high shrink-swell potential.�� The subsurface soils at the project site could,therefore,have a high <br /> expansion potential. Expansive soils shrink and swell as a result of moisture changes. These changes <br /> can cause heaving and cracking of slabs-on-grade,pavements, and structures built on shallow <br /> foundations. <br /> There are no unique geologic features on or adjacent to the site. Due to the flat topography of the <br /> project site,the potential for erosion or landslide on or adjacent to the site is low. <br /> 4.6.1.2 Seismicity and Seismic Hazards <br /> The project site is located within the seismically active San Francisco Bay Region. The Uniform <br /> Building Code designates the entire Bay Area as Seismic Activity Zone 4,the most seismically <br /> active zone in the United States. The project site is not within a defined Alquist-Priolo Earthquake <br /> Fault Zone and the risk of fault rupture at the site is low.�� The faults in the region are,however, <br /> capable of generating earthquakes of magnitude 7.0 or higher and strong to very strong ground <br /> shaking would be expected to occur at the project site during a major earthquake on one of the <br /> nearby faults. <br /> Faults that could affect the project site include the San Andreas fault,the Hayward fault,the <br /> Calaveras fault, and the Monte Vista—Shannon fault. The Monte Vista—Shannon fault is the <br /> nearest active fault to the project site,located approximately 4.5 miles from the site. <br /> 9 Redwood City General Plan. <br /> io USGS. National Geologic Map Database. 1991. <httt�://n�mdb.us�s.uov/n�m- <br /> bin/ILView.t�l?sid=519 1.sid�ivtvt�e=b�isfact=l.5> Accessed July 26,2012. Alluvial soils are fine-grained fertile soil deposited <br /> by water flowing over flood plains or in river beds. <br /> i i Redwood City General Plan. <br /> i�California Department of Conservation. fllquist-Priolo Earthquake Fault Zones. <br /> <htti�://www.quake.ca.gov/gma�s/ap/ap maps.htm> Accessed July 26,2012. <br /> Kensington Assisted Living Facility Project 36 Initial Study <br /> Redwood City Apri12013 <br />