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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-7 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx extends roughly 22 miles. The Marsh Creek-Greenville segment has an estimated reoccurrence interval of 4,200 years based on an assumed maximum event having a magnitude of 6.5 on the Richter Scale, and has a slip rate of 0.1-0.3 mm/year.14 The most recent notable event associated with the Marsh Creek-Greenville segment is the 1980 Livermore earthquakes with a magnitude of 5.8 and 5.1 with an epicenter near the Los Vaqueros Reservoir 15 and resulted in minor surface rupture.16 Concord Fault The Concord Fault is a major right lateral strike-slip fault that strikes in a northwest direction from the northwest corner of Mount Diablo in the south to the Suisun Bay in the north spanning approximately 12.4 miles in length and directly transects the City of Concord. The fault line is located approximately 13.3 miles north of the project site. The fault has no established maximum events or reoccurrence interval. However, the most recent notable earthquake occurred in 1955 with a magnitude of 5.4 on the Richter Scale and a swarm of smaller earthquakes occurred in 2015. In addition, the fault is highly characterized by aseismic creep with a slip rate between 3.0-3.5 mm/year.17 Liquefaction Liquefaction is a phenomenon that occurs in loose water-saturated sandy soils with a shallow water table. Strong seismic waves generated from an earthquake can cause the water table to temporarily rise, pulling aggregates apart and turning the soil from a solid into a temporary liquid-like state. Damage from strong seismic shaking is typically more severe in areas susceptible to liquefaction such as areas of artificial landfills and is usually associated with the subsidence or tilting of structures and may cause internal utility lines to rise to the surface. As previously mentioned, soils on the project site are mostly comprised of Diablo clay consisting of a heavy clay loam, silty clay or clay texture and the Updated Geotechnical and Geologic Report indicated subsurface conditions to comprise of a thin layer of residual soil and colluvium with stiff to stiff lean to fat clays and no groundwater to be present as identified in test pit excavation between 3-18 feet in depth. However, the northern edge and southeastern portions of the project site consist of younger alluvium consisting of sands, clays, and gravels with perched groundwater within the southeastern portion of the project site along Devany Creek.18 In addition, the California Geological Survey (CGS) identifies the northern edge and southeastern portions of the project site to be within a State-designated Liquefaction Zone of required investigation; see Exhibit 3.6-1.19 14 Bryant, W.A., S.E. Cluett. 2002. Greenville fault zone, Marsh Creek-Greenville section (Class A) No. 53b, Quaternary fault and fold database of the United States. June 25. 15 California Department of Conservation. 2017. Website: https://maps.conservation.ca.gov/cgs/historicearthquakes/. Accessed February 25, 2025. 16 Bryant, W.A., S.E. Cluett. 2002. Greenville fault zone, Marsh Creek-Greenville section (Class A) No. 53b, Quaternary fault and fold database of the United States. June 25. 17 Bryant, W.A., S.E., Cluett. 1998. Fault number 38a, Concord fault, Avon section, Quaternary fault and fold database of the United States. August 18. 18 Cornerstone Earth Group. 2020. Updated Preliminary Geotechnical and Geologic Report. March 31. 19 California Department of Conservation. 2025. Earthquake Zones of Required Investigation. 2025. Website: https://maps.conservation.ca.gov/cgs/informationwarehouse/eqzapp/. Accessed February 25, 2025.