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4.6 Geology and Soils 4.6 – Geology and Soils 4.6 Geology and Soils This section describes the existing geological setting of the proposed Sand Canyon Resort Project (project) site, identifies associated regulatory requirements, evaluates potential impacts, and identifies mitigation measures related to implementation of the proposed project. 4.6.1 Environmental Setting This section describes the existing conditions in the project area and also identifies the resources that could be affected by the proposed project. Regional Geology The project site is located in the Transverse Range Geomorphic Province of California, which is characterized by east– west trending mountains and faults. Sedimentary basins within the Transverse Range include the Ventura Basin, Soledad Basin, Ridge Basin, and the San Fernando Valley. The Ventura, Soledad, and Ridge sedimentary basins are the result of the interplay of the San Andreas Fault and the Transverse Range fault system. Seismic activity along the San Andreas Fault is in response to differential movement between the Pacific geologic plate (west of the fault) and the North American geologic plate (east of the fault). The project site overlies the western Soledad (sedimentary) Basin, along the northern flanks of the western San Gabriel Mountains. The San Gabriel Fault Zone, located approximately 3 miles southwest of the site, defines the southwestern boundary of the Soledad Basin. Local Geology The site topography is dominated by a major northwest-trending bedrock ridge between Sand Canyon and Oak Springs Canyon, which descends towards the Santa Clara River, located approximately 1 mile north of the site. Several minor westerly and easterly trending ridges descend onto the site from the main northwest-trending ridge. The natural slopes on site include gradients of approximately 4:1 (horizontal to vertical) to approximately 1.5:1. Site elevations range from approximately 1,590 feet above mean sea level in the northwest portion of the site to approximately 1,740 feet above mean sea level in the southeast portion (Appendix F, Geotechnical Report). Earth materials on site include artificial fill, alluvium, and bedrock units assigned to the Mint Canyon Formation. The latter underlie the project site at depth and are exposed at the ground surface in areas of higher topographic relief. The Mint Canyon Formation consists of fine- to coarse-grained sandstone, interbedded with conglomerate, siltstone, and claystone. Beds are several inches to several feet thick, with a few widely spaced (i.e., greater than 20 feet) joints, or fractures. The joints are tight, with no separation, and continuous over 3 to 10 feet in length. As a result of past site activities associated with grading and development of the golf course, there is a moderately thin (i.e., 0.5 to 3 feet thick) cover of artificial fill materials overlying some of the areas identified as Mint Canyon Formation (Appendix F). Prior to grading for the existing golf course, Holocene age alluvial deposits mantled all of the canyons and drainage courses within the project boundaries, but were either removed or blanketed by artificial fill. As observed in exploratory excavations for the project-specific geotechnical investigation (Appendix F), the alluvium consists of fine- to coarse-grained sand and silty sand. Artificial fill deposits are present over half the site, placed within previous canyon areas or to establish the various golf course features. The artificial fill is composed of sand and silty sand mixtures derived from the on-site or nearby alluvial and bedrock materials. Sand Canyon Resort Project Draft EIR 11285 November 2020 4.6-1 4.6 – Geology and Soils Seismicity and Faulting As is the case for all of Southern California, the project is located in a seismically active area. The California Geological Survey (CGS) (CGS 2018) classifies faults as follows: • Holocene-active faults, which are faults that have moved during the past approximate 11,700 years. These faults are capable of surface rupture. • Pre-Holocene faults, which are faults that have not moved in the past 11,700 years. This class of fault may be capable of surface rupture, but is not regulated under the Alquist-Priolo Earthquake Fault Zoning Act of 1972, which regulates construction of buildings to be used for human occupancy. • Age-undetermined faults, which are faults where the recency of fault movement has not been determined. Holocene-active faults have been responsible for large historical earthquakes in Southern California, including the 1971 San Fernando earthquake (moment magnitude [Mw] 6.6), the 1992 Landers earthquake (Mw 7.3), the 1952 Kern County earthquake (Mw 7.5), and the 1933 Long Beach earthquake (Mw 6.4). The Southern California region also includes blind thrust faults, which are faults that do not rupture all the way up to the surface, but are capable of substantial earthquakes. Examples include the 1987 Whittier Narrows earthquake (Mw 5.9) and the 1994 Northridge earthquake (Mw 6.7). Both of these earthquakes occurred on previously unidentified thrust faults (USGS 1971, 1988, 1994; SCEDC 2013). Prominent Holocene-active and pre-Holocene faults in the project region are listed in Table 4.6-1 and shown on Figure 4.6-1, Regional Faulting. Table 4.6-1. List of Earthquake Faults Fault Name Closest Distance from Project Site (in miles) Direction from Project San Gabriel 3 Southwest San Fernando 7 Southwest San Andreas 16 Northeast San Cayetano 21 West Verdugo 13 Southeast Sierra Madre 20 Southeast Malibu Coast 30 Southwest Santa Monica 25 South Raymond 23 Southeast Newport-Inglewood 24 South Whittier 29 Southeast Elsinore 62 Southeast Sources: CGS 2010, USGS 2018a. Based on the Alquist-Priolo Earthquake Fault Zoning Act, only those faults that have direct evidence of movement within the last 11,000 years are required to be zoned. The CGS considers fault movement within this period a characteristic of faults that have a relatively high potential for ground rupture in the present or future. As discussed in Section 4.6.2, Regulatory Framework, the Alquist-Priolo Earthquake Fault Zoning Act requires the State Geologist to establish earthquake fault zones around the surface traces of active faults and to issue appropriate maps to assist cities and counties in planning, zoning, and building regulation functions. These zones, which generally Sand Canyon Resort Project Draft EIR 11285 November 2020 4.6-2 4.6 – Geology and Soils extend 200 to 500 feet on each side of a known active fault based on location, precision, complexity, or regional significance of the fault, identify areas where potential surface fault rupture along an active fault could prove hazardous and identify where special studies are required to characterize hazards to habitable structures. If a site intended for human occupancy lies within an earthquake fault zone on an official CGS map, a geologic fault rupture investigation must be performed before issuance of permits to demonstrate that the proposed development is not threatened by surface displacement from the fault. The project site is not underlain by Holocene-active faults or Alquist-Priolo earthquake fault zones. The closest such faults are the San Gabriel and San Fernando faults, located approximately 3 miles and 7 miles southwest of the site, respectively. On-site faulting is confined to the Miocene age Mint Canyon Formation, which is considered pre- Holocene and inactive (RTF&A 2018; CGS 2010; USGS 2018a). Ground Shaking Seismically induced ground shaking is the primary cause of damage during earthquakes. Based on the proximity of the Northridge and San Fernando earthquakes, as well as the relative proximity to the San Andreas Fault, seismic parameters determined for the project site resulted in an anticipated peak ground acceleration (PGA) of 0.95 percent of gravity. This PGA value was based on a mean contribution from all earthquake sources of magnitude 6.9, at a distance of about 7 miles (Appendix F). For perspective, with respect to mortgage loans, the U.S. Geological Survey considers regions to have a high seismic risk if there is a 10% or greater probability of the maximum PGA being equal to or greater than 0.15 percent of gravity at any point in a 50-year period (Fannie Mae 2017). Liquefaction/Lateral Spreading Liquefaction occurs when partially saturated soil enters a liquid state, resulting in the soil’s inability to support overlying structures. Liquefaction typically occurs in areas where the groundwater is less than 30 feet from the surface and where the soils are composed of poorly consolidated fine to medium sand. Lateral spreading consists of lateral movement of gently to steeply sloping saturated soil deposits that is caused by earthquake-induced liquefaction. The Seismic Hazards Mapping Act of 1990 directs the California Department of Conservation, Division of Mines and Geology (now the CGS) to identify and mitigate seismic hazards. Based on the Seismic Hazards Zone Map for the Mint Canyon quadrangle (CGS 1999), the alluviated canyon bottoms within the western portion of the project site are considered potential liquefaction areas, as shown on Figure 4.6-2, Seismic Hazards. A project- specific liquefaction analysis, including completion of borings, laboratory testing, and engineering analysis, indicate that the maximum seismic-induced ground settlement associated with liquefaction is 2.85 inches (Appendix F). Landslides Slope failures include many phenomena that involve the downslope displacement and movement of material, triggered either by gravity or seismic forces. Exposed bedrock slopes may experience rockfalls, rockslides, rock avalanches, and deep-seated rotational slides, and soil slopes may experience soil slumps and rapid debris flows. Slope stability can depend on a number of complex variables, including the geology, structure, and amount of groundwater, as well as external processes such as climate, topography, slope geometry, and human activity. The factors that contribute to slope movements include those that decrease the resistance in the slope materials and those that increase the stresses on the slope.
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