ENVIRONMENTAL IMPACT ANALYSIS C. GEOLOGY

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1 IV. ENVIRONMENTAL IMPACT ANALYSIS C. GEOLOGY This section summarizes the results of a geotechnical evaluation conducted for the proposed project by Jerry Kovacs and Associates, Inc. in August The complete study is contained in the Technical Appendices to this EIR as Appendix E. Environmental Setting Regional The proposed project site is located within the northwest portion of the Los Angeles Basin, on the Santa Monica Plain. The Santa Monica Plain is dissected by streams originating within the Santa Monica Mountains to the north. The site is located along the border separating the Transverse Ranges Geomorphic Province to the north and the Peninsular Ranges Geomorphic Province to the south. The Santa Monica-Hollywood fault system is considered the boundary between these two geomorphic provinces. The Transverse Ranges are dominated by east-west trending, reverse and thrust faults (i.e., Hollywood and Santa Monica faults). The Peninsular Ranges are dominated by northwest-trending, strike-slip faults. Geologic Materials The site is underlain by late Pleistocene age older alluvium. The site was explored on July 17 and 18, 1996 by drilling seven exploratory borings to depths ranging from 50 to 70 feet below existing site grade. The exploratory borings were all excavated on portion of the project site located north of Gorham Avenue because of the inaccessibility of the portion of the site located south of Gorham Avenue. For purposes of this analysis, however, the general site conditions south of Gorham would be expected to be similar to those of the northern portion of the project site. Up to six feet of fill soils were encountered during exploration. The fill consists of clayey sand and sandy clay which are generally medium dense to locally dense, fine to medium grained and contain small amounts of gravel-sized material. The fill encountered was free of debris. Natural soils underlying the fill consisted of sands and clays, with zones of clayey and sandy gravels. The soils are generally dense to hard below a depth of about 10 feet. The natural soils consist primarily of detrital sediments deposited by river and stream action typical to this area of Los Angeles County. Groundwater Page IV.C-1

2 Groundwater was encountered in some borings as moderate seepage at depths of approximately 49 feet. Wet soils were also encountered in one boring at a depth of 22.5 feet, suggesting the presence of a minor seepage condition at these levels. The groundwater conditions indicated perched zones consisting of saturated granular soils. The amount of seepage in these zones may fluctuate seasonally. Well measurement records were researched at the County of Los Angeles Department of Public Works, Hydrologic Records and Water Conservation Division. Depth-to-groundwater information was obtained for three nearby wells: 2524; 2544D and 2535J. Well No is located approximately one-half mile to the northwest of the proposed project site, near the intersection of Bundy Drive and Rose Marie Lane. The depth to groundwater in this well was measured to be 72.3 feet on October 27, The ground surface elevation at the well is 353 feet. Well No. 2544D is located approximately one-half mile east of the proposed project site, on the Veterans Administration property near the San Diego Freeway. The depth to groundwater in this well was measured to be 70.6 feet on October 27, The ground surface elevation at this well is 333 feet. Well No. 2535J is located approximately miles to the south of the proposed project site, near the intersection of Bundy Drive and Texas Avenue. The depth to groundwater at this well was measured to be 76.6 feet on October 31, The ground surface elevation at this well is feet. Faults The numerous faults in Southern California include active, potentially active, and inactive faults. The criteria for these major groups are based on criteria developed by the California Division of Mines and (CDMG) for the Alquist-Priolo Earthquake Fault Zoning Program (Hart, 1997). By definition, an active fault is one that has had surface displacement within Holocene time (about the last 11,000 years). A potentially active fault is a fault that has demonstrated surface displacement of Quaternary age deposits (last 1.6 million years). Inactive faults have not moved in the last 1.6 million years. A list of nearby active faults and the distance in miles between the nearest point on the fault, the maximum credible earthquake, and the peak site acceleration for the fault is listed in Table IV.C-1. Active Faults Santa Monica Fault: The closest known fault to the project site is the potentially active Santa Monica fault. The Santa Monica fault is the western segment of the Santa Monica-Hollywood fault zone that trends east from the Santa Monica coastline (where it may extend west as the Malibu Coast fault) to the Hollywood area on the east. The fault is shown as two main strands west of Century City by Leighton (1990). The northern strand is approximately 0.7 miles south of the project site. Crook and Ward (1983) Table IV.C-1 Major Named Faults Considered To Be Active In Southern California (A) Fault Maximum Peak Distance From Direction Page IV.C-2

3 (in alphabetical order) Magnitude Accel (g) Site (Miles) From Site Anacapa-Dume 7.0 (b) RO WSW Cucamonga 6.9 (b) RO E Elsinore (Glen Ivy Segment) 7.5 (b) SS ESE Elysian Park Thrust 7.1 (b) RO E Hollywood 7.0 (b) RO NE Malibu Coast 6.9 (b) RO W Newport-Inglewood Zone 6.7 (b) SS SE Northridge 6.5 (b) RO N Oakridge 7.0 (b) RO NW Palos Verdes Hills 7.2 (b) SS S Raymond 7.5 (b) RO NE San Andreas (Southern Segment ) 8.0 (b) SS NE San Cayetano 6.9 (b) RO NW San Gabriel 7.4 (b) SS NE Santa Monica 7.0 (b) RO S Sierra Madre 7.3 (b) RO NE Simi-Santa Rosa 7.1 (b) RO NW Verdugo 6.7 (b) RO NE Whittier 7.1 (b) SS ESE (a) Slemmons, 1979 (b) California Division of Mines and, 1996 SS Strike Slip The strike is the direction, or trend, of the line marking the intersection of a fault plane with the horizontal. A strike slip fault is characterized by movement parallel to the strike of the fault. RO Reverse Oblique In geologic terminology, a hanging wall is the side of the fault above the fault plane. It is called the hanging wall because in places where faults have been filled in with mineral deposits and then mined, this is the side on which miners hanged their lanterns. A footwall is the side of the fault below the fault plane. The footwall is so named because this is the side on which miners walk. A normal fault is characterized by predominantly vertical displacement in which the hanging wall side is moved downward with respect to the footwall side of the fault. An oblique fault is characterized by a combination of movement both perpendicular and parallel to the strike of the fault a combination of strike slip (discussed above) and dip slip. (The dip is the angle between a fault plane and the horizontal and is always perpendicular to the strike). A reverse fault is characterized by movement which is predominantly vertical, with the hanging wall side moved upward in relation to the footwall side. concluded that the latest surface faulting episode on the Santa Monica Fault is pre-holocene based on unfaulted 10,000 year old soil. The closest known active faults to the project site are the Hollywood and Newport-Inglewood faults. Page IV.C-3

4 Hollywood Fault: The active Hollywood fault, located approximately one mile northeast of the site, trends approximately east-west along the base of the Santa Monica Mountains from the West Beverly Hills Lineament in the West Hollywood-Beverly Hills area (Dolan and Sieh, 1992) to the Los Feliz area of Los Angeles. The fault may act as a groundwater barrier within Holocene sediments (Converse et al., 1981). Scarps six to nine feet high in Holocene flood plain deposits have been suggested along the fault trace in the Atwater area (Weber et al. 1980). Studies by several investigators (Dolan and Sieh, 1992 and Crook and Proctor, 1992) have indicated that the fault is active, based on geomorphic evidence, stratigraphic correlation between exploratory borings, and fault trenching studies. Newport-Inglewood Fault Zone: The active Inglewood fault of the Newport-Inglewood fault zone is located approximately two miles east of the project site. This fault zone is composed of a series of northwest-trending echelon faults extending from the Ballona Gap (I-10 Freeway) southeastward to the area offshore of Newport Beach. This zone is reflected at the surface by a line of geomorphically young anticlinal hills and mesas formed by the folding and faulting of a thick sequence of Pleistocene age sediments and Tertiary age sedimentary rocks (Barrows, 1974). Fault-plane solutions for 39 small earthquakes (1977 to 1985) show mostly strike-slip faulting with some reverse faulting along the north segment (north of Dominguez Hills) and some normal faulting along the south segment (south of Dominguez Hills to Newport Beach) (Hauksson, 1987). Recent investigations by Law/Crandall (1993) in the Huntington Beach area indicate that the North Branch segment of the Newport-Inglewood fault zone offsets Holocene age alluvial deposits in the vicinity of the Santa Ana River. Other active faults in proximity to the project site include the Whittier Fault, the Sierra Madre-San Fernando Fault and the San Andreas Fault. Blind Thrust Faults Seismic forces other than faults with known surface expression are so-called buried, or blind, thrust faults. These faults are not exposed at the surface and are typically broadly defined based on the analysis of seismic wave recordings of several hundreds of small earthquakes in the southern California area. The risk for surface rupture potential of these buried thrust faults is inferred to be low (Leighton, 1990). The Wilshire Arch, Santa Monica Mountains Thrust, Compton-Los Alamitos Thrust, Elysian Park Thrust and the eastern blind portion of the Oak Ridge system, among others are located in relatively close proximity to the proposed project site. Compton-Los Alamitos Thrust: As defined by Dolan et al. (1995), the Compton-Los Alamitos Thrust is an inferred blind thrust fault located within the southwestern portion of the Los Angeles Basin. The thrust fault is suggested to extend over 50 miles from the Santa Monica Bay coastline southeast into northwestern Orange County and may connect with the Elysian Park thrust to the northwest along a detachment fault below Los Angeles. This thrust fault is not exposed at the surface and does not present a Page IV.C-4

5 potential surface rupture hazard; however, the Compton-Los Alamitos Thrust should be considered a potential source of ground shaking. Elysian Park Fold and Thrust Belt: The Elysian Park Fold and Thrust Belt as originally defined by Hauksson (1990) was postulated to extend northwesterly from the Santa Ana Mountains to the Santa Monica Mountains, extending westerly and paralleling the Santa Monica-Hollywood and Malibu Coast faults. The Elysian Park Fold and Thrust Belt is now thought to consist of two components known as the Santa Monica Mountains Thrust and the Elysian Park Thrust. The Elysian Park Thrust is the closest segment of the Elysian Park Fold and Thrust Belt. These thrust faults are not exposed at the surface and do not present a potential surface rupture hazard; however, the Elysian Park Fold and Thrust Belt should be considered an active feature capable of generating future earthquakes. Historic Seismicity As with all of southern California, the proposed project site has experienced historic earthquakes from various regional faults. The epicenters of recent recorded earthquakes in the vicinity of the site are shown in Table IV.C-2. Larger, more distant earthquakes, such as the 1857 Fort Tejon earthquake on the San Andreas Fault have also affected the site. Liquefaction Liquefaction involves a sudden loss of strength of a saturated, cohesionless soil that is caused by shock or strain and results in temporary transformation of the soil to a fluid mass. Factors that affect liquefaction include water level, soil type, particle size and gradation, relative density, confining pressure, intensity of shaking, and duration of shaking. Liquefaction potential has been found to be the greatest where the groundwater level is shallow and loose, fine sands occur within a depth of about 50 feet or less. Liquefaction potential decreases with increasing grain size and clay and gravel content, but increases as the ground acceleration and duration of shaking increase. Due to the depth of groundwater on the proposed project site and the firm nature of the soils, the potential for liquefaction is assessed to be low. Earthquake (Oldest to Youngest) Table IV.C-2 List Of Historic Earthquakes Distance to Epicenter Date of Earthquake Magnitude From Project Site (Miles) Direction to Epicenter From Project Site Long Beach March 11, 1933 (GCT) SE San Fernando February 9, N Page IV.C-5

6 Whittier Narrows October 1, E Sierra Madre June 28, NE Landers June 28, E Big Bear June 28, E Northridge January 17, N Hector October 16, NE Landsliding The topography at the site slopes gently to the south. As a result, the probability of seismically-induced landslides on the site is assessed to be low. Earthquake-Induced Flooding Earthquake-induced flooding is flooding caused by failure of dams or other water-retaining structures due to earthquakes. Review of the County of Los Angeles Flood and Inundation Hazards Map (Leighton, 1990) indicates that the site does not lie within an inundation boundary. Therefore, the risk from seismically-induced flooding is considered remote. Tsunami and Seiche Tsunamis are tidal waves generated by fault displacement or major ground movement below the ocean. The proposed project site is high enough and far enough from the ocean to preclude risk of exposure to hazards of tsunami. Seiches are large waves generated in enclosed bodies of water in response to ground shaking. No major water-retaining structures are located immediately up-gradient from the project site. The risk of flooding from a seismically-induced seiche is considered to be remote. Environmental Impact Threshold of Significance. In accordance with the Draft L.A. CEQA Thresholds Guide, May, 1998, a project would normally have a significant geologic hazard impact if it would cause or accelerate geologic hazards which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. Page IV.C-6

7 The geotechnical analysis conducted for the proposed project site concluded that construction of the proposed mixed-use development is feasible from a geotechnical engineering standpoint. Between zero and six feet of existing fill was encountered during exploration. Since the proposed project will include subterranean levels, it is expected that existing fill soils will be removed during excavation. A total of 58,000 cubic yards of export material is expected to be generated during the process of excavation and site preparation. The natural soils encountered at the proposed basement level are considered suitable for support of the proposed buildings and a conventional foundation system may be utilized for the proposed buildings. Shoring and dewatering will be implemented during construction until the proposed structures are in place. Grading and excavation plans are subject to the approval of the Department of Building and Safety through its ministerial permitting and inspection processes. No hazards would be associated with constructing the proposed project on the project site and no significant impacts would occur. Seismicity The hazards of ground shaking are common in seismically active Southern California. The project site is not located within an Alquist-Priolo zone. The site could be subjected to strong ground shaking. However, the proposed project would be designed and built to resist the highest peak horizontal acceleration projected to occur at the project site in conformance with the seismic design provisions of the current Uniform Building Code, based upon a deterministic analysis prepared for the project site. The project is not expected to cause or accelerate seismic hazards that would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. Additionally, the project would not pose an increased risk to public safety as a result of seismic hazards. Therefore, ground shaking will not be a significant impact at the site. Liquefaction Based on the dense nature of the alluvial soils encountered in the borings and the depth to groundwater, the potential for liquefaction to have an adverse impact on the proposed project is very low. According to the Environmental and Public Facilities Maps showing areas susceptible to liquefaction in Los Angeles and surrounding communities, the project site is not located in an area with the potential for liquefaction. Other Geotechnical Impacts Other potential geologic hazards such as slope stability, flooding, inundation, tsunamis, seiches, and volcanic hazards would have less than significant impacts on the proposed project. No other significant adverse geotechnical impacts are anticipated as a result of the proposed project. Cumulative Impacts Page IV.C-7

8 To minimize the potential for cumulative geotechnical impacts, which could cause geologic hazards, related projects would be developed in accordance with the provisions of applicable State and local laws and ordinances. The cumulative impacts of geologic hazards resulting from related projects are considered less than significant. Mitigation Measures Because no significant impacts related to geology or geotechnical considerations were identified, no mitigation measures are required. However, the project would still be subject to the following standard City geotechnical conditions: The design and construction of the project shall conform to the Uniform Building Code seismic standards as approved by the Department of Building and Safety. Building design considerations may include, but are not limited to: ground stabilization, selection of appropriate foundation type and depths, selection of appropriate structural systems to accommodate anticipated displacements or any combination of these measures. Prior to the issuance of building or grading permits, the applicant shall submit a geotechnical Report prepared by a registered civil engineer or certified engineering geologist to the Department of Building and Safety for approval. Level of Significance After Mitigation Impacts of the proposed project related to geology and geotechnical considerations would be less than significant. Page IV.C-8