Appendix 6A Geologic Information about the Project Area prepared by Ninyo & Moore October 2008

Transcription

1 Appendix 6A Geologic Information about the Project Area prepared by Ninyo & Moore October 2008

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3 Appendix 6A: Geologic Information about the Project Area 6A-1Existing Geologic and Subsurface Conditions Regional Geologic Setting The project site is located within the Los Angeles Basin within the Transverse Ranges geomorphic province of southern California (Norris and Webb, 1990). Geologically, the Los Angeles Basin and vicinity is a region divided into four blocks that include uplifted portions and synclinal depressions. The site is located in the northeastern block, which is situated between the Whittier fault zone and the base of the San Gabriel Mountains and is separated from the northwestern block by the Raymond fault (Norris and Webb, 1990). The northeastern block of the basin is primarily underlain by marine Cenozoic sedimentary rocks up to approximately 22,000 feet thick overlying basement rocks comprised of igneous and metamorphic rocks. More recent Quaternary sediments, primarily of alluvial origin, comprise the low-lying valley and drainage areas within the region, including the Los Angeles basin and the Plan area. Site Geology The project site is located along the Los Angeles River between the Elysian Park Hills and the Repetto Hills at the northern edge of the Los Angeles Basin. The Plan area is generally underlain by Quaternary alluvial soils overlying Tertiary age sedimentary deposits (Figure 3). The alluvium is generally comprised of both stream channel and floodplain deposits of the Los Angeles River consisting of unconsolidated silt, sand, and gravel (Dibblee, 1989). Older alluvium consisting of river terrace deposits (Yearkes and Campbell, 2005) is mapped along the east side of the river. These deposits are described as dissected silt, sand, and gravel. Fill soils may be present on the project site, related to previous site development. The sedimentary bedrock exposed in the hills bordering the northwest boundary of the Plan area has been mapped as the Upper Pliocene Puente Formation, a member of the Monterey Formation (Lamar, 1970; Dibblee, 1989). For the purpose of this report, the bedrock unit will be referred to as the Puente Formation (Figure 3). This unit has been described as a light brown to gray, well bedded, medium to coarse-grained sandstone (Yearkes and Campbell, 2005). The geologic structure in the area is dominated by a northwest-southeast plunging anticline (the Elysian Park Anticline) and the Elysian Park blind thrust fault. The Plan area borders the southwest limb of the anticline where the bedding of the Puente Formation typically dips to the southwest on the order of 30 or more degrees (Lamar, 1970; Dibblee, 1989). The anticline is concealed under the alluvium and its approximate location is inferred from nearby bedrock outcrops. While the subject site is generally flat-lying, there are steep slopes along the northwest boundary where the site is bordered by Elysian Park and Broadway Boulevard. There is a mapped landslide in Elysian Park across the Los Angeles River from the project site [Lamar,1970; Dibblee, 1989; California Division of Mines and Geology (CDMG), 1998b]. No landslides are mapped within the site boundaries, and no landslides were observed during our site reconnaissance. Groundwater Based on review of the State of California Seismic Hazard Report, the historic high groundwater level in the vicinity of the site is reported to be at a depth of approximately 20 Page 6A-1

4 feet below the ground surface (CDMG, 1998b). It should be noted that fluctuations in the level of groundwater at the subject site may occur due to variations in ground surface topography, groundwater pumping, subsurface stratification, rainfall, irrigation practices, and other factors which may not have been evident at the time of our evaluation. Shallow perched conditions may be present in places. 6A-2 Principal Regional Faults Upper Elysian Park Blind Thrust The Upper Elysian Park Blind Thrust is a reverse thrust fault that does not reach the surface and that is why it is referred to as blind, though its projected fault trace is mapped as crossing the northern portion of the Project Area (Yearkes and Campbell, 2005; Blake, 2001). Geomorphic evidence of this fault at the surface is the Elysian Park anticline that extends for approximately 12.4 miles from the Hollywood fault on the northwest through the Silver Lake district and the cities of South Pasadena and Alhambra to San Gabriel on the east [Southern California Earthquake Center (SCEC) Working Group C, 2001]. Although not presently designated an Earthquake Fault Zone due to the lack of a well-defined surface trace, this fault is considered active (Oskin, et al., 2000) and capable of producing moderate earthquakes of Maximum Moment Magnitude of every 500 to 1,300 years on average. The slip rate on this fault is estimated to be 1.7 millimeters per year (Shaw, 1996). It is likely, therefore, that this fault could generate high ground shaking in the Project Area. Puente Hill Blind Thrust Fault The Puente Hills Blind Thrust is an active reverse thrust fault that does not reach the surface; it extends for more than 25 miles in the northern Los Angeles Basin from downtown Los Angeles south to northern Orange County. Although not presently designated an Earthquake Fault Zone due to the lack of a well-defined surface trace, this fault is considered active (Shaw, et al., 2002) and capable of generating moderate (Mmax ) earthquakes every approximately 400 to 1,320 years for single-segment earthquakes and strong earthquakes (Mmax 7.1) every approximately 780 to 2,600 years for multi-segment earthquakes (Shaw, et al., 2002). The slip rate of the fault is estimated to be 0.7 mm per year (Cao, et al., 2003). The Puente Hills Blind Thrust rupture area is projected below the Project Area while the projection of the fault trace is located approximately 2.9 miles northeast of the site (Blake, 2001). Malibu Coast-Santa Monica-Hollywood-Raymond Fault Zone The Malibu Coast, Santa Monica, Hollywood, and Raymond faults are considered a fault zone that is a subsystem of the Transverse Ranges Southern Boundary fault system (Dolan, et al., 2000a). The fault zone extends sub-parallel to the Malibu coastline easterly along the south side of the Santa Monica Mountains through Malibu, Santa Monica, West Los Angeles, and Hollywood. The Hollywood fault portion of the zone is located approximately 3.5 miles northwest of the Project Area (Blake, 2001). The fault system exhibits reverse left-lateral movement and is considered capable of generating earthquakes ranging from Mmax 6.4 to 6.7 in the plan area (Cao, et al., 2003). Newport-Inglewood Fault Zone The Newport-Inglewood fault zone is a major tectonic structure in the Los Angeles Basin and consists of a series of northwest-trending, right-lateral, strike-slip fault segments that extend from the southern edge of the Santa Monica Mountains southeast to offshore from Newport Beach. The Newport-Inglewood fault zone was the source of the 1933 Long Beach earthquake with a magnitude of Magnitude (M) 6.4 (SCEC, 2004). The Newport-Inglewood fault is considered capable of generating a Mmax 7.1 earthquake (Cao, et al., 2003). The Page 6A-2

5 fault is approximately 41 miles in length and has a slip rate of approximately 1 mm per year (Cao, et al., 2003). The Newport-Inglewood fault is located approximately 8.6 miles southwest of the plan area (Blake, 2001). Sierra Madre Fault Zone The Sierra Madre fault zone is comprised of a series of active reverse faults. The approximately 35-mile-long fault zone is located approximately between the cities of Sunland and Azusa along the foothills of the San Gabriel Mountains. The Sierra Madre fault is considered capable of generating a Mmax 7.2 earthquake, and the slip rate of the fault is estimated to be 2 mm per year (Cao, et al., 2003). The Sierra Madre fault is located approximately 10.2 miles northeast of the plan area (Blake, 2001). Whittier Fault (Elsinore Fault Zone) The Whittier fault is a right-lateral, strike-slip fault zone that extends approximately 24 miles from Whittier Narrows in Los Angeles County, southeast to the Santa Ana Canyon where it merges with the Elsinore fault zone. The Whittier fault zone is considered capable of generating a Mmax 6.8 earthquake and has a slip rate of approximately 2.5 mm per year (Cao, et al., 2003). The Whittier fault is located approximately 13.5 miles southeast of the Project Area (Blake, 2001). Clamshell-Sawpit Fault The Clamshell-Sawpit fault is a reverse fault located near the communities of Sierra Madre and Monrovia. In 1991, an earthquake occurred in the Sierra Madre area which is believed to have originated on the Clamshell-Sawpit fault (SCEC, 2004). The fault is approximately 11 miles long, and the slip rate of the fault is estimated to be 0.5 mm per year (Cao, et al., 2003). The Clamshell-Sawpit fault is considered capable of generating a Mmax 6.5 earthquake. The Clam-shell-Sawpit fault is located approximately 15.3 miles northeast of the Project Area (Blake, 2001). Northridge (East Oak Ridge) Fault The Northridge (East Oak Ridge) fault is an active reverse thrust fault located on Oak Ridge near the communities of Santa Paula and Fillmore, northwest of the community of Northridge. This fault was associated with the 1994 M 6.7 Northridge earthquake (SCEC, 2004). The Northridge (East Oak Ridge) fault is considered capable of generating a Mmax 7.0 earthquake (Cao, et al., 2003). The fault is approximately 56 miles long, and the slip rate of the fault is estimated to be 1.5 mm per year. The Northridge (East Oak Ridge) fault is located approximately 15.6 miles northwest of the Project Area (Blake, 2001). Palos Verdes Fault Zone The active Palos Verdes fault zone is approximately 0.6 to 0.9 miles wide and trends northwesterly along the eastern flanks of the Palos Verdes peninsula. The fault zone extends offshore to the southeast as well as to the northwest where it is presumed to merge with the Santa Monica fault system. The fault zone includes a right-lateral strike slip and a right reverse separation type faulting. The slip rate is approximately 3 mm per year and is considered capable of generating a Mmax 7.3 earthquake (Cao, et al., 2003). The Palos Verdes fault zone is located approximately 18.6 miles south of the Project Area (Blake, 2001). San Andreas Fault Zone The San Andreas fault zone has long been recognized as the dominant seismotectonic feature in California. This right-lateral, strike-slip fault is over 700 miles long and strikes northwest through the state from the Gulf of California to north of San Francisco. Two of California s three largest historic earthquakes, the 1906 San Francisco earthquake (M 8.3) and the 1857 Forth Tejon earthquake (M 7.9), occurred along the San Andreas fault (SCEC, 2004). The segment of the San Andreas fault that ruptured during the 1857 earthquake is located approximately 33 miles northeast of the Project Area. The slip rate of the fault is estimated to be 30 mm per year (Cao, et al., 2003). The fault is considered capable of Page 6A-3

6 producing earthquakes in excess of M 7.4, and the average frequency of earthquakes along this segment of the San Andreas fault is approximately 140 years (SCEC, 2004). 6A-3 Potential Geologic and Seismic Hazards Based on our review of geologic and seismic background information and geotechnical site reconnaissance, future development at the project site is not anticipated to have a significant impact on the geologic environment. However, future improvements may be subjected to potential impacts from geologic and seismic hazards. Potential impacts on the future improvements are provided in the following sections. Surface Fault Rupture The projected fault trace of the Upper Elysian Park Blind Thrust fault is mapped as crossing the Plan area (Blake, 2001). In addition, the rupture surface of the Puente Hills Blind Thrust Fault is projected below the Plan area. Blind thrust faults, by definition, do not have surface or near-surface expression of fault rupture. Therefore, the distance from a project site to a blind thrust fault is difficult to evaluate. The distances of the Plan area to the Upper Elysian Park Blind Thrust and Puente Hills Blind Thrust, as indicated in Table 1, are based on a surface projection of the rupture area. Additionally, the Upper Elysian Park Blink Thrust projected fault trace is considered to cross the northern portion of the Plan area. The projected fault trace for the Puente Hills Blind Thrust, considering a fault dip of approximately 25 degrees (Cao, et al., 2003), is approximately 2.9 miles northeast of the Plan area. During an earthquake, a fault will generally tend to rupture along its existing rupture plane, though deviation in the rupture plane and/or other secondary ground deformation may occur. The site is not located within a State of California Earthquake Fault Zone (formerly known as Alquist-Priolo Special Studies Zone) and not located within a fault rupture study area as indicated in Exhibit A of the City of Los Angeles Safety Element (1996) (Figure 5). Recent earthquakes that occurred on blind thrust faults generally did not produce substantial surface faulting. Assessment of the potential for surface fault rupture would be evaluated during future design phases of development. Consequently, surface ground rupture is considered to have less than significant impact with mitigation incorporation. Seismic Ground Shaking The seismic hazard likely to impact the project site is ground shaking during an earthquake on one of the nearby or distant active faults. The level of ground shaking at a given location depends on many factors, including the size and type of earthquake, distance from the earthquake, and subsurface geologic conditions. The size and type of construction also affects how particular structures perform during ground shaking. In order to evaluate the level of ground shaking that might be anticipated in the widespread Plan area, the estimated peak horizontal ground acceleration (PGA) data were reviewed in the northern and southern portions of the Plan area. The 2007 California Building Code (CBC) recommends that the design of structures be based on the horizontal peak ground acceleration (PGA) having a 2 percent probability of exceedance in 50 years which is defined as the Maximum Considered Earthquake (MCE). The statistical return period for PGAMCE is approximately 2,475 years. The probabilistic PGAMCE for the project area was calculated as being in the range of 0.85g to 0.90g using the United States Geological Survey (USGS, 2008) ground motion calculator (web-based). The design PGA was estimated to be in the range of 0.55g to 0.60g using the USGS ground motion calculator. These estimates of ground motion do not include near-source factors that may be applicable to the design of structures. The potential impacts due to ground shaking should be evaluated prior to design and construction of project improvements and incorporated into the design. Therefore, the potential impacts due to Page 6A-4

7 ground shaking are considered to have a less than significant impact with mitigation incorporation. Liquefaction Liquefaction is the phenomenon in which loosely deposited granular soils with silt and clay contents of less than approximately 35 percent and non-plastic silts located below the water table undergo rapid loss of shear strength when subjected to strong earthquake-induced ground shaking. Ground shaking of sufficient duration results in the loss of grain-to-grain contact due to a rapid rise in pore water pressure, and causes the soil to behave as a fluid for a short period of time. Liquefaction is known generally to occur in saturated or nearsaturated cohesionless soils at depths shallower than 50 feet below the ground surface. Factors known to influence liquefaction potential include composition and thickness of soil layers, grain size, relative density, groundwater level, degree of saturation, and both intensity and duration of ground shaking. The Plan area is located in an area mapped as potentially liquefiable on the State of California Seismic Hazards Zones map (CDMG, 1999b) (Figure 6). Historical high groundwater is reported at a depth of approximately 20 feet below the ground surface. Assessment of the potential for liquefaction would be evaluated prior to design and construction of the project improvements and incorporated into the design, as appropriate. Consequently, the potential impacts due to liquefaction are considered to have a less than significant impact with mitigation incorporation. Landslides Landslides, slope failures, and mudflows of earth materials predominately occur where slopes are too steep and/or the earth materials too weak to support themselves. Landslides may also occur by seismic ground shaking, particularly where high groundwater is present. In general, there are no steep slopes within the boundaries of the Plan area. Slopes bordering the northern portion of the Plan area are relatively steep and may be subjected to instability. Assessment of the potential for landslides or mudflows would be evaluated prior to design and construction of future improvements and incorporated into the design. Therefore, the potential for landslides is considered to have a less than significant impact with mitigation incorporation. Soil Erosion Soil erosion refers to the process by which soil or earth material is loosened and removed from its original location. Erosion can occur by many different processes and may occur at the project site where bare soil is exposed to moving water or wind. Future construction activities at the project site may result in ground surface disruption during excavation, grading, and trenching that would create the potential for erosion to occur. However, the erosion potential when the improvements are developed will be relatively minor due to the anticipated covering of construction areas with structures, pavements, and associated hardscape and landscaped areas. Surface drainage provisions would also reduce the potential for soil erosion at the site. Potential soil erosion related to the project development is considered to have a less than significant impact with mitigation incorporation. Subsidence Subsidence is typically associated with areas of groundwater withdrawal or other fluid withdrawal from the ground such as oil and natural gas, and could cause damage to project improvements, including foundations, structures, pavements, and other hardscape features. Our background review did not indicate that subsidence has been reported in the project area. Consequently, potential subsidence is considered to have a less than significant impact. Page 6A-5

8 Soil Settlement Loose natural soils or undocumented/poorly compacted fill may be present in some areas at the site. Compressible natural soils and poorly compacted fills pose the risk of adverse settlement under static loads imposed by new fill or structures. Differential settlement of soils can cause damage to project improvements, including foundations, structures, pavements, and other hardscape features. Assessment of the potential for soils prone to settlement would be evaluated prior to design and construction of project improvements. Therefore, the potential for soil settlement is considered to have a less than significant impact with mitigation incorporation. Expansive Soils Expansive soils generally result from specific clay minerals that have the capacity to shrink or swell in response to changes in moisture content. The ability of clayey soil to change volume can result in uplift or cracking to foundation elements or other rigid structures, such as sidewalks or slabs, founded on these soils. Expansive soils may be present in geologic units that underlie the project site. Assessment of the potential for expansive soils would be evaluated during the design phase of the project. Therefore, the potential for expansive soils is considered to have a less than significant impact with mitigation incorporation. Corrosive Soils The project site is located in a geologic environment that could potentially contain soil conditions that are corrosive to concrete and metals. Corrosive soil conditions may exacerbate the corrosion hazard to pipelines, foundations, and other buried improvements. Assessment of the potential for corrosive soils would be evaluated during the design phase of the project. Therefore, potential soil corrosivity is considered to have a less than significant impact during construction with mitigation incorporation. Methane Zones Portions of the site in the western and northern areas are located in a City of Los Angeles methane zone and the City of Los Angeles methane buffer zone. Methane gas in the soil could have an impact on conditions during construction of the future development including trench excavations or other subsurface construction activities. Assessment of the potential for methane would be evaluated during the design phase of the project and monitored during construction, as appropriate. Therefore, the potential for methane is considered to have a less than significant impact with mitigation incorporation. Groundwater Based on our limited background review, relatively shallow groundwater is anticipated at the site. Historic high groundwater is reported to be approximately 20 feet deep (CDMG, 1998b). Groundwater levels may be influenced by seasonal variations, precipitation, irrigation, soil/rock types, groundwater pumping, storm water infiltration, and other factors and are subject to fluctuations. Shallow perched conditions or seepage may be present in places. Subsurface construction activities for future development at the site may consist of relatively shallow excavations for building pads, foundations, utilities, and other improvements. Shallow groundwater, if encountered, may have an impact on the construction activities. The impacts may include stability of excavations, means and methods of construction and additional pressures to the structures. In the event a future storm water infiltration system is implemented, the future performance of the improvements may be impacted. The impacts may include hydrocollapse and soil settlement of potentially compressible soils, mounding of groundwater and increase in the potential for liquefaction. Further study, including subsurface exploration, would be performed during the design phase to evaluate the presence of groundwater, seepage, and/or perched groundwater at Page 6A-6

9 the site as well as the permeability of the soils and the potential impacts on design and construction of project improvements. Therefore, shallow groundwater is considered to have a less than significant impact with mitigation incorporation. Distinctive Geologic or Topographic Features This potential geologic impact refers to the future development s potential to cover or modify one or more distinct prominent geologic or topographic features. Rock exposures or other prominent geologic features were not observed on the surface at the site and are not anticipated at shallow depth. The existing topography of the project site is comprised of relatively flat gradients, and most of the Plan area has been developed with buildings and pavements. Prominent topographic features were not observed at the site. Future construction is expected to result in minor grading and trenching activities but will be matched with surrounding street gradients and is not anticipated to significantly alter the existing topography. Therefore, future development would not result in significant impacts related to the alteration or modification of prominent geologic or topographic features. Excavations Earthwork associated with future construction is anticipated to include excavations for the creation of building pads, parking areas, and trench excavations for utility lines. Potential deeper excavations may be anticipated for deeper foundation work for structures, if needed. Based on our background review and site reconnaissance, we anticipate that the materials encountered in excavations will be comprised predominantly of loose to dense sand, silty sand, clayey sand and silt, and very stiff clay. We anticipate that excavations within these alluvial materials at the project site will be feasible with conventional grading equipment. However, areas of cemented soils could present excavation difficulty if encountered at the site. The excavatability of cemented materials at the site would result in a less than significant impact to the future development. Excavations for proposed project improvements adjacent to existing streets, sidewalks, or structures will need to be performed with care to reduce the potential for differential movement of existing improvements located near the excavations. With appropriate mitigation incorporation during construction, excavations at the site would result in a less than significant impact to surrounding improvements. We anticipate that future development will be fenced during construction operations so that the public will not be exposed to the impacts of excavations. Construction personnel may be exposed to the impacts of excavations, and appropriate mitigative safety measures would result in a less than significant impact to site personnel. Since excavations will be filled following construction, the future project would not result or expose people to impacts related to excavations after construction of the project. Dam Inundation Based on our review of the County of Los Angeles Department of Regional Planning s Safety Element (1990), portions of the plan area are located within several potential dam failure inundation zones, including the dams for Devil s Gate Reservoir, Elysian Park Reservoir, and Hansen Lake. Dams in California are monitored by various governmental agencies (such as the State of California Division of Safety of Dams and the U.S. Army Corps of Engineers) to guard against the threat of dam failure. Current design and construction practices, and ongoing programs of review, modification, seismic retrofitting or total reconstruction of existing dams are intended to see that dams are capable of withstanding the maximum credible earthquake for the site. Due to the regulatory monitoring of dams and typical flood control measures that exist, the impact of inundation due to dam failure is not considered a significant constraint to the project. Therefore, the proposed project would not likely result or expose people to impacts related to dam failure inundation. Page 6A-7

10 Seiches and Tsunamis A seiche is the seismically induced sloshing of water in a large enclosed basin, such as a lake, reservoir, or bay. While the Elysian Park Reservoir is located to the north and east of the Plan area, due to its distance from the project, the potential for damage from seiches to the proposed project is considered low. Therefore, the proposed project would not result in, or expose people to, significant impacts related to seiches. Tsunamis are open-sea tidal waves generated by earthquakes. Tsunami damage is typically confined to low-lying coastal areas. Water surge caused by tsunamis is measured by distance of run-up on the shore. The project site is not mapped within the area considered to be susceptible to tsunami inundation (City of Los Angeles, 1996). Therefore, the proposed project would not result in, or expose people to, significant impacts related to tsunamis. Mineral Resources The CGS and the State Mining and Geology Board (SMGB) classify the regional significance of mineral resources in accordance with the California Surface Mining and Reclamation Act of 1975 (SMARA). The SMGB uses a classification system that divides land into four Mineral Resource Zones (MRZ) that have been designated based on quality and significance of mineral resources (CDMG, 1983). According to the State of California (CDMG, 1994), the Plan area is located in an area classified as MRZ-2 and MRZ-3 (Figure 8). MRZ-2 is defined as areas where adequate information indicates that significant mineral deposits are present, or where it is judged that a high likelihood exists for their presence. MRZ-2 roughly follows the Los Angeles River which is known for aggregate. In light of the existing concrete lined river channel, the loss of aggregate on this area is remote. MRZ-3 is defined as areas containing mineral the significance of which can not be evaluated from available data. Due to the abundance of similar mineralogical materials in the Plan area and in the surrounding vicinity, the potential of the project to result in the loss of availability of a known mineral resource is not considered a significant impact. Page 6A-8