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4.2 Geology and Soils Rambla Pacifico Street Reconstruction EIR Section 4.2 Geology and Soils 4.2 GEOLOGY and SOILS This section discusses impacts associated with geology and soils associated with implementation of the proposed project. This section studies how the stability of the slopes would impact the proposed project and residential areas located nearby. A Preliminary Geotechnical Engineering Study (Jan, 2007) and a Engineering Geology Analysis (September, 2007) were prepared for the proposed project by E.D. Michael, Consulting Geologist. A follow-up report by Michael was completed in October 2007. Additionally, GeoKinetics prepared reports based on the geology and factors of safety in September 2007, October 2007, January 2008, February 2008, April 2008, May 2008, October 2008, January 2009, March 2009, April 2009, May 2009, August 2009, October 2009, and November 2009. The reports were reviewed by City of Malibu and comments were prepared by the City of Malibu Geotechnical Staff dated August 2007, October 2007, February 2008, March 2008, January 2009, May 2009 and October 2009. In response to City comments, GeoKinetics prepared responses addressing issues raised in the comment letters. The GeoKinetic responses are dated January 2008, February 2008, August 2009, and September 2009. Revised grading plans and grading yardage verification for the proposed project were submitted in December 2009 by JT Engineering. The following analysis is based on the abovementioned documents, all of which are included in Appendix C. 4.2.1 Setting The project site is located within the City of Malibu, which extends about 27 miles along the Pacific Ocean coastline within southwestern Los Angeles County. Malibu lies within the coastal and foothill areas on the south side of the Santa Monica Mountain Range. The project site is within the Malibu Beach Quadrangle, which covers approximately 25 square miles of land comprising noncontiguous tracts of public lands, the easternmost portion of Solstice Canyon County Park, Pepperdine University (outside the City), Malibu Lagoon State Beach and the community of Las Flores. The southernmost boundary of the project site is located approximately 750 feet as the crow flies north of the Pacific Ocean coastline in the Santa Monica Mountain Range, which is in the western part of the Transverse Ranges Geomorphic Province of California. The valleys and mountains of the Transverse Ranges are typically bounded by a series of east-west trending, generally north dipping reverse faults with left-lateral, oblique movement. Bedrock beneath this area consists of Miocene geologic-aged volcanic and marine sedimentary rocks. This province is considered seismically active. The project site is at an elevation that ranges from approximately 77 to 386 feet above mean sea level and is located on a moderate to steeply sloping hillside. The surrounding coastal area is generally characterized by broad, gently to moderately sloping, and relatively continuous City of Malibu 4.2-1 Rambla Pacifico Street Reconstruction EIR Section 4.2 Geology and Soils a. Regional Geology. The faulting and seismicity of southern California is dominated by the compressionary regime associated with the intersection of the San Andreas Fault Zone and the Garlock Fault. The San Andreas Fault Zone separates two tectonic plates. The western side of the fault is the Pacific Plate and the eastern side of the fault is the North American Plate. The Western Plate is moving in a northwesterly direction relative to the North American Plate. The San Andreas Fault generally trends northwest to southeast. However, north of the Transverse Ranges Province, the fault trends more in an east-west direction, causing the fault’s right-lateral strike-slip movement to produce north-south compression between the two plates. This compression has produced rapid uplift of many of the mountain ranges in Southern California. According to the Southern California Earthquake Center, north-south compression in southern California has been estimated at between 5 and 20 millimeters per year (SCEC, 1995). b. Site Geology. Malibu is bounded by the Santa Monica Mountains to the north, the Pacific Ocean to the south, the Santa Monica Fault to the east and Ventura County to the west. The project site lies on the southern side of the Santa Monica Mountain Range, between Las Flores Canyon Road and the easterly termini of Paseo Hidalgo Street and Rambla Orienta Street, approximately 1,000 feet north of the Rambla Pacifico Street/Pacific Coast Highway intersection to the southernmost portion of the project. The major drainage in the vicinity of the site is Las Flores Creek, which is located adjacent to the east of the site and flows southward from the Santa Monica Mountains. According to the State of California Seismic Hazard Zone Report for the Malibu Beach Quadrangle (CDMG, 2001), the historic highest ground-water depth in the small coastal stream canyons, such as the Las Flores Creek located east of the project site, is estimated to be approximately 10 feet below surface grade, with depth increasing in their upper reaches. According to the Geologic Map of the Malibu Beach Quadrangle (Dibblee, 1993), the project site is comprised of primarily Holocene geologic age landslide debris. The landslide movement on the project site is depicted as being within the proposed roadway alignment (Dibblee, 1993). Geokinetics, Inc., prepared a February 26, 2008 report for the City of Malibu in response to the City’s Review Comments of February 14, 2008. Plate 1 in the Geokinetics report is a geologic map of the project area that depicts the proposed roadway as crossing a portion of the active Rambla Pacifico Landslide. Landslide material is moving downslope, toward Las Flores Creek east of the project site. Numerous ground tension cracks are mapped as existing all along the proposed roadway. The geologic map depicts Conejo Volcanics and Calabasas Formation as comprising the hillside beyond the mapped landslide areas. Near Las Flores Creek, the native surface material is mapped as Quaternary age alluvium. Plate 1 also depicts a fault crossing the southern portion of the proposed roadway and numerous faults north of the proposed roadway. These faults are also depicted as crossing the existing Las Flores Canyon Road east of the project area. The faults are not depicted as offsetting the landslide mass. Also, the fault near the southern portion of the proposed roadway is depicted as extending below some of the houses southwest of the proposed roadway, between the existing roads Calle del Barco and Rambla Pacifico Street. The February 26, 2008 Geokinetics report also depicts geologic cross sections through the proposed construction area. These cross sections show the slip plane of the landslide and depth City of Malibu 4.2-2 Rambla Pacifico Street Reconstruction EIR Section 4.2 Geology and Soils to groundwater. Soil borings are shown on the cross section. Over 30 soil borings were drilled by different investigators to study the geology of the area. Several of these soil borings were drilled to depths of over 200 feet within the general area of the proposed roadway. The borings identified the landslide slip plane and depth to groundwater. The cross sections depict several slip planes below the area. Depth to groundwater is shown on the cross sections as being 40 feet or more below grade. An estimate of the groundwater level at time of landslide movement that occurred in the early 1990s is shown at over 50 feet below surface grade in the general location of the proposed roadway. c. Seismic Hazards. The U.S. Geological Survey defines active faults as those that have had surface displacement within Holocene time (about the last 11,000 years). Surface displacement can be recognized by the existence of cliffs in alluvium, terraces, offset stream courses, fault troughs and saddles, the alignment of depressions, sag ponds, and the existence of steep mountain fronts. Potentially active faults are ones that have had surface displacement during the last 1.6 million years. Inactive faults have not had surface displacement within the last 1.6 million years. The Homeowners Guide to Earthquake Safety (California Seismic Safety Commission, 2005) briefly mentions and depicts seismic zones within California. As referenced in the Homeowners Guide, the International Building Code ranks the different regions according to their seismic hazard potential. Four regions have been established, Seismic Zones 1 through 4, with Zone 1 having the least seismic potential and Zone 4 having the highest seismic potential. As depicted in the Homeowners Guide to Earthquake Safety, the project site lies within Seismic Zone 4. Several active and potentially active faults are located in the general site vicinity (see Figure 4.2- 1). These nearby faults include: • Malibu Coast Fault (Active) • Las Flores Thrust Fault (Active) • Santa Monica Fault (Potentially Active) • Palos Verdes Hills Fault (Active) • Newport-Inglewood Fault (Active) According to the Geologic Map of the Malibu Beach Quadrangle (Dibblee, 1993), a portion of the Malibu Coast Fault is depicted as a concealed fault that trends in a northwest/southeast direction and lies approximately 1,500 feet southwest of the southern portion of the project site. The Geologic Map of the Malibu Beach Quadrangle depicts the Las Flores Thrust Fault as generally trending in a northwest/southeast direction and lies approximately 500 feet northwest and east of the project site. In addition to the before-mentioned nearby faults,
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