Trancas Canyon Park EIR Section 4.5 Geology and Soils 4.5 GEOLOGY AND SOILS The following analysis is based in part on a Geotechnical Report (July 2007) and a Grading Plan Review Report (June 2008), both performed by Geolabs-Westlake Village for the Trancas Park Project, as well as subsequent technical memorandums from Geolabs supplementing these reports. These reports and memorandums are included in their entirety as Appendix G to the EIR. 4.5.1 Setting Trancas Canyon is located within the city of Malibu, which extends over 26 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 City is within the Topanga, Malibu Beach, Point Dume, and Triunfo Pass Quadrangles which cover approximately 50 square miles of land comprising noncontiguous tracts of public lands, the westernmost portion of Malibu Creek State Park, Point Dume State Beach, Zuma Beach County Park, and Robert H. Meyer Memorial State Beach. The project site is located approximately one-half mile 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 site is at an elevation that ranges between 100 and 300 feet above mean sea level within the Santa Monica Mountains. Although the project site is generally located in a moderate to steeply sloping canyon, the surrounding coastal area is characterized by broad, gently sloping, and relatively continuous terrace surfaces above narrow to moderately wide beaches. Trancas Canyon and the surrounding areas are composed of undifferentiated alluvial deposits (stream- deposited, unconsolidated) of gravels, sands, and silt, which extend down in the canyon. 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 to 20 millimeters per year (SCEC, 1995). b. Site Geology. The project site is located between Trancas Canyon Road and Paseo Canyon Drive, one half mile from the Pacific Coast Highway in the city of Malibu. The City of 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 is located at City of Malibu 4.5-1 Trancas Canyon Park EIR Section 4.5 Geology and Soils approximately 6050 Trancas Canyon Road within the City of Malibu on generally east-facing manufactured and natural slopes (Geolabs, June 30, 2008) on the southern side of the Santa Monica Mountain Range. The major drainage in the vicinity of the site is Trancas Creek, which is located adjacent to the east of the site and drains south from the Santa Monica Mountains. According to the State of California Seismic Hazard Zone Report for the Point Dume Quadrangle (CDMG, 2001), the estimated historic highest groundwater depth in Trancas Canyon was 10 feet below grade. According to the Geologic Map for the project area prepared by Geolabs (June 30, 2008, Plate 1.2), the site is underlain by artificial fill, topsoil, colluvium, landslide deposits, Zuma Volcanics, the Encinal Canyon Member of the Topanga Formation, and the Vaqueros Formation. According to Geolabs, several generations of artificial fill exist at the site, with a large canyon fill underlying the lower flat pad area. The large canyon fill was an engineered fill emplaced in 1964 according to the standards of care at that time. Based on Cross Section A-A’, shown on Plate 2.0 of Geolab’s June 30, 2008 report, the maximum depth of the large canyon fill is approximately 60 feet. Other areas of undocumented artificial fill are also present at the site. Topsoil, defined as residual soil that caps the bedrock, was observed up to two feet thick by Geolabs (June 30, 2008), and reported up to five feet thick by previous geological consultants of record. Colluvium consisting of sandy clay and clayey sand with locally derived rock fragments was observed on the lower portions of existing natural slopes and upslope (and offsite) of the existing canyon fill (Geolabs, June 30, 2008). Four landslides have been mapped in the northwest trending ravine north of the existing lower fill pad. According to Geolabs (June 30, 2008), three of the landslides are outside of the proposed park boundaries, but are elevated above the proposed park site. The fourth landslide extends onto the northwestern corner of the existing upper pad area onsite (Geolabs, June 30, 2008). Zuma Volcanics are exposed in the southwestern and southern portions of the project area, south of the Malibu Coast Fault. The Zuma Volcanics generally consist of fractured, orange- brown andesite with interbeds of shale, siltstone and sandstone of the Trancas Formation (Geolabs, June 30, 2008). The Encinal Canyon Member of the Topanga Formation, consisting of dark brown to black fractured siltstone and claystone, was encountered in the northwestern portion of the project area, north of the Malibu Coast Fault. The Vaqueros Formation, a medium to coarse grained sandstone and conglomerate with interbedded platy to shaly siltstone and mudstone exists in the northeastern portion of the project area, north of the Malibu Coast Fault (Geolabs, June 30, 2008). Perched groundwater was encountered by Geolabs during their exploratory excavations onsite (June 30, 2008). The perched groundwater was observed to be emanating from high-angle fractures in the bedrock, and was encountered between depths of 40 to 51 feet below ground surface. Geolabs indicated that the regional groundwater table would be expected to occur at or below the elevation of the thalweg of Trancas Canyon Creek (June 30, 2008). City of Malibu 4.5-2 Trancas Canyon Park EIR Section 4.5 Geology and Soils c. Seismic Hazards. Seismic Potential. 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. The proximity of active faults is such that the project area has experienced and will continue to experience strong seismically induced ground motion. 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 bluffs 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 those 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. Several active and potentially active faults are located in the general site vicinity (see Figure 4.5- 1). These nearby faults include: • Malibu Coast Fault (Active) • Santa Monica Fault (Potentially Active) • Palos Verdes Hills Fault (Active) • Newport-Inglewood Fault (Active) According to the Geologic Map for the site by Geolabs (June 30, 2008), which is a compilation of their work onsite in addition to work performed by previous geologists, the Malibu Coast Fault trends northwest/southeast through the subject property, with two splays mapped (one of which is shown as concealed). Although the proposed park site is not located within a State- defined Alquist-Priolo Earthquake Fault Zone, the Malibu Coast Fault Zone is considered active by the City of Malibu and the Los Angeles County Seismic Safety Element unless proven otherwise (Geolabs, June 30, 2008). Other faults in the Southern California area that have the potential to seismically affect the site include the San Gabriel Fault, the San Andreas Fault, and the probable existence of large blind thrust faults currently undocumented. Faults generally produce damage in two ways: surface rupture and seismically induced ground shaking. Surface rupture is limited to areas very near the fault and ground shaking covers a wide area. Surface Rupture. Surface rupture along a fault is the surface expression of fault displacement. Fault displacement occurs when material on one side of a fault moves relative to the material on the other side of the fault. Surface displacement can range from a few inches to tens of feet during a rupture event. This can have disastrous consequences, including injury and loss of life, when buildings are located within the rupture zone.