J. Geology, Soils, and Seismicity
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IV. Environmental Setting, Impacts, and Mitigation Measures J. Geology, Soils, and Seismicity This section describes geologic and seismic conditions in the project vicinity and evaluates the potential for the proposed project to result in significant impacts related to exposing people or structures to unfavorable geologic hazards, soils, and/or seismic conditions. Potential impacts are discussed and evaluated, appropriate standard conditions of approval are identified, and mitigation measures are prescribed, as necessary. Setting Topography The city of Oakland includes the mountainous uplands of the Oakland-Berkeley Hills and an alluvial plain that slopes gently westward away from these hills to meet the flat marginal baylands of the San Francisco Bay. The project area is located on the alluvial plain, approximately ½ mile north of the Oakland Estuary. The ground surface in and around the project site is relatively flat and slopes gently southwest towards Highway 880. Ground elevations range from approximately 30 to 40 feet above mean sea level (msl). Geology The project area lies within the geologic region of California referred to as the Coast Ranges geomorphic province.1 The natural region of the Coast Ranges is between the Pacific Ocean and the Great Valley and stretches from the Oregon border to the Santa Ynez River near Santa Barbara. Discontinuous northwest-trending mountain ranges, ridges, and intervening valleys characterize this province. Much of the Coast Range province is composed of marine sedimentary and volcanic rocks that form the Franciscan Assemblage. The Franciscan Assemblage in this region of California represents some of the oldest rocks in the region, and consists primarily of greenstone (altered volcanic rocks), basalt, chert (ancient silica-rich ocean deposits), and sandstone that originated as ancient sea floor sediments. The San Francisco Bay is located in a broad depression in the Franciscan bedrock resulting from an east-west expansion between the San Andreas and the Hayward fault systems. The bedrock surface can be found at elevations of 200 to 2,000 feet below msl across the Bay Area. Sedimentary deposits overlie the Franciscan bedrock that originated from millions of years of erosion, deposition, and changes in sea level. Geologists categorize these sedimentary deposits into geologic formations based on the period of deposition and material type, as described below for the San Francisco Bay region. • The Alameda Formation is the deepest and oldest of these sedimentary deposits and consists of a mixture of clay, silt, sand, gravel, and some shells with predominantly silt and clay sediments surrounding discontinuous layers of sand and gravel; 1 A geomorphic province is an area that possesses similar bedrock, structure, history, and age. California has 11 geomorphic provinces. Gateway Community Development Project IV.J-1 ESA / 204358 Draft Environmental Impact Report August 2007 IV. Environmental Setting, Impacts, and Mitigation Measures J. Geology, Soils, and Seismicity • Overlying the Alameda Formation is the San Antonio Formation which consists of sandy clays, gravelly clays, clayey sands and gravels with interbedded silty clay deposits. • Younger alluvial deposits once referred to as the Temescal Formation are deposited on top of the San Antonio and consist of sandy clays, clayey sands, sands and gravels. The source material for these alluvial deposits comes from the Berkeley Hills. The underlying geology of the project site is mapped as alluvial fan and fluvial deposits of Holocene times. These deposits are characterized as unconsolidated, plastic, moderately to poorly-sorted silt and clay rich in organic material that formed from streams draining the nearby hillsides and standing floodwaters from the Bay (USGS, 2000). Site Soils According to the Soil Survey of Alameda County, Western Part, site soils belong to the Urban land-Clear Lake complex. This complex is comprised primarily of approximately 55 percent Urban land and 35 percent Clear Lake clay. Urban-land Clear Lake complex is characterized as very deep and poorly drained soils having no hazard of erosion, high shrink-swell potential, and high potential for differential settlement. The Urban land soil mapping unit occurs in areas where the soil material has been altered or mixed during urban development and consists of soils that are covered by structures and other development. The Clear Lake soil unit consists of clay and silty clay formed in alluvium that derived mainly from sedimentary rock (USDA, 1981). Seismicity The San Francisco Bay Area region contains both active and potentially active faults and is considered a region of high seismic activity (Figure IV.F-1).2 The 1997 Uniform Building Code locates the entire Bay Area within Seismic Risk Zone 4. The U.S. Geological Survey (USGS) Working Group on California Earthquake Probabilities has evaluated the probability of one or more earthquakes of Richter magnitude 6.7 or higher occurring in the San Francisco Bay Area within the next 30 years. The result of the evaluation indicated a 62 percent likelihood that such an earthquake event will occur in the Bay Area between 2003 and 2032 (USGS, 2003). The magnitude (M) is a measure of the energy released in an earthquake. The estimated magnitudes, described as moment magnitudes (Mw) represent characteristic earthquakes on particular faults (Table IV.J-1).3 Intensity is a measure of the ground shaking effects at a particular location. However, ground movement during an earthquake can vary depending on the overall magnitude, 2 An “active” fault is defined by the State of California as a fault that has had surface displacement within Holocene time (approximately the last 10,000 years). A “potentially active” fault is defined as a fault that has shown evidence of surface displacement during the Quaternary (last 1.6 million years), unless direct geologic evidence demonstrates inactivity for all of the Holocene or longer. This definition does not, of course, mean that faults lacking evidence of surface displacement are necessarily inactive. “Sufficiently active” is also used to describe a fault if there is some evidence that Holocene displacement occurred on one or more of its segments or branches (Hart, 1997). 3 Moment magnitude is related to the physical size of a fault rupture and movement across a fault. The Richter magnitude scale reflects the maximum amplitude of a particular type of seismic wave. Moment magnitude provides a physically meaningful measure of the size of a faulting event (CDMG, 1997b). The concept of “characteristic” earthquake means that we can anticipate, with reasonable certainty, the actual earthquake that can occur on a fault. Gateway Community Development Project IV.J-2 ESA / 204358 Draft Environmental Impact Report August 2007 IV. Environmental Setting, Impacts, and Mitigation Measures J. Geology, Soils, and Seismicity TABLE IV.J-1 ACTIVE FAULTS IN THE PROJECT SITE VICINITY Maximum Moment Distance and Magnitude Direction from Recency of Fault Historical Earthquake Fault Project Area Movement Classificationa Seismicityb (Mw)c Hayward 3 miles east Historic (1836; Active M6.8, 1868 7.1 1868 ruptures) Many <M4.5 Holocene Calaveras 16 miles east Historic (1861 Active M5.6–M6.4, 1861 6.8 rupture) Holocene M4–M4.5 swarms 1970, 1990 San Andreas 18 miles west Historic (1906; Active M7.1, 1989 7.9 1989 ruptures) M8.25, 1906 Holocene M7.0, 1838 Many <M6 Marsh Creek - 29 miles east Historic (1980 Active M5.6 1980 6.9 Greenville rupture) Holocene Concord - Green 22 miles Historic (1955) Active Historic active 6.9 Valley northeast Holocene creep Rodgers Creek 28 miles north Historic Holocene Active M6.7, 1898 7.0 M5.6, 5.7, 1969 a See Footnote 4. b Richter magnitude (M) and year for recent and/or large events. The Richter magnitude scale reflects the maximum amplitude of a particular type of seismic wave. c Moment magnitude (Mw) is related to the physical size of a fault rupture and movement across a fault. Moment magnitude provides a physically meaningful measure of the size of a faulting event (CDMG, 1997). The Maximum Moment Magnitude Earthquake, derived from the joint CDMG/USGS Probabilistic Seismic Hazard Assessment for the State of California, 1996. (USGS, 1996). SOURCES: Hart, 1997; Jennings, 1994; Peterson et al, 1996. distance to the fault, focus of earthquake energy, and type of geologic material. The composition of underlying soils, even those relatively distant from faults, can intensify ground shaking. The Modified Mercalli (MM) intensity scale (Table IV.J-2) is commonly used to measure earthquake effects due to ground shaking. The MM values for intensity range from I (earthquake not felt) to XII (damage nearly total), and intensities ranging from IV to X could cause moderate to significant structural damage.4 For comparison, the 1906 San Francisco earthquake (Mw 7.9) produced strong (VII) shaking intensities, while the 1989 Loma Prieta earthquake, with an Mw of 6.9 produced moderate (VI) shaking intensities in the project area. (ABAG, 2005a,b). Regional Faults The two main earthquake faults in the region are the San Andreas Fault Zone on the San Francisco Peninsula and the Hayward Fault Zone that extends along the east bay plain. These two faults are within the San Andreas Fault System, which marks the boundary between two 4 The damage level represents the estimated overall level of damage that will occur for various MM intensity levels. The damage, however, will not be uniform. Some buildings will experience substantially more damage than this overall level, and others will experience substantially less damage. Not all buildings perform identically in an earthquake. The age, material, type, method of construction, size, and shape of a building all affect its performance. Gateway Community Development Project IV.J-3 ESA / 204358 Draft Environmental Impact Report August 2007 PROJECT SITE Gateway Community Development Project . 204358 SOURCES: California Department of Conservation, Division of Mines and Geology (After Jennings, 1994) Figure IV.J-1 Regional Fault Map IV. Environmental Setting, Impacts, and Mitigation Measures J.