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1. Geology and Seismic Hazards 8 PUBLIC SAFETY ELEMENT As required by State law, the Public Safety Element addresses the protection of the community from unreasonable risks from natural and manmade haz- ards. This Element provides information about risks in the Eden Area and delineates policies designed to prepare and protect the community as much as possible from the effects of: ♦ Geologic hazards, including earthquakes, ground-shaking, liquefaction and landslides. ♦ Flooding, including dam failures and inundation, tsunamis and seiches. ♦ Wildland fires. ♦ Hazardous materials. This Element also contains information and policies regarding general emer- gency preparedness. Each section includes background information on the particular hazard followed by goals, policies and actions designed to minimize risk for Eden Area residents. The Public Safety Element establishes mechanisms to prepare for and reduce death, injuries and damage to property from public safety hazards like flood- ing, fires and seismic events. Hazards are an unavoidable aspect of life, and the Public Safety Element cannot eliminate risk completely. Instead, the Element contains policies to create an acceptable level of risk. 1. GEOLOGY AND SEISMIC HAZARDS Earthquakes and secondary seismic hazards such as ground-shaking, liquefac- tion and landslides are the primary geologic hazards in the Eden Area. This section provides background on the seismic hazards that affect the Eden Area and includes goals, policies and actions to minimize these risks. 8-1 COUNTY OF ALAMEDA EDEN AREA GENERAL PLAN PUBLIC SAFETY ELEMENT A. Background Information As is the case in most of California, the Eden Area is subject to risks from seismic activity. The Eden Area is located in the San Andreas Fault Zone, one of the most seismically-active regions in the United States, which has generated numerous moderate to strong earthquakes in northern California and in the San Francisco Bay Area. The region experienced large and destruc- tive earthquakes in 1838, 1868, 1906 and 1989. Future earthquakes of equally grave proportions are a certainty in the San Francisco Bay region according to the Working Group on California Earthquake Probabilities (WG 02), estab- lished by the U.S. Geological Survey (USGS).1 Earthquakes can also give rise to various secondary seismic hazards, including ground shaking, liquefaction and subsidence, ground rupture and slope insta- bility. These seismic hazards can cause damage to structures, and risk the health and safety of citizens, particularly in unreinforced masonry buildings. 1. Measuring Earthquake Magnitudes The strength of an earthquake is generally expressed in two ways: magnitude and intensity. Magnitude, which is expressed in whole numbers and decimals (e.g. 7.1), is a measure that depends on the seismic energy radiated by the earthquake as recorded on seismographs. The original magnitude scale is the Richter scale (M),2 which has no upper limit and is not used to express dam- age.3 Earthquakes with magnitude of about 2.0M or less on the Richter scale are usually called microearthquakes and are not commonly felt by people. Events with magnitudes of about 4.0M and up are felt by most people. 1U.S. Geological Survey, 2003, Earthquake Probabilities In the San Francisco Bay Region: 2003-2032 by the Working Group on California Earthquake Probabilities. Open-File Report 03-214, page 1. 2 California Geological Survey, How Earthquakes and Their Effects are Meas- ured, Note 32, Revised April 2002. 3 USGS Earthquake Hazards Program (http://neic.usgs.gov/neis/general/ richter.html, accessed on January 27, 2005). 8-2 COUNTY OF ALAMEDA EDEN AREA GENERAL PLAN PUBLIC SAFETY ELEMENT The most commonly used magnitude scale today is the Moment Magnitude (Mw) scale, which is related to the physical size of fault rupture and the movement across a fault. Mw is based on the seismic moment4 at the source, or epicenter, of the earthquake. The Moment Magnitude scale is a way of rating the seismic moment of an earthquake with a simple, logarithmic nu- merical scale similar to the original Richter magnitude scale. Because it does not “saturate” the way local magnitude does, it is used for large earthquakes— those that would have a local magnitude of about 6.0M or larger.5 The force of an earthquake at a particular place is measured on the Modified Mercalli Intensity Scale, which is a subjective ranking of an earthquake’s ef- fects on persons and structures. It is expressed in Roman numerals from I to XII. Lower numbers on the scale indicate less severe shaking. Table 8-1 summarizes the Modified Mercalli Intensity Scale in relation to the Richter Scale. 2. Earthquake Faults The Hayward Fault, one of ten major faults that comprise the San Andreas Fault Zone, runs along the eastern edge of the Eden Area and links with the Rodgers Creek Fault to the north. Although the last major earthquake gen- erated by the Hayward Fault was in 1868, pressure is slowly building again and will begin to overcome the friction and other forces that are causing the fault zone to stick. Eventually, the accumulated energy will be released in another big earthquake.6 4 The seismic moment of an earthquake is determined by the strength or re- sistance of rocks to faulting multiplied by the area of the fault that ruptures and by the average displacement that occurs across the fault during the earthquake. (Source: Cali- fornia Geological Survey, How Earthquakes and Their Effects are Measured, Note 32, Revised April 2002.) 5 Sources: California Geological Survey, How Earthquakes and Their Effects are Measured, Note 32, Revised April 2002; http://www.seismo.berkeley.edu/ seismo/faq/magnitude.html; http://www.scecdc.scec.org/Module/sec3pg19.html. 6U.S. Geological Survey, USGS Open House 2000 Field Trip – Hayward fault zone, Hayward., http://wrgis.wr.usgs.gov/wgmt/sfbay/oh2000-ha-ft.pdf. 8-3 COUNTY OF ALAMEDA EDEN AREA GENERAL PLAN PUBLIC SAFETY ELEMENT TABLE 8-1 MODIFIED MERCALLI AND RICHTER SCALES Modified Expected Modified Definition Richter Mercalli Mercalli Maximum for this Plan Magnitude Category Intensity at Epicenter Usually detected only 2 I-II by instruments Minor 3 III Felt indoors Felt by most people; 4 IV-V slight damage Felt by all; many frightened Moderate 5 VI-VII and run outdoors; minor to moderate damage Everybody runs outdoors 6 VII-VIII moderate to major damage Major 7 IX-X Major damage 8+ X-XII Total and major damages Source: ABAG (http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html). According to WG 02, the fault system that includes the Hayward and Rod- gers Creek faults has a 27 percent probability of generating an earthquake with a magnitude greater than or equal to 6.7 on the Mercalli Richter Scale in the next 30 years. It is also the most likely fault in the Bay Area to be the site of a major earthquake in this time period. 7 The Hayward Fault is of particular concern to WG 02 because of the dense urban fabric of the communities adjacent to it, and the major infrastructure lines that cross it. A large earthquake on the Hayward Fault would, in all probability, cause extensive damage throughout the Eden Area. 7 U.S. Geological Survey, USGS Open House 2000 Field Trip – Hayward fault zone, Hayward., http://wrgis.wr.usgs.gov/wgmt/sfbay/oh2000-ha-ft.pdf, page 4. 8-4 COUNTY OF ALAMEDA EDEN AREA GENERAL PLAN PUBLIC SAFETY ELEMENT A moderate to major earthquake along the Hayward Fault is most likely to generate the strongest ground shaking in the area, but other regional faults, including the San Andreas, Calaveras, Rodgers Creek and others could also affect the Eden Area. Regional faults are shown in Figure 8-1. A moderate to major earthquake on any of these faults could topple buildings, disrupt infra- structure, cripple the transportation system and trigger landslides. 3. Secondary Earthquake Hazards Earthquakes often result in secondary hazards that may cause widespread damage. The three most likely secondary earthquake hazards in the Eden Area are ground-shaking, liquefaction and landslides. The USGS and the As- sociation of Bay Area Governments (ABAG) have worked together to map the likely intensity of ground-shaking throughout the Bay Area under various earthquake scenarios. The most intense ground-shaking scenario mapped in the Eden Area assumes a 6.9 magnitude earthquake on the Hayward Fault system. The predicted ground-shaking from such an earthquake would be “very violent” or “violent” throughout most of the Eden Area.8 The California Geological Survey has mapped liquefaction and landslide haz- ards. Liquefaction occurs when the strength of saturated, loose, granular ma- terials, such as silt, sand or gravel, is dramatically reduced as a result of an earthquake. This earthquake-induced deformation transforms a stable mate- rial into a temporary fluid-like state in which solid particles are virtually in 8 Association of Bay Area Government’s Earthquake Maps and Information website. http://www.abag.ca.gov/bayarea/eqmaps. Violent shaking (IX Mw) would produce heavy damage and general panic. Some masonry and frame buildings would face serious structural and foundation damage. Reservoirs and underground pipes would also be at serious risk. Conspicuous cracks and ground disruption particularly in alluvial areas, are anticipated. Very Violent shaking (X Mw) would result in ex- treme damage. Most masonry and frame structures would be destroyed with their foundations. Some well-built wooden structures and bridges destroyed. There could be serious damage to dams, dikes, embankments and large landslides. Water may also be thrown from waterbodies and sand and mud would shifted horizontally on beaches and flat land. Rails would be bent slightly by the motion. 8-5 H e Yolo a ld El s b u r g - 505 R ¦¨§ o d g er Davis Sonoma s C r e e Napa Sacramento k F a 80 u ¦¨§ Sacramento l ut101 t Amador Napa S a G n r e Solano A e n n 5 d ¦¨§ r V e Fairfield a a l s l F e a y (!99 u F Calaveras lt a u C San Marin l o t n Joaquin c o §80 rd ¦¨ F a u lt Stockton Contra Costa Mt.
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