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72 5.3.3 SURFACE FAULT RUPTURE A fault is a point of displacement along the fractures of the earth’s crust caused by shifting tectonic plates. When an earthquake occurs, there is a rupture on a fault as built-up energy is suddenly released. Active faults are those that have ruptured in the past 11,000 years.12 Often the rupture occurs deep within the earth, but it is possible for the rupture to extend to the surface and create visible above-ground displacement, called “surface rupture.” The California Geological Survey (CGS) publishes maps of active Bay Area faults that could produce surface rupture, as required by the Alquist-Priolo Earthquake Fault Zoning Act (1972).13 These maps show the most comprehensive depiction of fault traces that can rupture the surface, and the zones directly above and surrounding the fault traces. Cities and counties require special geologic studies within these zones to prevent construction of human- occupied structures. PAST OCCURANCE AND POTENTIAL FOR OCCURANCE: As previously indicated, The Hayward Fault runs directly through San Leandro, and has the potential for significant damage to the city if a major earthquake were to occur. In the 1868 Hayward earthquake San Leandro experienced major damage to infrastructure, since that time significant improvements in building safety has been made. The San Andreas Fault is 14 miles west of San Leandro, depending upon the magnitude and epicenter of the earthquake, an earthquake along the San Andreas could produce similar damage to the City as the 1989 Loma Prieta earthquake did. Due to the proximity of the Hayward Fault, and its history of producing large destructive earthquakes, San Leandro faces a potential threat of surface fault rupture. The City’s primary concerns after a large earthquake would be:  Loss of life and injury due to infrastructure failure  Water main breaks due to aging water line infrastructure  Fire due to ruptured gas lines and infrastructure failure  Roadway failure  Need for mass shelter 5.3.4 GROUND SHAKING When faults rupture, the slip generates vibrations or waves in the earth that are felt as ground shaking. Larger magnitude earthquakes generally cause a larger area of ground to shake, and to shake more intensely. As a result, one principal factor in determining anticipated levels of shaking hazard in any given location is the magnitude of expected earthquakes. The intensity of ground shaking felt in one area versus another, however, is based on the magnitude and other factors including distance to the fault; direction of rupture; and, the type of geologic materials at the site. For example, softer soils tend to amplify ground shaking, while denser materials limit ground shaking impacts at the site surface. 111