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<br /> <br />5. Hazard Identification, Analysis, Assessment <br />5.1 Hazard Characterizations <br />5.1.1 Earthquake <br />Earthquakes occur when two tectonic plates slip past each other beneath the earth’s <br />surface, causing sudden and rapid shaking of the surrounding ground. Earthquakes <br />originate on fault planes below the surface, where two or more plates meet. As the plates <br />move past each other, they tend to not slide smoothly and become “locked,” building up <br />stress and strain along the fault. Eventually the stress causes a sudden release of the <br />plates, and the stored energy is released as seismic waves, causing ground acceleration to <br />radiate from the point of release, the “epicenter.” <br />The Bay Area is in the heart of earthquake country. Major faults cross through all nine Bay <br />Area counties. Every point within the Bay Area is within 30 miles of an active fault, and 97 <br />of the 101 cities in the Bay Area are within ten miles of an active fault. <br />The total amount of energy released in an earthquake is described by the earthquake <br />magnitude. The moment magnitude scale (abbreviated as M) is logarithmic; the energy <br />released by an earthquake increases logarithmically with each step of magnitude.3 For <br />example, a M6.0 earthquake releases 33 times more energy than a M5.0, and a M7.0 <br />earthquake releases 1,000 times more energy than a M5.0 event. <br />The quantified size or measurement of an earthquake is dependent on factors that include <br />the length of the fault and the ease with which the plates slip past one another. In the Bay <br />Area, technical specialists have observed varied fault behaviors, giving some sense of which <br />faults may or may not produce a large, damaging earthquake. Earth scientists are most <br />concerned about the San Andreas and Hayward faults, believed most likely to produce <br />large, regionally damaging earthquakes. There are, however, many other Bay Area faults <br />that can produce localized damage. <br />Additionally, earthquakes are often not isolated events, but are likely to trigger a series of <br />smaller aftershocks along the fault plane, which can continue for months to years after a <br />major earthquake, producing additional damage. <br /> <br /> <br /> <br /> <br />3 USGS (2014)