Item 26 - Adoption of the City of Santa Ana Emergency Operations Plan

Home Browse Search

City of Santa Ana Emergency Operations Plan Part I Basic Plan Earthquake as a Threat to Orange County - There are a large variety of earthquake events that could potentially affect Orange County and the City of Santa Ana. Figure 34 above shows the location of known faults in or near Orange County. Predicted ground shaking patterns throughout Southern California for hypothetical scenario earthquakes are available from the United States Geological Survey as part of their on -going "ShakeMap" program. A ShakeMap is a representation of ground shaking produced by an earthquake. The information it presents is different from earthquake magnitude and epicenter because ShakeMap focuses on the ground shaking produced by the earthquake, rather than the earthquake source or energy released. Therefore, while an earthquake has one magnitude and one epicenter, it produces a range of ground shaking levels at sites throughout the region depending on distance from the earthquake, the rock and soil conditions at specific locations, and variations in the seismic waves from the earthquake due to the structure of the Earth's crust. Earthquake scenarios are not earthquake predictions. That is, no one knows in advance when, where or how large a future earthquake will be. However, if we make assumptions about the size and location of a hypothetical future earthquake, we can make a reasonable prediction of the effects of the assumed earthquake, particularly the way in which the ground will shake. This knowledge of the potential shaking effects is the main benefit of the earthquake scenario for planning and preparedness purposes. The following maps describe earthquakes in terms of Instrumental Intensity, which is a Modified Mercalli Intensity estimated from instrumental ground motion recordings. Figure 42 below provides the Modified Mercalli Intensity (MMI) Scale: 41