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| Quake & Shake Press Release |
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News Release U.S. Department of the Interior U.S. Geological Survey Western Region Center 345 Middlefield Road Menlo Park, CA 94025-3561 Release Upon Receipt Sept. 10, 2000 __________________________________________________________________________ NAPA DAMAGE LINKED TO UNUSUALLY HIGH LEVELS OF SHAKING Strong motion instruments recorded unusually high levels of shaking in the city of Napa as a result of the September 3, Magnitude-5.2 earthquake near Yountville, Calif. Recordings of strong shaking by stations operated throughout the region by the U.S. Geological Survey (USGS), the California Department of Conversation's Division of Mines and Geology (CDMG), and UC Berkeley demonstrate that peak shaking levels in the city of Napa were amplified five to eight times relative to a station located in the mountains less than a mile from the earthquake epicenter. Both the high levels and local amplification help explain the surprising concentration of earthquake damage throughout the city, according to scientists from the three agencies. The highest recorded level of shaking came from an instrument located in a fire station north of the city of Napa, approximately six miles from the epicenter. The shaking there was 50 percent of the force of gravity. This means that buildings at the site were subjected to a horizontal force that was 50 percent of the building's weight. A station located south of town, near Napa College, recorded a peak shaking level nearly as high. "Both recordings are substantially higher than expected for a magnitude 5.2 earthquake and are consistent with the significant damage that the city suffered," said Dr. Mary Lou Zoback, chief of the USGS Earthquake Hazards Team in Menlo Park. While earthquake shaking levels depend on the distance from the earthquake source, the high level of ground shaking in Napa also appears to be the result of two other factors: first, the amplification of shaking by young sediments along the Napa River, and second, the focusing of strong motion to the southeast, the direction the earthquake rupture propagated. A regional map of shaking levels, "ShakeMap," can be viewed on the web at "Moderate earthquakes like this can occur anywhere in California. They serve as a reminder of California's earthquake risk and need for preparedness," said Dr. James Davis, State Geologist of California. All three agencies are actively upgrading seismic instrumentation in the greater San Francisco Bay region to obtain high-fidelity strong-motion recordings of local earthquakes, according to Zoback and Davis. "Data such as those collected in the Yountville earthquake enable seismologists to construct detailed models that more accurately predict ground motions for large earthquakes forecast for the region. These results, in turn, can guide the revision of building codes that ultimately prevent failures of structures," they noted. As the Nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the Nation's natural resources, and enhance the quality of life by monitoring water, biological, energy and mineral resources. More information on USGS programs is available online at www.usgs.gov.
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