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Open-File Report 96-532

National Seismic Hazard Maps: Documentation June 1996

By Arthur Frankel, Charles Mueller, Theodore Barnhard, David Perkins, E.V. Leyendecker, Nancy Dickman, Stanley Hanson, and Margaret Hopper

Combining the Models

The inclusion of background zones lowers the probabilistic ground motions in areas of relatively high historic seismicity while raising the hazard to only low levels in areas with no historic seismicity. Figure 12 shows a hazard map (PGA 2% PE in 50 years) without the background zones (using models 1-3 and 5 with weights of 0.5,0.25,0.25,1., respectively), while Figure 13 contains a hazard map derived solely from the background zones (model 4 only). The probabilistic ground motions from the background zones are not small, relative to the values in much of the original map. In areas of low seismicity such as Iowa, Wisconsin, and Florida, the ground motions from the background zone are much higher than those from models 1-3, and 5 (compare Figures 13 and 12.). We combined the five models using weights of 0.4, 0.2, 0.2, 0.2, and 1.0 , respectively, to make the map in Figure 14. Note that this decreases the values relative to Figure 12 in areas of higher seismicity such as eastern Tennessee, New York City, and the northern New York State-Ottawa region. Now the mapped values (Figure 14) are significantly higher in quiescent areas such as Florida, Texas, Iowa, and Wisconsin than the map without the background zones (Figure 12). This increase is at the expense of the more active areas, which are lower in Figure 14 than in Figure 12.

The effects of including the background zones can also be visualized from the hazard curves. Figure 15 displays two sets of hazard curves for six CEUS cities (Memphis, New York City, Chicago, Houston, St. Paul, and Miami): one set made with the background zones (models 1-5; dashed lines) and one set made without them (models 1-3,5; solid lines). Adding the background zones slightly lowers the hazard curves for Memphis and New York City, while substantially raising the hazard curves in Houston, St. Paul, and Miami. Although the hazard curves in the latter cities are still not high after including the background zones, their PE's are increased by a factor of 2-3 relative to the values found without the background zones. The hazard curve for Chicago is not significantly affected by the inclusion of the background zones. Note the difference in the slope of the hazard curves for St. Paul and Miami. This may be caused by the difference Mmax values, since St. Paul is in the craton zone and Miami is in the extensional zone.

 

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