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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

Additional Notes for CEUS

One criticism of the use of gridded historic seismicity is that it assumes that seismicity is stationary in time, so that areas of high activity in the past will remain so in the near-future. It has been suggested that using areal source zones based on geologic or geophysical criteria obviates the problem of non-stationary seismicity. We disagree. When a-values are determined for areal source zones they are necessarily based on the historic catalog. Therefore, they are still at the mercy of non-stationarity. This problem becomes even more severe as one tries to pick small source zones over localized features which may be tectonic. The more localized the source zone, the more likely you will not have a representative sampling of the a-value for this feature from the historic seismicity. This is one reason why we preferred to use large background source zones. This is one way to combat any non-stationarity in the seismicity by assuming that seismicity rates averaged over large areas are more stable in time.

It is useful to compare our maps to the source zones used in the EPRI (1986) study. For the areas to the north and west of New Madrid, most of the six EPRI teams had three source zones in common: 1) the Nemaha Ridge in Kansas and Nebraska, 2) the Colorado-Great Lakes lineament extending from Colorado to the western end of Lake Superior, and 3) a small fault zone in northern Illinois, west of Chicago. Each of these source zones are apparent as higher hazard areas in the our maps. The Nemaha Ridge is outlined in the maps because of magnitude 4 and 5 events occurring in the vicinity. Portions of the Colorado-Great Lakes lineament show higher hazard in the map, particularly the portion in South Dakota and western Minnesota. The portion of the lineament in eastern Minnesota has been historically inactive, so is not apparent on the our maps. The area in western Minnesota shows some hazard because of the occurrence of a few magnitude 4 events since 1860. A recent paper by Chandler (1995), argues that the locations and focal mechanisms of these earthquakes are not compatible with them being on the lineament, which is expressed as the Morris Fault in this region. The area in northern Illinois has relatively high hazard in the our maps because of M4-5 events that have occurred there.

Frankel (1995) found good agreement in the mean PE's and hazard curves derived from models 1-3 and 4 and those produced by the EPRI (1986) study, when the same PGA attenuation relations were used.

 

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