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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

Background Source Zones (model 4)

The background source zones are intended to quantify seismic hazard in areas that have not had significant historic seismicity, but could very well produce sizeable earthquakes in the future. In essence, these zones address the possible non-stationarity of seismicity. Background source zones were largely agreed upon in the Memphis workshop and are shown in Figure 11. They consist of a cratonic zone, an extended margin zone, a Rocky Mountain zone, and a Colorado Plateau zone. The latter zone was discussed at the Salt Lake City workshop. The Rocky Mountain zone was not discussed at any workshop, but is clearly defined by the Rocky Mountain front on the east and the areas of extensional tectonics to the west, north and south. As stated above, the dividing line between the cratonic and extended margin zone was drawn by Rus Wheeler based on the westward and northern edge of rifting during the opening of the Iapetan ocean. One justification for having craton and extended crust zones is the work done by Johnston et al. (1994). They compiled a global survey of earthquakes in cratonic and extended crust and found a higher seismicity rate (normalized by area) for the extended areas.

Some of the boundaries of the background zones are drawn along the international borders or along the coastline. Although these may seem arbitrary, we think it is valid to use the international borders since we want the hazard maps to reflect the area-normalized rates of seismicity in the broad area of interest for this set of maps, i.e., the United States. The boundary along the coast is necessary because the catalog is obviously incomplete for offshore earthquakes.

a-values for each background zone were determined by counting the number of mb3 and larger events within the zone since 1924 and adjusting the rate to equal that since 1976. The area-normalized a-value was then disaggregated into a set of grid cells, so that we could calculate the hazard using the same computer code used with the smoothed historic seismicity. A b-value of 0.95 was used for all the background zones, based on the b-value found for the entire CEUS.

 

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