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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

The background source zones for the WUS (model 2) were based on broad geologic criteria and were developed by discussion at the Salt Lake City (SLC) workshop (except for the Cascades source zone). These zones are shown in Figure 18. Note that we do not have background source zones west of the Cascades and west of the Basin and Range province. For those areas, we simply used model 1 with a weight of 1.

At the SLC workshop, we started with one large source zone encompassing the Basin and Range Province as well as other areas of extensional tectonics and the region of compressional tectonics in eastern Washington. Then we asked workshop participants to choose areas that they thought should be separate source zones because of their tectonic uniqueness. There was substantial sentiment for a Yellowstone Parabola source zone (see, e.g., Anders et al., 1989) that would join up seismically-active areas in western Wyoming with the source areas of the Borah Peak and Hebgen Lake earthquakes. It was felt that the relatively seismically-quiescent areas consisting of the Snake River Plain and Colorado Plateau should be separate source zones because of the geologic characteristics. An area of southwest Arizona was suggested as a separate source zone by Bruce Schell, based partly on differences in the age and length of geologic structures compared with the Basin and Range Province (see Euge et al., 1992). We have since added a Cascades source zone since we felt that was a geologically-distinct area.

The remaining background source zone includes the Basin and Range Province, the Rio Grande Rift, areas of Arizona and New Mexico, portions of west Texas, and areas of eastern Washington and northern Idaho and Montana. The northern border of this zone follows the international border. As stated above, we think this is a valid approach since we are interested in basing the hazard maps on the seismicity rate in the area of interest.

This large background zone is intended to address the possibility of having large earthquakes (M6 and larger) in areas with relatively low rates of seismicity in the brief historic record. It is important to have a large zone that contains areas of high seismicity in order to quantify the hazard in relatively quiescent areas such as eastern Oregon and Washington, central Arizona, parts of New Mexico, and west Texas. One can see the effect of this large background zone by noting the contours on the hazard maps in these areas. Of course the prominence of the background zones in the maps is determined by the weighting of models 1 and 2.

a-values were determined for each background zone by counting the number of events with M>=4.0 since 1963 in each zone. As with the CEUS, the area-normalized a-values were distributed into a set of grid cells. The hazard was calculated directly from these gridded a-values. Fictitious finite faults centered on each grid cell were used for M>=6.0.

In Figures 19, 20, 21, we show the effect of adding the background zones to one of the hazard maps made from the WUS runs. Figures 19 is the hazard map made from model 1 (the gridded seismicity), not including faults. Figure 20 contains the hazard map constructed just from model 2, the background zones. The combination map (using weights of 0.67 and 0.33 for models 1 and 2, respectively) is shown in Figure 21. The combined map significantly raises the hazard in eastern Washington and Oregon. The map exhibits a zones of higher hazard crossing Arizona from the northwest to southeast. It also tends to emphasize the hazard along the Rio Grande Rift in New Mexico and southern Colorado. It displays a zone of higher hazard that extends past El Paso into west Texas, including the sites of the 1931 Valentine and 1995 Alpine earthquakes. The combined map also shows somehat higher hazard along the Yellowstone Parabola than the map based on just the gridded seismicity.

The combined hazard map (Figure 21) does lower the hazard somewhat in areas of high seismicity such as western Nevada and central Utah. This is the price to be paid for raising the hazard in areas of low seismicity, while maintaining the total seismicity rate. This is essentially insurance for possible non-stationarity in earthquake occurrence.

Figure 22 shows the mean hazard curves for several western cities (Salt Lake City, Reno, Las Vegas, Phoenix and El Paso) for the cases with and without the background zones (both cases include the hazard from the faults). The hazard curve for Reno is the highest of this group and it decreases somewhat when the background zones are included. The hazard curves for Salt Lake City and Las Vegas are barely affected by the inclusion of the background zones. The curve for Phoenix is raised substantially by the background zones, since it is in an area of relatively low historic seismicity. The curve for El Paso is raised the most by the background zones. El Paso is in an area of relatively low seismicity, but certainly has the potential for large earthquakes with active faults nearby. The peculiar shape of the hazard curve for El Paso for the case without the background zones is caused by its proximity to Quaternary faults with relatively long recurrence times.

 

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