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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

CEUS Catalogs and b-value Calculation

The primary catalog used for the CEUS for longitudes east of 105 degrees is Seeber and Armbruster (1991), which is a refinement of the EPRI (1986) catalog. We supplemented this with the PDE catalog from 1985-1995. In addition, PDE, DNAG, Stover and Coffman (1993), Stover, Reagor, and Algermissen (1984) catalogs were searched to find events not included in Seeber and Armbruster (1991). Mueller et al. (1996) describes the treatment of catalogs, adjustment of rates to correct for incompleteness, the removal of aftershocks, and the assignment of magnitudes.

One of the most crucial parameters affecting the hazard maps is the magnitude of the events. For the CEUS, we used a catalog based on the mblg magnitude scale. We assumed that the preferred magnitudes in Seeber and Armbruster represented mblg. and that mblg equal mb in the CEUS. Treatment of magnitudes in the other catalogs is described in Mueller et al. (1996).

In a few cases, events were deleted or moved. For example the 1909 mb 5.0 event in northern Illinois was moved to the location specified in Stover and Coffman (1993) and consistent with felt reports. The preferred location is 1 degree north of that in Seeber and Armbruster(1991), placing it just west of Chicago. Based on information provided by Martin Chapman we moved the location of the 1916 Skyland event in western North Carolina further to the west. We also removed from the catalog earthquakes induced by fluid injection such as the events near Denver's Rocky Mountain Arsenal, events near Rangely Colorado, and events near Cogdell, Texas (Davis and Pennington, 1989).

Aftershocks and other dependent events were removed from the catalog before calculations of completeness times, and a and b-values (see Mueller et al., 1996).

Completeness times for different magnitude ranges (say events greater than mbref) were determined by plotting cumulative numbers of events against time for events with magnitude between mbref and mbref+0.5 (e.g. 3.0 and 3.5). An example of such a plot is shown in Figure 5a, for mb3.0-3.5 in the major portion of the CEUS. There is an increase of slope or detection rate at about 1924 and another increase of slope at about 1975. The rate appears to taper off from about 1985-1995. We use 1924 as the beginning of the counting period for events with magnitudes greater than 3.0. Next we adjusted the total rate of mb 3.0 and larger events to equal the rate observed since 1976 (see Mueller et al., 1996 for more details). The cumulative number plots indicate that mb 3.0 events are complete since about 1976. Figure 5b shows the cumulative number versus time plot for mb3.0-3.5 in the New Madrid area. There is a strong increase in detection rate starting about 1927. The rate then decreases until about 1950 when it increases to a fairly constant rate. Therefore, using 1976 as a start of the completeness period for mb 3.0 and larger events is reasonable for this region. Veneziano and Van Dyck (1986) and Bollinger et al. (1989) found a similar completeness time for mb3 events in specific areas in the CEUS.

For the entire CEUS catalog east of 105 longitude (excluding Charlevoix events), we calculated a b-value of 0.96±0.02, based on the maximum likelihood (ML) of Weichert (1980) using different completeness periods for different magnitude ranges (see Figure 6). The beginning of the completeness periods were: 1976 for mb3.0-3.9, 1940 for mb4.0-4.9, 1860 for mb5.0 to 7.0 and 1800 for mb>=7.0. A least-squares fit to the data also yields a b-value of 0.96 (Figure 6). Therefore, we used a regional b-value of 0.95 to calculate the hazard in the CEUS, based on rounding the calculated b-value of 0.96 to the nearest 0.05. For the Charlevoix region we used the b-value of 0.76 based on the work of John Adams, Stephen Halchuck and Dieter Weichert of the Geologic Survey of Canada (see Adams et al., 1996). This lower b-value was assigned to a roughly elliptical area about 40 km by 70 km encompassing the cluster of seismicity around Charlevoix.

Using a regional b-value of 0.95 ensures that the sum of the a-values in each of models 1 and 2 extrapolates to a rate of mb5 and greater events about equal to the historic rate (see Figure 6). Of course, model 3 is already based on the historic rate of mb5+ events. Thus, the maps are essentially constrained by the historic rates of mb5 and greater events in the CEUS.

 

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