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OFR 97–464: Page [5]

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DNAG catalog.) To fix this, we used only the high-preference CDMG catalog in most of California. Finally, we removed man-made seismic events in the following areas: several coal-mining districts in central Utah since 1900 (Wong et al, 1989; W. Arabasz and J. Pechmann, personal communication, 1996) and the Nuclear Test Site in southern Nevada.

The final WUS catalog contained 2896 earthquakes with magnitude 4.0; the algorithm eliminated approximately 830 "foreshocks" and 4400 aftershocks (Appendix 1). Figure 2 shows the entire WUS catalog (file Figures 3a, 3b, 3c, 3d, and 3e, compare the contributions from each original catalog during 1964-1985, a period when all the catalogs were active.

We counted earthquakes on a 0.1o-by-0.1o grid, and normalized by the counting window duration to get a seismicity rate in each grid cell in WUS. In a zone encompassing most of California we counted earthquakes with 4 <= magnitude < 5 since 1933, 5 <= magnitude < 6 since 1900, and magnitude >= 6 since 1850. Considering the poorer catalog completeness in the rest of WUS, we used 1963, 1930, and 1850 for these magnitude ranges, respectively. This scheme is shown in Figure 4. The resulting catalog used for computing the hazard is plotted in Figures 5a, 5b and 5c

Central and Eastern United States Catalog

We included the Rocky Mountain and Colorado Plateau regions in CEUS, because we assumed that CEUS attenuation rules would be more appropriate there than WUS rules. The catalog preference order in CEUS was: NEWMEX (highest preference) > NCEER > USHIS > SRA > PDE > DNAG. We wanted to combine these lists in such a way that the final CEUS catalog would be dominated by NCEER east of -105o longitude before 1985, and SRA and PDE east of -105o after 1984. SRA and PDE would dominate west of -105o. (Due to late settlement of the western areas, such a western-extended CEUS catalog would not be uniformly complete, something we would need to keep in mind when adjusting seismicity rates for completeness -- see below.) The final CEUS catalog needed to be complete down to magnitude 3.

During the reformatting step each magnitude value was converted to an equivalent body-wave magnitude (called mb*), and for catalogs with multiple magnitude entries a weighted sum of these was used to compute a single body-wave magnitude value as follows.

mb: mb*=mb (3.0,4.0,6.8,7.0)
downweight for mb<4.0 (s/n) and mb>6.8 (saturation); downweight for year<1975...>>NEXT

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