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OFR 97–464: Page [3]

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agency are listed. There are approximately 180 contributed moment magnitudes. This catalog contains approximately 17,000 records from 1568 to December 1986.

No single catalog was sufficient for our task. Although PDE and DNAG might claim to be complete, we often favored CDMG, NCEER, NEWMEX, SRA, or USHIS because of the detailed research that went into them. We decided to build a new catalog by combining the original ones, and using our opinions about catalog reliability to eliminate duplicate records when an earthquake was listed in more than one catalog. Since attenuation relations, completeness, and magnitude-conversion rules all vary regionally, we decided to build two catalogs: a moment-magnitude (Mw) catalog for the western United States (WUS) and a body-wave-magnitude (mb) catalog for the central and eastern United States (CEUS). The WUS/CEUS catalog boundary (Figure 1) is the same as the WUS/CEUS attenuation boundary (Frankel et al, 1996).

We used a four-step algorithm to build new catalogs. First, we reformatted the original catalogs, writing each record in a common format that included its catalog provenance. For catalogs with multiple magnitude entries (PDE, DNAG, USHIS, and SRA) a single magnitude value was computed at this step (see below). Second, we concatenated the reformatted catalogs, and sorted the full catalog into chronological order. Third, based on our preferences, we chose a single survivor record when an earthquake was listed in more than one catalog (the provenance information was used here). Earthquakes were considered duplicates when their origin times were within one minute; times seemed more reliable than locations for this purpose. Fourth, we removed aftershocks and foreshocks using the sliding-time-and-distance-window algorithm of Gardner and Knopoff (1974). An earthquake was declared a "foreshock" when a larger event was encountered in its aftershock window. (For very old earthquakes the origin times reported in different catalogs can differ by more than one minute. In these cases one event is declared an aftershock or a foreshock of another, a somewhat roundabout way to avoid duplication.)

Western United States Catalog

The catalog preference order in WUS was: CDMG (highest preference) > USHIS > SRA > PDE > DNAG. We wanted to combine these lists in such a way that the final WUS catalog would be dominated by CDMG in California, and USHIS, SRA, and PDE elsewhere. The final WUS catalog needed to be complete down to magnitude 4....>>NEXT

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