91-0021 NCEER-91 Earthquake Catalog: Improved Intensity-based Magnitudes and Recurrance Relations for U.S. Earthquakes East of New Madrid L.Seeber, J.G.Armbruster 1991 Lamont Doherty Geological Observatory 110 PB92-176742 15 A new catalog of earthquakes in the eastern U.S. entitled The NCEER-91 Earthquake Catalog stems from existing compilations and from new data. NCEER-91 is derived from the EPRI (1987) catalog, and is based on new data from archival searches and on published compilations not fully incorporated in EPRI. This paper presents results from work in progress and is primarily concerned with earthquake magnitudes. The data discussed is from the area east of the New Madrid seismic zone (east of 85.5W and north of 30.0N) and magnitudes M greater than or equal to 3. In this area, NCEER-91 drastically reduces the number of magnitudes based on maximum intensity (Mmi) by providing data for more reliable felt-area magnitudes (Mfa), most of them during the periods of completeness. B-values and repeat times vary more smoothly with different low-magnitude limits and are internally more consistent in NCEER-91 than in EPRI/V. Differences in b-value and repeat times between NCEER-91 and EPRI/S, however, are insignificant. Overall rates of seismicity in sub-regions (1) east and (2) west of the Appalachian crystalline front are similar and no significant differences in their statistical parameters are detected. Considering statistical confidence limits as well as variations derived from alternative procedures, the authors conclude that b-values are approximately 1.05, and repeat times for M greater than 6 are approximately 200 years in each of these regions. Eastern United States, Magnitude, Central United States, NCEER-91 Earthquake Catalog, B-Values, Seismicity, Seismic Hazard Assessment, Recurrence Intervals, and Earthquake Catalogs.