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INPUT TO ANALYSIS <br /> Seismic Sources <br /> Seismic source characterization is concerned with three fundamental elements: (1) the <br /> identification of significant sources of earthquakes and the characterization of their source <br /> geometry; (2) the maximum size of these earthquakes; and (3) the rate at which they occur. <br /> The source parameters for the significant faults in the study region (defined in this study as <br /> generally within about 100 km; Plate 1) and an areal source, the Colorado Rocky Mountains, <br /> were estimated and used in the probabilistic analysis. <br /> Faults In North-Central Colorado <br /> Nine faults were identified within the study area in central Colorado that are thought to be <br /> active or potentially active (Plate 1 and Table 1). For this study, a fault is classified as <br /> "active" and is considered to be a potential source of future earthquakes if there is evidence <br /> for surface displacement in deposits younger than about 130,000 years (late Quaternary) <br /> and/or if it is associated with a moderate- to large-magnitude historical earthquake or pattern <br /> of microearthquakes suggestive of an active fault. A fault is classified as "potentially active" <br /> and is considered to be a potential source of future earthquakes if there is evidence for <br /> surface displacement during the Quaternary, but the age of the most recent event is <br /> unknown, or if it is likely to have been associated with a moderate- to large-magnitude <br /> historical earthquake or pattern of microseismicity suggestive of an active fault. <br /> Our characterization of these faults included an assessment of the following parameters: (1) <br /> potential rupture lengths; (2) fault dips; (3) thickness of the seismogenic crust; (4) estimated <br /> slip rates; (5) maximum magnitudes; and (6) fault activity. Parametric uncertainty is <br /> explicitly shown by assigning weights to each of the values. The potential rupture lengths <br /> were based on existing published and unpublished data. A fault dip of 60° ± 10' (weighted <br /> 0.3, 0.4, and 0.3 respectively; Figure 1) was estimated for all the faults in the study region. <br /> This range of dips is typically observed for Basin and Range normal faulting. The <br /> seismogenic depths for the faults and the areal source zone were estimated to be 15 km ± <br /> 5 km. The uncertainty reflects the range in the seismogenic crustal thickness which is <br /> HACONTRACT\TENMILE\8 8 M0412951500 <br />