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<br /> <br /> <br /> <br /> <br /> <br />r <br /> <br />LJ <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />h <br />~~ <br /> <br />~~ <br />INPUT TO ANALYSIS <br />Seismic Sources <br />Seismic source chazacterization is concerned with three fundamental elements: (1) the <br />identification of significant sources of earthquakes and the chazacterization 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 Rcek}~ 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 amoderate- to large-magnitude historical earthqual:e or pattern <br />of micrcearthquakes 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 unknown, <br />or if it is likely to have been associated with amoderate- to lazge-magnitude historical <br />earthquake or pattern of microseismicity suggestive of an active fault. <br />Our characterization of these faults included an assessment of the following pazauneters: (1) <br />potential rupture lengths; (2) fault dips; (3) thickness of the seismogenic crust; (•A) estimated <br />slip rates; (5) maximum magnitudes; and (6) fault activity. Pazametric un~certalnty 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were based on existing published and unpublished data.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° t 1(i° (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 intermountain U.S. normal faulting (Doser and <br />Smith, 1989). The seismogenic depths for the faults and the areal source zone were estimated <br />to be 15 km * 5 km. The uncertainty reflects the range in the seismogenic crustal thickness <br />which is generally thicker beneath the Colorado Rocky Mountains than beneath the Rio <br />H:\CON7RAC1\274561DUR8 g M0309951G09 <br />