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<br />Reclamation (USBR), and other government agencies and private <br />industry for seismotectonic hazard evaluations. The scope of work <br />included synthesis of previous seismotectonic studies in Colorado and <br />description of the regional seismotectonic setting of the dam site. The <br />PSHA consisted of three parts: (1) seismic source chazacterization <br />including location, geometry and earthquake recurrence; (2) definition <br />of an applicable attenuation relationship to provide estimates of peak <br />horizontal ground accelerations; and (3) combination of the contribu- <br />tion of all sources to seismic exposure at the dam site. The results of <br />the probabilistic analysis aze expressed as a relationship between peak <br />(bedrock) ground acceleration and probability of non-exceedance for <br />time periods of interest, ranging from 50 to 100,000 years. The <br />completed PSHA defines seismogenic structures/zones, recurrence <br />relationships, probabilistic ground motions, and response spectra for <br />use in engineering analysis and design. <br />During the course of our investigations, existing geological and <br />seismological literature in our files and the files of federal and state <br />agencies was reviewed. Potentially active faults and seismotectonic <br />provinces defined by the Colorado Geological Survey in the region <br />surrounding the site were examined in view of current data relating to <br />late Quaternary tectonic development of the Southern Rocky <br />Mountains. In addition to geological studies, historic seismicity data <br />(magnitude/intensity, geographic coordinates, focal depth, date, etc.) <br />within Colorado were compiled from files of the National Earthquake <br />Information Center, U.S. Geological Survey. From these data, we <br />assigned maximum earthquakes and recurrence relationships to each <br />known areal source zone and seismogenic fault likely to affect the <br />project azea. <br />seismogenic (active) faults and azeal source zones in Colorado incor- <br />porated in the PSHA for Trout Creek dam are illustrated on Figure 1. <br />Historic seismicity data including earthquake magnitudes and dates of <br />occurrence are tabulated in Appendix I. Earthquake recurrence <br />relationships developed from these data are illustrated on Figures 2 <br />through 7. Figure 8 presents seismic hazard curves for peak and effec- <br />tive accelerations with a 90% probability of non-exceedance in various <br />time periods of interest. Figures 9, 10 and 1 l present maximum earth- <br />quake ground motions and response spectra for 5%, 7% and 10% of <br />critical damping at the 50th and the 84th percentiles. The output from <br />SEISRISK III is included as Appendix II. Appendix III contains a <br />glossary of terms (Hays, 1980) commonly used in earthquake hazard <br />3 <br />