Laserfiche WebLink
<br />Colorado. Based on these workshops and reevaluation of seismicity <br />data, Algermissen and others (1982) published revised probabilistic <br />estimates of maximum accelerations and velocities for the United <br />States. A maximum earthquake magnitude of 7.3 is postulated for the <br />general project azea; and maximum bedrock accelerations aze <br />estimated to range between 0.09 and 0.138 with a 90% probability of <br />non-exceedance in 250 yeazs (return period = 2373 yeazs). <br />Currently, the USGS has developed probabilistic estimates of ground <br />motion for either geographic coordinates or zip codes within the <br />United States. A seazch based on zip code for Buena Vista azea listed <br />the following peak ground accelerations (firm rock sites) for various <br />levels of non-exceedance: <br />TABLE I <br />PROBABILISTIC GROUND MOTIONS <br />USGS NATIONAL HAZARD MAPPING <br />Probability of 10% in 5% in 2% in <br />Non-Exceedance 50 years 50 years 50 years <br />Peak Ground 0.0332 g 0.0552 g 0.0966 g <br />Acceleration <br />Approximate 475 years 975 years 2475 years <br />Return Period <br />6.3 SEISMICITY, TECTONICS AND FAULTING <br />Correlation of diffuse background seismicity with geologic structure in <br />the Southern Rocky Mountains is exceedingly difficult. The appazent <br />lack of correlation is believed to be related to four factors (Arabasz and <br />Smith, 1981): (1) uncertain subsurface structure; (2) apparent discor- <br />dance between surface fault patterns and fault slip at depth; (3) a lack <br />of historic (within last 140 years) surface rupture; and (4) inadequate <br />focal depth resolution from regional seismic monitoring. Accordingly, <br />historic seismicity is of little value in defining seismogenic faults or <br />seismic risk within the Southern Rocky Mountains. Moreover, statisti- <br />cal relationships based on historic seismicity alone do not accurately <br />22 <br />