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-6- <br />ground acreleration at the site caused by this earthquake would have <br />been about 0.039. In our opinion, it is likely that future • <br />earthquakes will occur in the Rangely Oil Field as Tony as secondary <br />oil recovery operations are under way; however, we do not feel that <br />earthquakes ouch stronger than those which have occurred in the past <br />will occur in the future. <br />Algermissen and Perkins (1976) have prepared probabilistic <br />estimates of naximnm peak ground acceleration for the United States <br />based on historic seismic activity. Their study indicates that <br />maximum horizontal ground acceleration with a recurrence interval of <br />475 years for the Rangely area would be 0.049. <br />Potentially Capable Faults: Studies done by the Colorado Geological <br />Survey indicate that the faults adjacent to the Uinta Ards to the <br />north of the site may be potentially capable; however, more detailed • <br />field investigtions would be required to confirm this. For the <br />purpose of our investigation, we have considered these fault zones to <br />be capable of generating earthquakes during the service life of the <br />project. The most significant fault for seismic exposure at the <br />disposal site is the Yampa Fault which is located about 18 miles to <br />the north. The Yampa Fault has a total length of 72 miles. It is <br />estimated that a maximum credible earthquake for this fault zone would <br />be a magnitude 6.4. The maximum credible earthquake was calculated <br />based on fault length and magnitude correlations given by Slemorons <br />(1977). The rupture length used in calculating the maximum credible <br />magnitude was one-half the total fault length. Based on magnitude and _ <br />peak horizontal grourd acceleration-attenuation relationships, Donovan <br />• <br />