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The estimated maximum credible earthquake (MCE) for the Colorado Plateau Province in Colorado <br />is in the range of M 5.5 to 6.5 (Kirkham and Rogers, 1981). This range coincides with a probabilistic <br />epicenter distance of 18 to 30 km for the MCE random earthquake in this azea of Colorado, as <br />reported by GEI (2006). <br />7.1.1 Historic Earthquakes <br />The earthquake history of an area is very important in the evaluation of future seismicity because <br />past earthquakes suggest both the location and size of future earthquakes. There are, however, <br />serious limitations in using the historic earthquake record by itself to predict future seismicity. Allen <br />(1975) discusses such limitations and suggests the coordinated use of recent geologic history and <br />earthquake history. <br />Colorado has a relatively short earthquake history dating back only some 140 years. The first <br />reported earthquake occurred on December 7, 1870, when an observer at Fort Reynolds, 32 km east <br />of Pueblo, noted moderate shaking of the structure. Father Armand W. Forstall installed the first <br />Colorado seismometer at Regis College in 1909. This instrument has provided valuable data, but the <br />system has generally operated at low gain and is capable of detecting only lazge events. A <br />seismograph was in operation at the University of Colorado at Boulder from 1954 to 1959. Events <br />were recorded on 35 mm film from one vertical and two horizontal seismometers. These records, <br />along with those from Regis College, are the main sources of pre-1961 instrumental data in <br />Colorado. <br />' In December of 1961, the Colorado School of Mines installed athree-component seismograph in the <br />Cecil H. Green Observatory at Bergen Park, about 14 km southwest of Golden. This system has been <br />in continuous high-gain operation since installation and is now the primary source of instrumental <br />' data on Colorado earthquakes. The Bergen Park station, designated GOL by the worldwide <br />standazdized seismograph network (WWSSN), is thought to be capable of locating Colorado <br />' earthquakes to within 15 km of their epicenters. During parts of 1971 and 1972, the Colorado School <br />of Mines and NOAA jointly operated aseven-station, state-wide monitoring network. Colorado <br />earthquakes were located with much greater accuracy (to within 1 to 5 km) during this period. <br />Figure 1 shows earthquakes felt in Colorado from 1867 to 1996. It also shows the geographic <br />' distribution of the more significant events and their relationship to the six seismotectonic provinces. <br />As seen in Figure 1, several moderate-sized earthquakes and numerous small earthquakes have been <br />felt and/or recorded during the 129-year period. <br />7.1.2 Ground Motions <br />' The rules for the SEO establish that high hazard dams with a height greater than 30 feet should be <br />designed to withstand the MCE. As previously indicated, the proposed design is associated with an <br />earthquake with a 6.5 magnitude. To estimate the ground motions at the site, the attenuation <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc <br />13 <br />