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7 Seismic Hazard <br />Colorado was considered to be an area of relatively low natural seismicity with only a minor <br />potential for earthquakes. However, recent geological and geophysical investigations discovered <br />several active faults capable of generating damaging earthquakes (Kirkham and Rogers, 1981). <br />These investigations suggest that Colorado is an area of moderate earthquake potential. <br />Mine-induced seismicity is considered to be another aspect that poses a hazard to Monument Dam. <br />This is due to the relatively short distance from the dam to the coal mining at the E Seam. <br />This section of the report discusses the seismic hazard at Monument Dam. It includes both natural <br />and mine-induced seismicity. The objective in both cases is to estimate ground motions for analysis <br />and design of the proposed dam enhancement. <br />7.1 Natural Seismicity <br />Natural seismicity refers to seismic events caused by natural tectonic movements, which occur due to <br />slip of the earth's crust along a fault. These events produce ground shaking that typically lasts for <br />seconds to minutes with predominant periods between 0.2 and 2.0 seconds. <br />A major period of extensional faulting was initiated in Colorado about 28 million years ago and has <br />continued to the present, at least to a limited extent. According to Kirkham and Rogers (1981), faults <br />that have moved during this period of Neogene tectonism are classified as potentially active faults. <br />These faults are suspected of being capable of future movement and thus damaging earthquakes. <br />Other faults, however, appear to be inactive for millions of years and pose no serious earthquake <br />hazard. <br />Colorado has been divided into six seismotectonic provinces based on the distribution and <br />characteristics of Neogene faults, historic earthquakes, and major structural and physiographic <br />regions as shown on Figure 1 (Kirkham and Rogers, 1981). These seismotectonic provinces include <br />the Rio Grande Rift, Eastern Mountains, Plains, Western Mountains, Uinta-Elkhead, and Colorado <br />Plateau. Most of the Neogene faulting and associated igneous activity in Colorado appears to be <br />related to the Rio Grande Rift. Monument Dam is located in the eastern portion of the Colorado <br />Plateau Province near the boundary with the Western Mountain Province. <br />The Colorado Plateau Province in western Colorado includes the northwest and west flanks.of the <br />San Juan Mountains, the salt anticline region of the Paradox Basin, the Uncompahgre uplift, and <br />much of the Piceance Basin, which includes Grand Mesa, Battlement Mesa, and the Roan Plateau. <br />Neogene faults are relatively scarce in this province except for those associated with the salt <br />anticlines and the Uncompahgre Plateau (Kirkham and Rogers, 1981). Numerous earthquakes have <br />been felt in the Colorado Plateau and the adjoining Western Mountain Provinces as seen in Figure 1. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 12 <br />