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' 7.2.2 Anticipated Seismicity Associated with E Seam Mining <br />The proposed E Seam mine is overlain by mountainous terrain that is gently rolling, which <br />' contributes to moderate changes in confining stresses at mine depth. The overburden depth above the <br />E Seam generally increases from about 300 to 600 feet at the closest approach to Monument Dam to <br />' about 1,400 feet at a horizontal distance of about 12,000 feet from the dam. Consequently, very little <br />mine-related seismic activity would be expected for the E Seam mine. About 20 percent of the <br />E Seam area is overlain by 1,000 feet to 1,400 feet of overburden, which would be capable of an <br />' event in the range of M 2.0 to 2.3 based on the findings of Ellenberger and Heasley (2000). <br />Correspondingly, the remaining approximately 80 percent of the E Seam area that has less than 1,000 <br />' feet of overburden would be capable of a microseismic event of M 2.0 or less. <br />In addition, favorable E Seam mine conditions are expected to limit the potential for mine-induced <br />' seismicity. Abrupt changes in confining stress are not anticipated because of the relatively gently <br />transitions in overburden depth related to the overlying terrain. The stratigraphic column (see <br />Appendix A) indicates the roof over the E Seam includes more ductile siltstone, shale strata, and less <br />massive sandstone strata than overlie the coal seams at greater depth, which would be expected to <br />lessen the potential for energy releases. <br />7.2.3 Ground Motions for Mine-Induced Seismicity <br />The attenuation relationship developed by Sadigh (1995) was selected to calculate peak ground <br />accelerations assuming a reverse fault type and a hard rock site. The peak ground acceleration at the <br />dam site for the expected maximum M 2.3 event for maximum E Seam overburden conditions (1,000 <br />to 1,400 feet) at an average horizontal distance of about 9,000 feet from the dam is 0.06g. The peak <br />ground acceleration at the dam site for the expected M 2.0 event for area of the E Seam that has <br />overburden depths ranging from 300 to 1,000 feet at an average horizontal distance of 3,500 feet <br />from the dam is 0.O77g. Calculated accelerations for selected mine-related earthquakes are <br />summarized in Table 2. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 16 <br />