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Geotechnleal Evaluation of Mine-Induced Seismicity on Monument Dam Dorsey 8 Whitney, LLP Year Latitude Longitude Magnitude Distance (km) Depth (km) 2004 38.83 -107.43 3.30 7 1 2004 38.88 -107.50 3.20 2 1 2004 38.81 -107.45 2.00 8 1 2005 38.92 -107.39 3.10 7 1 2005 38.79 -107.39 3.20 11 1 2005 38.83 -107.37 3.30 9 1 2005 38.89 -107.47 3.30 1 1 2005 38.95 -107.53 3.50 9 1 2005 39.03 -107.37 2.80 19 1 2005 38.91 -107.40 2.80 7 1 2005 38.83 -107.42 3.10 7 1 4.3 Anticipated Seismicity Associated with E Seam Mining The proposed E Seam mine is overlain by mountainous terrain that is gently rolling, which contributes to expected moderate changes in confining stress at mine depth. The overburden depth above the E Seam generally increases from about 300 to 600 feet at the closest approach to Monument Dam to about 1,400 feet at a horizontal distance of about 12,000 feet from the dam. Consequently, very little mine-related seismic activity would be expected for the E Seam mine. About 20 percent of the E Seam area is overlain by 1,000 feet to 1,400 feet of overburden, which would be capable of an event in the range of M 2.0 to 2.3 based on the findings of Ellenberger, et. al, 2001. Correspondingly, the remaining approximately 80 percent of the E Seam area that has less than 1,000 feet of overburden would be capable of a microseismic event of M 2.0 or less. Favorable E Seam mine conditions are expected to limit the potential for mine-induced seismicity. Abrupt changes in confining stress are not anticipated because of the relatively gently transitions in overburden depth related to the overlying terrain. The stratigraphic column (see Figure 3.2) indicates the roof over the E Seam includes more ductile siltstone, shale strata, and less massive sandstone strata than overlie the coal seams at greater depth, which would be expected to lessen the potential for energy releases. The E Seam support pillars have been designed by MCC to provide stable support conditions for the maximum overburden height and have incorporated a stiffer system of dual abutment entry pillars, all of which contributes to likelihood of infrequent, yielding, plastic-type deformation behavior for mine openings as opposed to sudden releases of energy. 4.4 Attenuation Relationship for Mine-Induced Seismicity A widely known attenuation relationship developed by Sadigh (1995) was selected to calculate peak ground accelerations assuming a reverse fault type and hard rock site. Pga at the dam site for the expected maximum M 2.3 event for maximum E Seam overburden GEI~ 14 06035006-03-16Monummi DarnGm[cchwralEval March 2006