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GA-84-0005 <br />RICHARD D. LAMM <br />GOVERNOR <br />June 5, 1984 <br /> <br />COLORADO GEOLOGICAL SURVEY <br />DEPARTMENT OF NATURAL RESOURCES <br />718 STATE CENTENNIAL BUILDING - 1313 SHERMAN STREET <br />DENVER, COLORADO 80203 PHONE (303) 868-2811 <br />Mr. Jim Pendleton <br />Mined Land Reclamation Division <br />1313 Sherman Street, Room 423 <br />Denver, Colorado 80203 <br />Dear Mr. Pendleton: <br />RE: STORM KING COAL PROJECT, NEW CASTLE, COLORADO <br />~II ~I~~I~~I~~~~~~I~ <br />JOHN W. ROLD <br />DiR ECTOR <br />We have reviewed the section of the Chen and Associates report (Job No. <br />25,999A) concerning seismic exposure and design earthquake for the Coal <br />Ridge No. 1 Mine. This review included the assessment of earthquake <br />activity and potentially active faults. <br />Historical and instrumental events have occurred in the general area. <br />Magnitude of the largest of these events is a Modified Mercalli Intensity <br />of VI. Most recently a series of events has occurred in close proximity <br />to the Grand Hogback, south of Carbondale. Approximately 15 of these <br />events have been felt in the Carbondale area and range in Richter magnitude <br />from 2.1 to 3.2. The historical and instrumental record suggests that the <br />general area may, in the future, be subject to moderate size events. <br />As noted in the Chen report, potentially active faults displace Miocene <br />basalts along the Grand Hogback. Faults probably also displace Quaternary <br />sediments south of the basalt flows. These faults coupled with the recent <br />earthquake activity suggests that structural movement and associated earth- <br />quake activity is possible along the Grand Hogback. <br />The earthquake event (near field) used in the design of the coal refuse <br />impoundment should probably be in the range of a magnitude 5.5 (intensity <br />VII). Such an event would be comparable to a Seismic Zone 2 of <br />Algermissen (1969) and a corresponding seismic coefficient of 0.10. We <br />recommend that such a coefficient be used in the stability analysis for <br />operational conditions. <br />bj <br />WRJ-84-009 <br />G E O L O G Y <br />STORYOFTHEPAST...KEYTO THEFUTURE <br />Walter R. Junge <br />Engineering Geologist <br />