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00123Q Earthquakes that have occurred near Quaternary faults in east-central Colorado Maximum intensity Date Time Location Lat.(N) Long. (W) (Mercalli) -- 10/11/1916 2241 Boulder 40.0 105.0 IllY 1/05/1965 1626 Rocky Flats 39.9 105.3 IllY Source: Hadsell, 1968 !!Intensity III indicates slight movement. The Boulder earthquake occurred approximately 11 miles from the proposed Coal Creek damsite, and the Rocky Flats earthquake occurred approxi- mately 6 miles from the proposed site. The Crescent site 1 ies southwest of Boulder. Faults throughout the mountain area are northwest-trending. Some mineralization has occurred wi th faul ti ng in the area west of Boul der. The Magnol i a mi ni ng di s- trict, approximately 4-1/2 miles from the Crescent site, has produced gold, silver, and some tungsten. No mineralization has occurred in the Crescent area. The Livingston fault crosses South Boulder Creek within the alternative reservoir site. In 1962, the Department of Defense drill ed a 12,000-foot-deep well at the Rocky Mountain Arsenal. Toxic wastes were injected into this well, and shortly thereafter, earthquakes occurred. I n the foll owi ng 3-1/2 years, over 700 nondevastating earthquakes occured in the Denver area (only three exceeded a Reichter scale magnitude of 5). When the deep di sposal was halted, the shocks decreased markedly (Mears, 1978). This well is approximately 20 miles southeast of the proposed Coal Creek site. In addition to the above earthquakes, there were 31 seismically active earthquakes in the Golden Fault in 1967 (Reichter scale magnitude less than 4) (Simon, 1969). An examination of the reports of intensity and of sei smographs (100-year record) shows that no earthquake havi ng an estimated modified Mercalli intensity greater than VI is known at or near any of the faults described. The mountain-front faults, such as the Golden Fault, have a long history of movement beginning probably in Precambrian time. An earthquake of intensity V is likely to be generated by movement on a fault having a length of at least 10 km (Scott, 1970; Hadsell, oral communication, 1967). By thi s formul a, the fault near Morrison and the Valmont Fault can be discounted. Only the Golden Fault is eligible for an earthquake intensity of V. Geologic information and seismic records suggest that earthquakes having intensities of IV or less on the modified Mercalli scale will continue, and a rare earthquake of intensity VI or VII is probable. 7