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.- .' .. ""'" ~..Jl;;) . MEMORANDUM To: Files Subject: Response to Geological Survey, June 13, 1974, Letter of Comment on the Draft Environmental Statement, Fryingpan- Arkansas Project, Colorado . 1. Comment - Overall, the environmental impacts related to geologic conditions in the project area appear to have been throughly C~n- sidered in the draft statement. However, the description of seismi- city, faulting, and potential earthquake hazards is inadequate (p. 111-63 to 65). There are a number of faults in this part of Colorado that cut Pleistocene sediments and the movement on these faults was too recent for them to be considered inactive. With respect to faulting, the environmental statement (p. 111-65) refers to two U.S. Geological Survey Professional Papers, one by Scott and one by Tweto. Writing of faults that he studied, Scott states: "I believe that future displacement on some of the faults described could result in earthquakes large enough in magnitude to endanger property or people." He concludes his paper by stating that, ". . . although the available data are inadequate to permit pre- diction of time of earthquakes in east-central Colorado, the geologic information and seismic record suggest that earthquakes having intensities of IV or less on the modified Mercalli scale will continue as recorded by Simon (1969), and a rare earthquake of intensities VI or VII is probable." These remarks place a different emphasis on faulting and seismic activity in the project area than is presented in the environmental statement. Re-evaluation of potential impacts of earthquakes and damages that might result in project component failure is suggested. Response: All major Bureau of Reclamation structures are designed to withstand earthquakes. The exact magnitude or "design earthquake" depends on two factors: the location of the structure in relation to known active faults and the seismic history of the area. Infor- mation concerning these two factors is obtained through detailed field work by USBR geologists working in close cooperation with experts of the USGS. In addition, the "design earthquake" also includes a safety factor. After obtaining all known available data, a comprehensive analysis is performed on each major structure accocding to the Bureau's publication "Method for Estimating Design Earthquake Rock Motions, ,. dated April 1972. Additional information is available in Chapter III, Section A3c and Chapter IV, Section A2v. XI-265 .