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,. I . . . . . . . . I . . . I I I I . I 5,3,1,4 Earthquake Potential In recent studies completed by the U,S. Department of the Interior, Geologic Survey, estimates of horizontal acceleration in bedrock are less than 4 percent gravity and approximately 4 percent gravity wtth 90 percent probability of not being exceeded In a 10 and 50 year time period, respectively. A horizontal acceleration of between 6 and 9 percent gravity wtth 90 percent probability of not being exceeded is estimated for a time period of 250 years. The maximum magnitude used for the project seismic zone was 6,1 In the 1981 pUblication 'Earthquake Potential in Colorado', the magnttude of the maximum credible earthquake for the Eastern Mountain Province, which includes the project area, is estimated between magnitude 6,0 and 6,75, The project area is located in Zone 1 of the Seismic Zone Map of the United States in the 1988 publication 'Advanced Dam Engineering.' Zone 1 is defined as an area which may experience minor earthquake damage and has an assigned seismic coefficient of 0,05. For seismic stability analysis, a peak horizontal acceleration of 0.10 was chosen to allow for uncertainties. 5,3,2 INVESTIGATIONS 5,3,2.1 Field Investigations Field investigations for the feasibHity study were conducted by WCC between October 23 and 25, 1989. This work involved drilling five test holes from the crest of the embankment. Piezometers were installed the fIVe test holes. The location of these test holes is shown on Figure 1, and summary logs are shown on Figure 2, WCC also conducted field investigations for the reconstruction of the dam in 1965. This work involved drilling 9 test holes near the embankment and excavating 12 test pits in the reservoir area. The approximate location of these test holes and test pits are also shown on Figure 1, Summary logs of these test holes are shown on Figure 3 and summary logs of the test ptts are shown on Figure 4, 5-9