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R. J. Irish Consulting Engineering Geologist, Inc. result, the risk oflandslides induced by reservoir water saturation or by reservoir water wave action is assessed to below. SITE SEISMICITY Colorado in general, end the upper Arkansas Yellay in particular, ere not considered to be e seismical) y active area, at least relative to the major earthquake belts of the world end of the United States of America; but that statement does not imply that the region is free of earthquake activity. In fact, some seismologists have suggested that the earthquake potential of the Colorado section of the Rio Grande Rift has been underestimated. Faults bordering the graben formed within the rift zone were initially activated during the early Miocene Period some 20 to 25 million years ago. Linear scarps in geologically young alluvium, hot springs, and other manifestation of reactivation for some of those faults, particularly in the Sen Luis Valley, have been interpreted by some researchers to evidence fault movements that would have generated earthquakes essentially into historical time; but reactivations during the lest 10,000 years have not been proven, except possibly at one site on the Sangre de Cristo fault on the eastern side of the San Luis Valley, where a reactivation date of about 2000 to 4700 years ego has been interpreted. A recurrence rate of 3000 to 5000 years has been suggested for that fault, but could be more li ke t 5,000 years. The Sangre de Cristo fault may have been the most active fault in Colorado during the past few hundred thousand to several million years, with fault movement-generated earthquakes having a maximum magnitude of about 7.5. It lies within 50 to 60 miles of the planned damsite. In the vicinity of Buena Vista and the Trout Creek damsite, vertical displacement on the Sawatch fault on the western side of the Arkansas Valley duri ng the past 20 to 25 million years has been about 12,000 feet. On the east side of the valley the graben-bounding faults adjacent to the mouth of the canyon section of the Trout Creek Yellay appear to be hidden beneath all uvisl soils deposited during the Ice-Age {2 million years ago to about 10,000 years ago} and after. Evidence of fault reactivation younger than 2 million years has not been documented. On the western side of the graben near Buena Vista, fault scarps in dateable soils suggest the most recent fault reactivation was 4000 to 30,000 years ago, with a recurrence level of 10,000 to 40,000 years. The hot springs along the Sawatch Fault, notably et Chalk Creek west of Buena Yiste,suggestcontinuing stress build-up. All in ell, the evidence for relatively recent earthquake activity in the Arkansas Valley in the vicinity of the damsite is more compelling than in the rest of the State of Colorado; but, even so, the risk of a large earthquake impacting the dam during its life is considered to be low. Conceivably a magnitude 6 or 7 earthquake could be generated within 5 miles of the damsite (e. g., on the Sawatch Fault},and that could subject the dam foundation to a peak particle acceleration on the order of 0. 3 to 0.4 q; but we consider the risk of occurrence of that size eerlhqueke to be low 7