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7.1.1 Potentially Active and Active Faults Colorado State Engineer regulations require consideration of poten- tially active faults defined by Kirkham and Rogers (1981) for design of all Class [and lazge to intermediate-size Class II dams. A map published by Kirkham and Rogers (1981), showing potentially active faults in Colorado, provides the basis for meeting State Engineer requirements. A potentially active fault, according to Kirkham and Rogers (1981), is any fault that has experienced displacement in the last 28 million yeazs. This criterion is extremely conservative and although acceptable for a regional evaluation of potential seismic hazards, is untenable for site-specific assessments. For example, the Corps of Engineers (COE) and Nuclear Regulatory Commission (NRC) generally consider seismogenic (activefcapable) faults for use in estimating design earthquakes to have experienced movement in the last 35,000 yeazs. NRC extends the limit to 500,000 years in the case of documented recurrent movement on the same fault. California regulations define an active (seismogenic) fault as one that has experi- enced movement in the last 10,000 yeazs. A number of faults identified as potentially active by Kirkham and Rogers (1981) are present in north-central Colorado. The most impor- tant of these faults relative to the Trout Creek dam site include numer- ous mapped and inferred faults associated with the development of the Upper Arkansas Valley in the north and south grabens (Sawatch fault); lazge regional faults bounding or traversing the crest of the Mosquito/Pazk Range and Arkansas Hills (Mosquito, Weston and London faults); the Sangre de Cristo and Villa Grove faults on the east side of the San Luis Valley; faults near Spinney Mountain about 31 miles (52 km) east of the Trout Creek site; and postulated basement faulting associated with the Rocky Mountain Arsenal northeast of Denver; and numerous other potentially active faults mapped by Kirkham and Rogers (1981) in Colorado. Many of these other faults, however, exhibit only circumstantial evidence for potential activity based on displacement of Miocene-Pliocene units or geometric relationships with faults that displace such units. The designation of these faults as potentially active does not mean, in fact, that they aze seismogenic, pose a hazazd to engineered works, or should be consid- ered in the current seismic hazard analysis. In the following sections, we discuss criteria for identification of active or seismogenic faults (Figure I) for input to the PSHA for Trout Creek dam. 26