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reported Pleistocene to historic movement along the Frontal fault of the Gore Range about 70 miles (I 16 km) west of Denver and about 60 miles (100 km) north of Buena Vista. Based lazgely on these published papers, Matthews (1973) called for reappraisal of seismic risk in Colorado based on geologic evidence rather than historic seismicity. Questions were also raised concerning the seismic safety of the Rocky Flats Weapons Plant northwest of Denver. Detailed studies of Neogene (<27 mybp) tectonics and Quaternary faulting in Colorado reached print in the middle 1970s. The Geological Society of America (Curtis, ed., 1975) published a series of papers describing significant Neogene structural deformation in Colorado. A year later, Witkind (1976) of the USGS published anopen-file report showing known and suspected active faults in Colorado. At about the same time, West (1978) conducted a study of recent movement along the Frontal fault of the Gore Range (Tweto and others, 1970); and the Colorado Geological Survey (CGS) released an open-file report providing a preliminary overview of earthquake hazards in Colorado. This latter report was finalized in 1981 and published as CGS Bulletin 43 (Kirkham and Rogers, 1981). Largely as a result of the work by Scott (1970), Witkind (1976) and Kirkham and Rogers (1981) and an increased public awareness of the earthquake hazazd issue, site-specific seismotectonic hazard evalua- tions were performed for a number of projects in the state. Converse, Wazd, Davis, Dixon (1980), in conducting studies for Spinney Mountain dam 60 miles (f00 km) southwest of Denver and 31 miles (52 km) east of the Trout Creek dam site, found strong evidence of late Pleistocene (13-30 thousand years before present) faulting and proposed a local maximum credible earthquake (MCE) of Richter magnitude M,,=6.2. The U.S. Bureau of Reclamation (USBR) found evidence of Quatemary fault displacement near Buena Vista in Chaff ee County, 4 miles (6.5 km) west of Buena Vista, and neaz Ridgway in Ouray County. Trenching of fault scarps near Buena Vista disclosed evidence of recurrent fault movement with 0.1 meter of Holocene (<10,000 ybp), single-event displacement (Ostenaa and others, 1980). A 5-station seismic net surrounding the Ridgway site detected over 50 events (Richter magnitude M,, -0.25 to 2.73) during a three month period of operation (Sullivan and others, 1980). In 1979, the upper Arkansas Valley became the focus of comprehen- sive seismotectonic studies conducted by the U.S. Bureau of 6