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Seismic Risk Evaluation <br />The earthquake potential of the Glenwood Springs area has <br />been considered by Kirkham and Rogers (1978). They support the <br />theory that flexural slip due to the unfolding of the Grand <br />Hogback monocline is the most likely cause of earthquakes in the <br />area. They conclude that because of the nature of the <br />deformation, large earthquakes caused by this process would seem <br />unlikely. <br />Recent seismic events centered in the Carbondale area have <br />generated renewed interest in earthquake activity in the area. <br />The principal seismic "event" consisted of hundreds of small <br />seismic events with a maximum recorded magnitude from the <br />national recording system of 3.1 on the Richter scale. A local <br />seismic net established by the U.S.G.S. to provide detailed <br />information on events in the Carbondale area between April 12 and <br />May 31, 1984, recorded a maximum magnitude -event of between 2.3 <br />and 2.5 on the Richter scale. <br />The preliminary estimate of the epicenter location is <br />approximately 5 miles west to southwest of Carbondale, although <br />all data have not yet been analyzed. The Aspen to Rifle area is <br />known as an area of low level seismic activity, where series of <br />• low magnitude events occur at relatively widely spaced time <br />intervals (Presgrave, 1984). While the 1984 Carbondale earth- <br />quakes are of technical interest, they are not considered to <br />alter previous evaluations of expected earthquake magnitudes or <br />frequencies in the area. <br />Quantitative data on recommended design earthquake <br />accelerations are available from a number of sources. Comaarison <br />of these sources enables development of a realistic design <br />acceleration. <br />The Uniform Building Code (1982 edition) includes Coal Basin <br />in Zone 1 of its Seismic Risk Map. This represents an area of <br />potential minor damage and corresponds to intensities of V and VI <br />on the Modified Mercalli Scale. These intensities correspond to <br />peak horizontal accelerations of 34 cm/sec (0.035 g) and 66 <br />cm/sec (0.067 g) according to the correlations presented by <br />Trif unac and Brad y (1975). <br />The Corps of Engineers (1982) presents a seismic zone map <br />where Coal Basin is located in a Zone 1 seismic probability area. <br />This corresponds to minor potential earthquake damage and a <br />recommended earthquake coefficient of 0.025 for dam design. <br />Much recent ear*_hquake prediction work is based on prob- <br />. abi 1 istic esti:,~ates. In the most cu: rent of this cork, Alger:ais- <br />sen and others (1982) present maps showing horizontal a~- <br />1 <br />113 <br />