1987-12-01_PERMIT FILE - C1981017 ‎(104)‎

• Permit No: C1981017
• IBM Index Class Name: PERMIT FILE
• Doc Date: 12/1/1987
• Doc Name: Rock Tunnel Project
• Section_Exhibit Name: Chapter II Appendix II-B-3 ‎(Part 2 of 2)‎
• Media Type: D
• Archive: No

Seismic Risk Evaluation
The earthquake potential of the Glenwood Springs area has
been considered by Kirkham and Rogers (1978) . They support the
theory that flexural slip due to the unfolding of the Grand
Hogback monocline is the most likely cause of earthquakes in the
area. They conclude that because of the nature of the
deformation, large earthquakes caused by this process would seem
unlikely.
Recent seismic events centered in the Carbondale area have
generated renewed interest in earthquake activity in the area.
The principal seismic "event" consisted of hundreds of small
seismic events with a maximum recorded magnitude from the
national recording system of 3. 1 on the Richter scale. A local
seismic net established by the U.S.G.S. to provide detailed
information on events in the Carbondale area between April 12 and
May 31, 1984, recorded a maximum magnitude event of between 2.3
and 2. 5 on the Richter scale.
The preliminary estimate of the epicenter location is
approximately 5 miles west to southwest of Carbondale, although
all data have not yet been analyzed. The Aspen to Rifle area is
known as an area of low level seismic activity, where series of
low magnitude events occur at relatively widely spaced time
intervals (Presgrave, 1984) . While the 1984 Carbondale earth-
quakes are of technical interest, they are not considered to
alter previous evaluations of expected earthquake magnitudes or
frequencies in the area.
Quantitative data on recommended design earthquake
accelerations are available from a number of sources. Comparison
of these sources enables development of a realistic design
acceleration.
The Uniform Building Code (1982 edition) includes Coal Basin
in Zone 1 of its Seismic Risk Map. This represents an area of
potential minor damage and corresponds to intensities of V and VI
on the Modified Mercalli Scale. These intensities correspond to
peak horizontal accelerations of 34 cm/sec (0. 035 g) and 66
cm/sec (0.067 g) according to the correlations presented by
Trifunac and Brady (1975) .
The Corps of Engineers (1982) presents a seismic zone map
where Coal Basin is located in a Zone 1 seismic probability area.
This corresponds to minor potential earthquake damage and a
recommended earthquake coefficient of 0. 025 for dam design.
Much recent earthquake prediction work is based on prob-
abilistic estimates. In the most current of this work, Algermis-
sen and others (1982) present maps showing horizontal ac-
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