Paleoseismology of Utah, Volume 4: Seismotectonics of north-central Utah and southwestern Wyoming (SS-82)

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Paleoseismic Studies

By: M. W. West

Geomorphic evidence of late Quaternary faulting and related tectonic deformation is present north of the Uinta Mountains in Uinta County, Wyoming and Summit County, Utah. A major zone of late Quaternary normal faulting, termed the Bear River fault zone, extends over 25 miles from southeast of Evanston, Wyoming to an apparent complex intersection with the North Flank fault of the Uinta Mountains in north-central Utah. The Bear River fault zone consists of well-defined scarps each about 1.9 to 2.2 miles in length arranged in a right en echelon pattern. Major scarps trend N. 20° W. to N. 20° E. and show consistent, down-to-the-west displacement. Short, down-to-the-east scarps trend N. 15-20° W. and are interpreted to be antithetic faults. Near the south end of the fault zone, scarps in Pleistocene glacial deposits show strong angular discordance (70°) with the main north-northeast pattern of faulting. Late Quaternary movement is indicated by scarps ranging from <3 to 49+ feet high in till, outwash, alluvium, and bedrock of the Eocene-age Wasatch Formation; beheading and reversal of streams and numerous sag ponds.

This report describes results of neotectonic studies conducted in Summit County, Utah and Uinta County, Wyoming. The work: was based originally on a seismotectonic hazard evaluation for the U.S. Bureau of Reclamation's Meeks Cabin and Stateline dams located about 26 miles and 35 miles, respectively, southeast of Evanston, Wyoming. A moderate to large earthquake in the vicinity of either dam would pose a threat to dam safety. Accordingly, the seismogenic potential of major faults in northcentral Utah and southwestern Wyoming required evaluation.

Other Information:
Published: 1994
Pages: 93 p.
Plates: 5 pl.
Scale: 1:100,000
Location: Rich County and Summit County
Media Type: Paper Publication

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