Dating a 20th-century fault, Elk Summit talus apron, Big Creek area, Valley County, Idaho

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Abstract:

A young fault cuts obliquely across the talus apron near Elk Summit in central Idaho. Twenty-five or thirty years ago, the fault was marked by a bare, brown, east-trending scarp that slashed across talus covered with lichens. The scarp, extending several hundred meters from a hinge point near Elk Summit, showed at its east end a maximum displacement of about 1 m, south block down, scarp dip perhaps 60°-70° S. The scarp has since degenerated to a narrow, discontinuous bench on which the talus blocks have few lichens. Comparative lichenometry of Rhizocarpon geographicum and Lecidea atrobrunnea from talus blocks at Elk Summit and headstones in local cemeteries indicates that the fault formed sometime between the 1960's and the first decade of the century. Aerial photographs indicate that, though the fault may have begun to develop before 1946, its surface expression did not become conspicuous until sometime between 1954 and 1964. We believe that surface expression of the fault is earthquake related. No major earthquake epicenters have been reported from the Elk Summit area, but effects of the August 1959 earthquake at Hebgen Lake, Montana, were felt at an intensity VI (modified Mercalli scale) within 9-30 km of Elk Summit, 320 km distant from Hebgen Lake. Accordingly, the most likely date of the fault near Elk Summit is 1959.

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Reference Code: U94LEO01IDUS

Full Citation: Leonard, B. F. and R. Rosentreter. 1994. Dating a 20th-century fault, Elk Summit talus apron, Big Creek area, Valley County, Idaho. U.S. Geological Survey Bulletin 2101. U.S. Government Printing Office, Washington, DC. 13 pp.

Location: EARTH SCIENCES REPRINT FILE (2 copies)