The main fault scarp of the M = 7.0 Borah Peak, Idaho, earthquake of October 28, 1983 (view is to the east, from a point just north of Doublespring Pass Road). The Lost River Range, in the background, rises to Borah Peak (elevation 12,655 ft), the highest point in Idaho, just to the right of the field of view. Over millions of years, the range has been uplifted to its present elevation and backtilted to the east along west-facing normal faults, such as the one which produced the scarp pictured here. The scarp cuts an alluvial fan of Pinedale age (12,000 to 15,000 years old). The beveled surface that intersects the 1983 scarp surface from above was, prior to the 1983 earthquake, the degraded fault scarp of a single earthquake that occurred several thousand years ago. Subsurface expression of this prehistoric earthquake was studied in detail on the walls of a trench excavated in 1976. This trench, located by the slightly darker and more erodable backfill material in the scarp face near the right margin of the cover, was displaced during the 1983 earthquake.