Lost River Fault Scarp

September 03, 2019



September 2019 Viewer's ChoicePhotographer: Rod Benson
Summary Author: Rod Benson 

The thin tan line in this photo is the Lost River Fault Scarp, which runs for over 20 miles (32 km) along the base of the Lost River Range in central Idaho. This scarp formed as result of a 6.9 magnitude earthquake that occurred at 8:06 a.m. on October 28, 1983. The quake was named the “Borah Peak Earthquake” because it happened near Borah Peak (12,662 ft. or 3,859 m), the highest mountain in Idaho. Hundreds of hikers come every year to begin their ascent of Borah (known locally as Mount Borah) at the Birch Springs Trailhead. This photo was taken along the road to the trailhead.

The Lost River Range is a fault-block mountain range on the northeastern edge of the Basin and Range Province. Like Basin and Range Mountains in other states such as Utah and Nevada, these mountains basically formed one earthquake at a time over millions of years. During the 1983 quake, the valley side of the fault dropped 9 feet (3 m) and the block that includes the Lost River Range rose 6 inches (15 cm), leaving the offset (scarp) shown in the bottom photo. 

The epicenter was located along the fault somewhere between the small towns of Mckay and Challis, Idaho. It was the most energetic earthquake in the lower 48 since the 1959 Hebgen Lake Earthquake in southwestern Montana. Shaking was felt in 8 states and 2 Canadian provinces, lasting from 30-60 seconds.

Top photo taken on July 7, 2019. Bottom photo, taken by Bruce Railsback, University of Georgia, in 1987, four years after the earthquake occurred. 

Photo Details: Top - Camera: Panasonic DMC-ZS60; Software: iPhoto 9.6.1; Exposure Time: 0.0010s (1/1000); Aperture: ƒ/4.1; ISO equivalent: 80; Focal Length (35mm): 58.