23.237.412.10/98

BORAH PEAK, elevation 12,662

FAULT SCARP

BIRCH SPRINGS MUDFLOW

ACCESS ROAD

MULTIPLE SCARPS

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Lost River Ranger District

P.O. Box 507/716 W. Custer

Mackay, Idaho 83251

(208) 588-2224 The Day

The Earth

United States

Department

of Agriculture

Forest

Service

Intermountain

Region

Salmon-Challis

National Forest

At 8:06 a.m., Friday, October 28, 1983, an 

earthquake measuring 7.3 on the Richter 

scale rocked the West. It was the worst 

earthquake to hit Idaho in a century and 

the most severe in the Mountain West 

since 1959. Tragically, two children were 

killed in Challis, Idaho.

With the epicenter near Challis, Idaho, 

the earthquake toppled buildings, sent 

boulders crashing into homes, released 

new springs and geysers, and caused 

an estimated $12.5 million in damage 

in sparsely-populated Custer and Butte 

Counties. A scarp 21 miles long and up to 

14 feet high near the base of Borah Peak 

is a haunting reminder of that day.
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Earthquake

Visitors Site

Willow

Creek

Summit To Challis

Hwy 93

Dickey Peak

11141'

Thousand Springs Valley

Borah Peak

12662'

Birch

Springs

Chilly Buttes

Chilly To Mackay

Hwy 93

Highway 93

EARTH

Q

U

AKE

SCARP

Doublesprings Road

Region are “high 

angle dip slip faults” 

that have moved one 

or more times during 

the past 10,000 years.

The 1983 earthquake also 

caused new springs and sand 

cones to develop north and 

east of Chilly Buttes (west of 

Highway 93 in Thousand Springs 

Valley). Sand cones are groundwater eruptions that develop a cone or 

tubular orifi ce. The geyser brings sand 

from the subsurface to form a low cone. 

Some of the new springs will be active for 

several years.

In the 1970’s, geologists 

recognized that the existing 

scarp at the foot of Borah 

Peak was as high and fresh as 

any other along the 100-mile-long Lost River 

fault and might be the site of a future earthquake. Not only 

did that earthquake occur, but the new scarp created by the 

1983 Borah Peak earthquake is identical to its predecessor. 

The scarp is now twice its former height and 21 miles long.

The Lost River fault thus created two nearly identical earthquakes during the past two ruptures at Thousand Springs 

Valley. But are all ruptures of the fault at the Valley similar 

to the 1983 event? Borah Peak now stands one and 

one-half mile above the deepest deposits in the valley, 

although they are composed of the same rock and once 

stood at the same level. If each earthquake increased the 

separation between Borah Peak and the Valley by six feet, then 

a Borah Peak earthquake must have struck 1,500 times since 

the birth of the fault about fi ve million years ago.

T

here is a great deformation called the Western Cordillera (principal mountain range of a continent) that runs 

from the southern tip of Chile to Alaska. The Lost River 

Range in Idaho, rising over 6,000 feet above the valley fl oor, 

is in the central portion of the Cordillera. The Idaho Batholith 

lies to the west and the Overthrust Belt to the east.

The Lost River Range is a triple-ridge range. Borah Peak 

(12,662 feet) is in the central ridge and is the highest point 

in Idaho. Most of the rocks that outcrop in the range consist 

of lithifi ed ocean sediments, even to the top of Borah Peak. 

It is possible to collect bivalve shells and corals from many 

locations in the Lost River Range, including peaks more than 

12,000 feet above sea level.

The Lost River Range is composed of Paleozoic and Precambrian sedimentary rocks, complexly folded and thrust faulted 

during the Mesozoic Era. Much of the present topography of 

the Lost River Range is probably from displacement from the 

late Pliocene and Pleistocene Epochs. Over the last 5 million 

years there has been over 8,000 feet of displacement on the 

Lost River Fault, forming the Lost River Range and spectacular Borah Peak.

In the 1930’s, Dr. Richter developed a system that was useful 

in the State of California to recognize variations in the energy 

released during an earthquake. The system has been used in 

the United States ever since. Each whole number is 10 times 

the energy release of the next smaller number; hence a Richter 5 earthquake has 10 times more energy than a Richter 

4 earthquake. A Richter 7 earthquake has 1,000 times more 

energy than a Richter 4 earthquake.

In 1983, a 7.3 earthquake formed a new rupture which followed an ancient scarp near the base of Borah Peak. The 

principal movement of the new rupture was up and down. The 

angle at the break is 60o to 80o and is called a “high angle 

dip slip fault.” Most of the active faults in the Intermountain