Idaho Geologic Survey GeoNote 05
BORAH PEAK EARTHQUAKE

The Borah Peak earthquake was the largest earthquake ever recorded in Idaho. At magnitude 7.3 it was the strongest earthquake in the lower 48 states since the 7.5 magnitude event at Hebgen Lake, Montana, in 1959. Moderate earthquakes are not unusual in Idaho. Residents of Cascade, Stanley, Pocatello, and Malad City may feel several a year, and scores of other tremors too small to be felt are recorded annually. The Borah Peak earthquake shook residents of eight western states and two Canadian provinces, and its consequences have made Idahoans dramatically aware of the seismic setting of the state,

The Borah Peak event occurred on a known fault along the eastern margin of the Lost River Range, which is southwesternmost of three major parallel "basin and range" structures north of the Snake River Plain. Although the quake was centered in a relatively unpopulated part of the state, falling masonry took the lives of two children and injured a woman in towns nearby. Mackay (population 541), the nearest town, sustained the most damage to schools, homes, and businesses. Farther away in Challis (population 758), buildings were affected to a lesser extent, but rockfalls posed a significant hazard.

The Borah Peak earthquake is the best documented and most studied of all previous quakes in the intermountain west. It is also being used as a seismic model for potential earthquakes alon the heavily populated Wasatch Front in Utah. Geologic effects such as ground rupture, ground-water discharge, liquefaction, mass movements, and ground shaking have seldom been better displayed. Studies of the seismic waves and aftershocks are revealing more about the crustal structure of this area and are leading to new tectonic interpretations of the Basin and Range region.

Earthquake Statistics

Date and Time: Friday October 28, 1983 8:06 a.m. Mountain Daylight Time (7:06 a.m. PDT in northern Idaho)
Magnitude. 7.3 M_s, Richter scale, 6.9 M_b
Epicenter: 44.046 degrees north latitude, 113.887 degrees west longitude
Depth: 16 + 4 kilometers
Strongly Felt Area: Intensity VI over 55,000 square kilometers
Intensity Modified Mercalli Scale:
VII Mackay, Challis
VI Boise
V Pocatello, Moscow, Libby (MT), Spokane (WA)
IV-II Seattle (WA), Salt Lake City (UT), Portland (OR)
Foreshocks: No earthquakes greater than magnitude 2 for two months prior to event
Aftershock Sequence: Five earthquakes; magnitude 5.8 within 48 hours.
Hundreds of snialler aftershocks in the following months.
Area still experiencing an above average number of minor shocks.

Geologic Effects

Faulting

Name: Lost River Fault, named in 1951
Surface Rupture in 1983: 36 + 3 kilometers long
Maximum Offset: 3 meters scarp height
Relative Movement (throw): 2.7 meters
Motion: normal fault; sinistrai (left) motion dip of fault plane:
45-53 degrees southwest. Absolute Uplift: 0.3 meter elevation gain at fault. Borah Peak rose about 0.2 meter
Last Movement: Recurrent over last million years; latest probably 15,000 years ago
Eyewitnesses at Scarp: Four elk hunters: Lawana Knox, Bill Knox, John Turner, and Don Hendriksen

Hydrology and Liquifaction

Eruption of 30 sand spouts and numerous spnngs at Chilly Buttes
Formation of temporary "Thousand Springs Lake"
Mine level 1100 at the Clayton Silver Mine flooded temporarily
Challis Hot Springs dried up, and later returned to three times norinal flow
One cubic kilometer of excess ground water estimated in Big Lost River drainage
Changed eruption of Old Faithful in Yellowstone National Park

Water levels in wells fluctuated throughout interstate region

Landslides and Rockfalls

Lupine Creek mudflow, a 200,000-cubic-meter flow, formed three days later due to increased pore pressure
Several large boulders in Challis rolled into residential area
Boulder hit Clayton school and rocks littered school area
Numerous rockfalls on highways and roads
Rockfall damaged the concrete spillway at Mackay Dam

Damage Reported to Buildings and Structures

12.5 million dollars to buildings and structures
11 businesses and 39 homes with major damage
200 homes with moderate damage
90% of chimneys in Mackay cracked or toppled
Four schools damaged and rebuilt: Mackay Junior and Senior High,
Challis High, Arco High, and the State School for the Blind at Gooding
Disruption to recreation facilities, trails, roads and range in Challis National Forest estimated at $255,000
Mackay City Hall and jail damage
Ground motion triggered reactor shut-downs at Idaho National Engineering and Environmental Laboratory
Challis Irrigation Company tunnel caved in

Prepared by Roy M. Breckenridge 0286

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