1983 Borah Peak earthquake

1983 Borah Peak earthquake

Borah Peak with fault scarp seen
near base of tan hill in foreground
Date October 28, 1983
Origin time 14:06:09 UTC [1]
Magnitude 6.9 Mw [1]
Depth 16 km (9.9 mi) [2]
Epicenter 44°05′N 113°48′W / 44.08°N 113.8°W / 44.08; -113.8Coordinates: 44°05′N 113°48′W / 44.08°N 113.8°W / 44.08; -113.8 [2]
Type Dip-slip [3]
Areas affected Central Idaho
United States
Total damage $12.5 million USD [4]
Max. intensity IX (Violent) [4]
Landslides Yes
Aftershocks 5.6 Mw Oct 28 at 19:51 UTC [2]
5.5 Mw Oct 29 at 23:29 UTC [2]
Casualties 2 killed, three injured [2]

The 1983 Borah Peak earthquake occurred on October 28 at 8:06:09 local time in the Lost River Range at Borah Peak in central Idaho, United States. The shock measured 6.9 on the moment magnitude scale and had a maximum Mercalli intensity of IX (Violent).


The event is the largest and most significant to strike in the state of Idaho. Two children were killed by falling masonry while walking to school in Challis, about 120 miles (200 km) northeast of Boise, the state's capital. Twelve and a half million dollars in damage took place in the Challis-Mackay region. As a result of extreme surface faulting, a maximum Mercalli intensity of IX (Violent) was decided upon, while vibrational damage was at a Mercalli intensity of VI (Strong) to VII (Very strong).[4]

Surface faulting

The rupture caused clear surface faulting. A 21-mile (34 km) long northwest-moving zone of fresh scarps and ground ruptures was present on a slope of the Lost River Range. Extensive breakage occurred along a 5-mile (8 km) zone between West Spring and Cedar Creek; ground surface was literally "shattered" into tilted blocks, each meters in width. These scarps were as broad as 330 feet (100 m).[4]


The Challis-Mackay region experienced rather thorough damage, with 11 commercial buildings and 39 homes with major damage; while another 200 houses were damaged, minor to moderately. Mackay in particular, about 50 miles (80 km) southeast of Challis, experienced the most severe damage. Most of the city's large buildings on its Main Street were damaged, to some extent; eight of these buildings were deemed condemned and closed down. Most of these buildings were built from materials such as brick, concrete block, and stone, each varying.[4]

Sand blows

Further information: Sand volcano

Near Chilly Buttes of Thousand Springs Valley, a series of artesian fountains/sand blows erupted immediately after the main shock. Groundwater gushed from these fountains forming small craters and depositing aprons of light-colored sandy sediment around each crater.


External links

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