Borah Peak:
1983
Oct. 28, 1983. Borah Peak, Custer County, Idaho.
The Borah Peak earthquake is the largest ever recorded in Idaho -
both in terms of magnitude and in amount of property damage. It caused two deaths in
Challis, about 200 km northeast of Boise, and as estimated $12.5 million in damage in the
Challis-Mackay area.
A maximum MM intensity IX was assigned to this earthquake on the
basis of surface faulting. Vibrational damage to structures was assigned intensities in
the VI to VII range.
Spectacular surface faulting was associated with this earthquake - a 34 km long northwest
trending zone of fresh scarps and ground breakage on the southwest slope of the Lost River
Range.
The most extensive breakage occurred along the 8 km
zone between West Spring and Cedar Creek. Here, the ground surface was shattered into
randomly tilted blocks several meters in width. The ground breakage was as wide as 100 km
and commonly had four to eight scarps as high as 1-2 m. The throw on the faulting ranged
from <50 cm on the southern-most section to 2.7 m south of rock creek at the western
base of Borah Peak.
Other geologic effects included rockfalls and landslides on the steep slopes of the Lost
River Range, water fountains and sand boils near the geologic features of Chilly Buttes
and the Mackay Reservoir, an increase or decrease in flow of water in springs, and
fluctuations in water levels. A temporary lake was formed by the rising water table south
of Dickey.
The most severe property damage occurred in the towns of Challis and Mackay, where 11
commercial buildings and 39 private houses sustained major damage and 200 houses sustained
minor to moderate damage.
At Mackay, about 80 km southeast of Challis, most of the commercial structures on Main
Street were damaged to some extent; building inspectors condemned eight of them. Damaged
buildings were mainly of masonry construction, including brick, concrete block, or stone.
Visible damage consisted of severe cracking or partial collapse of exterior walls,
cracking of interior walls, and separation of ceilings and walls at connecting corners.
About 90 percent of the residential chimneys were cracked, twisted, or collapsed.
At Challis, less damage to buildings and chimneys was sustained, but two structures were
damaged extensively: the Challis High School and a vacant concrete-block building (100
years old) on Main Street. Many aftershocks occurred through 1983.
Also felt in parts of Montana, Nevada, Oregon, Utah,
Washington, Wyoming, and in the Provinces of Alberta, British Columbia, and Saskatchewan,
Canada.
Source
Information
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