1983 Borah Peak earthquake

Earthquake

The Friday morning earthquake was caused by a slip on the preexisting Lost River Fault. The event is the largest and most significant to strike in the state of Idaho. 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). Three weeks later on November 18, President Ronald Reagan declared the earthquake a major disaster.

Aftershocks were felt for a year afterwards; nearly ten months later, a 5.4 aftershock was recorded on August 22, 1984.

Surface faulting

The rupture caused clear surface faulting; a 21 mi long northwest-trending zone of fresh scarps and ground ruptures was present on a slope of the Lost River Range. Extensive breakage occurred along a 5 mi zone between West Spring and Cedar Creek; ground surface was literally "shattered" into tilted blocks, each several meters in width. These scarps were as broad as 330 ft.

The ground breakage was 60 mi, and the throw on the faulting ranged from -0.5 to.

Damage

The Challis-Mackay region experienced rather thorough damage, with eleven commercial buildings and 39 homes sustaining major damage while another 200 houses suffered minor to moderate damage. Mackay in particular, about 50 mi 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.

An estimated $12.5 million in property damage was recorded. In some places, the water grounds shifted.

Fatalities and injuries

In Challis, two children were killed when a stone storefront collapsed on them while walking to school; two others suffered minor injuries. In Mackay, a woman was hospitalized due to her injuries.

Old Faithful

After the earthquake and aftershocks, the eruption intervals of Old Faithful geyser in Yellowstone National Park, about 150 mi east, were noticeably lengthened.

Sand blows

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. The blows were noted largely along waterways, especially where draws, or small streams, enter into larger ones. Observers noted that some blows had black, sediment-laden water while others ran mostly clear. Some blows continued for several minutes after the shaking stopped. The Big Lost River rose several inches as a result of this water being expelled from the ground. The eruptions were likely a response to liquefaction of a water-laden underground sediment layer.

File:Fault Scarp Borah Peak Earthquake 1983.jpg|Fault scarps that outline the graben produced near Willow Creek File:Borah Peak earthquake artesian fountain 1983.jpg|Crater caused by an artesian fountain File:Borah Peak earthquake artesian fountain 1983b.jpg|Another view of the same fountain

References

References

1. (October 28, 1983). "Idaho quake measures 6.9, two killed". Spokane Chronicle.
2. (October 29, 1983). "Quake jolts 8 states, kills 2 children". Eugene Register-Guard.
3. Sher, Tom. (October 29, 1983). "Killer quake rocks Northwest". Spokesman-Review.
4. (29 October 1983). "Two Children Die As Quake Hits Northwest And Canada". The New York Times.
5. "Observatory Seismology".
6. (November 19, 1983). "Quake disaster area declared in Idaho". Lewiston Morning Tribune.
7. "Idaho Earthquake History".
8. (January 25, 1984). "4.5 aftershock quietly shakes Challis-Mackay". Deseret News.
9. (August 23, 1984). "Idaho quake jolts, but not like 1983's". Deseret News.
10. (August 23, 1984). "Earthquake jolts Idaho". Spokesman-Review.
11. "Historical Earthquakes in Idaho {{!}} Idaho Geological Survey".
12. (October 29, 1983). "Idaho quake cuts a swath of death and destruction". Deseret News.
13. Shaughnessy, Rick. (October 29, 1983). "Quake rocks mountain states; Strong shock kills 2 children". [[Times-News (Idaho).
14. Reid, T.R.. (October 7, 1984). "Idaho quake upsets, yet befriends Old Faithful geyser". Lewiston Morning Tribune.
15. Youd, T. Leslie. (November 1985). "The Borah Peak, Idaho earthquake of Oct. 28, 1983". Earthquake Spectra.
16. ISC. (2015). "ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009)". [[International Seismological Centre]].
17. (September 4, 2009). "PAGER-CAT Earthquake Catalog". United States Geological Survey.
18. (1993). "Seismicity of the United States, 1568–1989 (Revised) – U.S. Geological Survey Professional Paper 1527". [[United States Government Printing Office]].
19. (1997). "The Geology of Earthquakes". [[Oxford University Press]].