Hi All. How many of you have heard of Borah Peak, Idaho? That is the location of a pretty strong quake in Idaho. It was a 7.3ML. I guess that would make it the strongest quake ever recorded in Idaho.

Borah was rocked at 8:06 a.m., Friday, Oct. 28, 1983 by a 7.0 quake. While there has been debate about exactly how powerful or important the quake was (initial reports had always called it a 7.3 quake), the single most dramatic event associated with it was that Borah when surveyed afterwards increased in height by 7 feet — however even that claim may turn out to be an urban legend.

Confused yet? If not, read on, as it gets very interesting. It turns out that maybe Borah didn’t rise 7 feet, but that the valley dropped about 5 to 9 feet. I guess it boils down to whom you want to believe or who has the best data, or maybe something else.

The quake occurred near the small town of Challis, Idaho (population, 1200). Officially, the quake became known as the Borah Peak Earthquake and it sent shockwaves over an area including not only surrounding parts of Idaho and nearby Montana, but places as far away as Boise, Salt Lake City, and portions of eastern Oregon and Washington as well as southern Canada. Tremors were also felt in some locations in the Dakotas. More than 150 aftershocks rumbled through the area over the next several days and weeks. In all, it was estimated that people living in an expanse encompassing over 500,000 square miles experienced some aspects of the quake.

The quake was dramatic in that its faulting was up and down (most quakes are not as visible on the fault line as they slide sideways such as the San Andreas where San Francisco and Los Angeles are inching toward each other). The Borah Quake produced a 21-mile long scarp that was raised at points as much as 14 feet and is still visible today. That’s a pretty good indication that it was a thrust quake however the quake occurred on a normal fault, or so it has been said.

From the Desert News October 31, 1983:

“Portable seismograph stations were set up near the quake’s epicenter, located near Mount Borah, Idaho’s tallest peak at a previous height of 12,662 feet above sea level. There is a 10-mile-long gash on the flanks of the mountain and its sisters in the Pahsimeroi range.”

“The quake raised the mountain about 15 more feet above the valley floor, but officials said they would not be sure if the valley is lower or the mountain is higher until the area is resurveyed.”

See what I mean about confusing. Don in creepy town

http://americasroof.com/archives/2005/02/12/borah-quake/

http://eqinfo.seis.utah.edu/NEHRP_HTM/1983bora/c1983bo1.htm

http://www.idahogeology.org/Services/GeologicHazards/Earthquakes/