1983 Borah Peak Earthquake

Normally, we don’t think about the state of Idaho having big earthquakes. They do get hit on a fairly regular basis, but the quakes are usually in the 2 to 3 magnitude range. There are 2,928 earthquake incidents in Idaho on record since 1931. The state averages just 35 earthquakes per year. The 1983 Borah Peak earthquake, measured 6.9 on the moment magnitude scale, struck Idaho on Friday, October 28th. It was at a depth of 16.0 miles and caused significant damage, resulting in two fatalities. Some readings showed a magnitude of 7.3 on the Richter scale in Custer County, Idaho. That made the 1983 Borah Peak earthquake almost a one-of-a-kind event for Idaho.

The Borah Peak earthquake struck at 8:06 am MDT in Idaho’s Lost River Range, registering a magnitude of 6.9 and becoming the state’s largest recorded quake. Triggered by a slip along the Lost River Fault, it reached a Mercalli intensity of IX (Violent), causing severe shaking and damage. The towns of Challis and Mackay were hit hardest, with the property damage estimated between $12.5 million and $15 million. Tragically, two schoolchildren lost their lives, and others were injured. The quake also caused significant surface faulting and created a 22-mile-long zone of fresh scarps and ground breaks, with vertical displacements up to 2.7 meters.

The Challis-Mackay region faced significant damage, with 11 commercial buildings and 39 homes severely affected, while another 200 houses had minor to moderate damage. Mackay, located about 50 miles southeast of Challis, suffered the worst. Many large buildings along its Main Street were damaged, with eight of them condemned and shut down. These structures were mostly made of materials like brick, concrete block, and stone, each varying in size and type.

The earthquake led to significant geological changes, such as surface faulting with visible ruptures, ground displacement, and tilted blocks. It caused groundwater fluctuations, altering water levels in springs and wells, while temporary lakes formed due to rising water tables. Numerous aftershocks occurred, including a notable 5.8 magnitude event almost a year later, on August 22, 1984.