The 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. Two children were killed by falling masonry while walking to school in about northeast of Boise, the state capital. Twelve and a half million dollars in damage took place in the Challis-Mackay region in Custer County. As a result of extreme surface faulting, a maximum Mercalli intensity of IX was decided upon, while vibrational damage was at a Mercalli intensity of VI to VII. Three weeks later on November 18, President Ronald Reagan declared the earthquake a Aftershocks were felt for a year afterwards; nearly ten months later, a 5.4 aftershock was recorded on
Surface faulting
The rupture caused clear surface faulting; a 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 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. The ground breakage was, and the throw on the faulting ranged from -0.5 to 2.7 m.
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 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. Two other people suffered minor injuries. In Mackay, a woman was hospitalized due to her injuries.
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.
