Idaho is a seismically active state, experiencing hundreds of earthquakes annually, though most are too small to be felt. Its geographical position within a major zone of seismic activity means that significant, damaging events are a documented part of its history. The state sits at the intersection of powerful tectonic forces, resulting in a complex pattern of crustal movement and faulting. This activity makes Idaho one of the most seismically active states in the United States.
The Tectonic Forces Shaping Idaho
The primary driver of seismic activity in Idaho is the ongoing crustal stretching associated with the Basin and Range Province. This geological phenomenon involves the North American tectonic plate being slowly pulled apart, which extends the crust and creates numerous fractures. This stretching motion causes the formation of normal faults, where one block of land drops down relative to the other. This continuous extension builds strain in the Earth’s crust until it is suddenly released as an earthquake.
This extensional movement links Idaho to the Intermountain Seismic Belt (ISB), a broad zone of earthquake activity extending from Montana through Utah and Nevada. Most of Idaho’s earthquakes are shallow, occurring at depths less than 35 kilometers, which results in more intense surface shaking. A secondary influence on the state’s seismicity comes from the Yellowstone Hotspot, particularly in eastern Idaho. The interaction between the hotspot’s deep magmatic activity and regional crustal stretching creates a complex pattern of faulting known as the Yellowstone Tectonic Parabola.
Major Seismic Zones Within the State
The majority of the state’s earthquake activity is concentrated in Central and Eastern Idaho, forming a branch of the Intermountain Seismic Belt. This region is often referred to as the Central Idaho Seismic Zone or Centennial Tectonic Belt. The zone contains multiple major active fault systems capable of producing large-magnitude earthquakes, including the Lost River Fault and the Sawtooth Fault. The Lost River Fault Zone is noted for its potential to generate significant surface-rupturing events.
Southeastern Idaho is also highly active, lying directly within the Intermountain Seismic Belt and sharing major fault systems with neighboring states. The Bear Lake fault zone is recognized as one of the most active structures in this region. Further west, the Western Idaho Seismic Zone, located between McCall and Boise, presents a seismic hazard. Active faults here, such as the Squaw Creek Fault, lie close to the state’s largest urban areas, posing a threat of significant damage.
Notable Historical Earthquakes
The most significant and damaging event in the state’s recorded history was the 1983 Borah Peak Earthquake. This magnitude 6.9 shock occurred on October 28, 1983, along the Lost River Fault in Central Idaho. The earthquake produced a surface rupture, creating a zone of fresh scarps over 35 kilometers long between Challis and Mackay. Vertical displacement along the fault reached up to 2.7 meters in some places.
The quake caused approximately $12.5 million in property damage, primarily in Challis and Mackay, where many unreinforced masonry buildings collapsed. Two children in Challis were killed when a stone storefront collapsed on them, making them the only fatalities directly attributed to the event. Geological effects included extensive landslides, rockfalls, and liquefaction that produced sand boils near the Mackay Reservoir.
The 1959 Hebgen Lake Earthquake, a magnitude 7.3 event just across the border in Montana, was strongly felt in Idaho and caused widespread effects. More recently, the magnitude 6.5 Stanley earthquake in March 2020 was the second largest recorded in Idaho. Although it caused minimal damage, it generated thousands of aftershocks, illustrating the state’s continued seismic unrest.