Top States with Earthquake Risks: #6-8 Wyoming, Idaho, Montana
States generally ranked as the sixth through eighth highest earthquake risks are Wyoming, Idaho, and Montana.
Earthquakes have occurred in every county of Wyoming. However, the most seismically active area is the northwest region, where Yellowstone National Park is located. Historical records indicate that earthquakes as strong a magnitude 6.5 are possible in most parts of the state. Several faults in western Wyoming are capable of magnitude 7.2 to 7.5 ruptures, including the Teton Fault, Star Valley Fault, Greys River Fault, Rock Creek Fault, and Bear River Fault. Recent studies have led researchers to conclude that many of these fault systems are past due for big quakes.
The largest recorded earthquake in Wyoming occurred on August 18, 1959, in Yellowstone National Park. It was a magnitude 6.5 event and was considered an aftershock of the magnitude 7.2 Hebgen Lake earthquake that struck southwestern Montana the night before.
Despite the headline grabbing attention of the threat of the Yellowstone supervolcano, or another volcanic eruption in the area, the reality is, a large earthquake poses a higher risk to the area.
On the morning of October 28, 1983, a magnitude 6.9 earthquake shook Central Idaho at Borah Peak. It was the largest quake to strike Idaho in recent history. Like any major tremor, aftershocks rattled the area for months afterwards, including a 5.4 quake. Two children died as a result of falling masonry while they were walking to school. While 200 homes received minor to moderate damage, 39 houses and 11 commercial buildings had major damage.
In August 1959, the southeastern corner of the state experienced the magnitude 7.2 Hebgen Lake, Montana, earthquake.
The area near the city of Soda Springs, in southeastern Idaho, experienced an earthquake swarm in September 2017. The largest of the tremors was a moderate 5.3, but it was enough to make the few in the area wary of a bigger one. The nearby Eastern Bear Lake Fault could rupture in the future with a magnitude 7.3 earthquake, although the U.S. Geological Survey estimates a 63 percent chance of a magnitude 6.0 or larger earthquake to hit the area within 50 years.
Should a large earthquake hit a small, rural community, the area could potentially become isolated with the destruction of roads, bridges, cell and radio towers, and electrical power transmission lines.
A magnitude 7.2 earthquake, named the Hebgen Lake earthquake, struck southwestern Montana on August 17, 1959. The earthquake was the deadliest and strongest on record to strike Montana. The tremor killed 28 and caused a huge landslide that blocked the Madison River and created Quake Lake, which is five miles long and a third of a mile wide.
The second largest quake in Montana’s recent history was a magnitude 6.2 that hit near Helena on October 18, 1935. A large aftershock of magnitude 6.0 struck on October 31. Those two quakes killed four people. Before the October 18 main shock, a small foreshock happened on October 3, and a more damaging 5.9 foreshock occurred on October 12.
Helena is at the northern end of the Intermountain Seismic Belt, which is an area of seismic risk that runs from northwestern Arizona through Utah and Idaho and into Montana. Near Helena is the Lewis and Clark Fault Zone, which typically has west-northwest trending faults.
If a magnitude 6.3 earthquake were to strike the Helena area today, it would likely result in over $500 million in property damage. North of the city, where the water table is high and alluvial soil is common, damage would likely result from liquefaction.