While Montana has never been struck by an earthquake as large as the 7.8 that struck New Zealand recently, the state has experienced its fair share of earth-rattling shakes. In fact, Montana is the 8th most earthquake-prone state in the union, and experiences between 7 to 10 quakes a day.
Many of those quakes are concentrated near the Helena area, which probably experiences a perceptible tremor every few years. The worst quakes in Helena’s history occurred in October of 1935. Between October 1935 and April 1936, the city experienced 1,800 tremors. The largest was a 6.2, which was proceeded by a 5.9 and was followed by a 6.0 aftershock. The 1935 quake destroyed the brand-new Helena High School, leveled the Lewis and Clark County Hospital, and toppled over 200 chimneys. The high school students had to attend classes in railroad coaches for the rest of the year.
In 2010, the Helena Independent Record collected some memories of the 1935 quake, certainly worth a read.
Montana experiences so many earthquakes because we sit right on top of the Intermountain Seismic Belt. Roughly 100 km wide, the ISB follows the Rocky Mountains through the U.S. and Canada. The instability of the earth around the Rockies is caused by the interaction between the Pacific and North American tectonic plates along the West Coast. This interaction causes the North American plate to stretch, resulting in the ISB. Nearly all of the most earthquake-prone states straddle the belt. As a result, nearly all of the largest earthquakes in Montana have occurred in the western third of the state. The one exception was a shocker of a quake in the far east of the state—Sheridan County—in 1943.
The Helena earthquake season of 1935-36 did not contain the state’s largest earthquake. Nor was it the deadliest, but, like all surprising acts of nature it continues to linger in the memories and imaginations of the state.