In October of 1935, a series of earthquakes occurred in the Helena Valley, Montana, that caused widespread damage. The quakes occurred along the Lewis and Clark Zone, which is recognized as a 30-mile wide meagshear in the earth’s crust, covering some 240 miles along the borders of northern Idaho and northwestern Montana.

While there is historical precedent for dangerous inland earthquakes in the region, more recent quakes that have struck parts of the northwestern states appear to be more mysterious in origin.

Ken Sprenke, a professor of geophysics at the University of Idaho, recently told the Idaho Spokesman Review that one theory, which involves the Yellowstone hot spot, can be ruled out due to the distance. “These are a real mystery,” he said of four earthquakes that occurred late last week.

David Wasson writes in the S-R:

The Sandpoint-area earthquakes occurred along what’s known as the Hope Fault, which is at the northern edge of a large seismic zone stretching from Helena to Coeur d’Alene, with possible extensions into Spokane. But pinpointing the specific cause is difficult, Sprenke said: “It’s harder to say with these earthquakes.”

So what is the origin of the inland earthquakes that have been occurring in this region?

One theory involves similarities between the recent quakes and a series of minor earthquakes that occurred in 2001 around Spokane, Washington. Researchers believe that these commonalities could indicate that some fault lines that have long existed in the region could extend further west than previously thought.

CBC reported last week that coinciding with the inland quakes, an earthquake measuring 6.1 occurred off the southern tip of Haida Gwaii on Friday morning. The USGS reports a total of three small earthquakes that occurred in the northwestern US and Canada around this time.