Stickney said the quakes are being produced by different triggers, indicating a complex process that's poorly understood. The swarms are occurring within the Centennial Tectonic Belt extending west from Yellowstone National Park. The Intermountain Seismic Belt extends north and south from Yellowstone through multiple states.

The park is well known as a geologic hotspot, sitting atop magma. But scientists said that doesn't explain the earthquake swarms in Idaho.

"It clearly is somehow related," Stickney said. "I'm not blaming the belts on Yellowstone, but the fact that they extend into and out of Yellowstone is not a coincidence."

Also perplexing, scientists say, is that the earthquakes are so far from tectonic plates on the West Coast.

"That's the fundamental question that we don't know," Stickney said. "It's related to plate tectonics, but no one knows why this is occurring so far from a plate boundary. Nobody understands why these belts are located where they are."

Kris Pankow, associate director of the University of Utah Seismograph Stations, with Stickney earlier this month presented their findings about last summer's Challis Swarm, as it's called, to fellow scientists at the 2014 American Geophysical Union in San Francisco.