“It’s an extension to the southwest of that same swarm — the Maple Creek Swarm,” Smith said. “It was an intense swarm that started last year, then it died down, but it’s been kicking off earthquakes in this area for almost two years.”

The recent quake swarm started on Feb. 8 and by noon Sunday had produced 180 earthquakes that topped out at magnitude 2.9, according to the U.S. Geological Survey. Hundreds smaller earthquakes have also occurred, but were too small to be located with precision.

If the latest rumblings are considered a continuation of the Maple Creek swarm, it would mean the subterranean instability has been going on for eight months.

Smith could recall only one swarm that persisted for longer: a 1985 event that started in the fall and lasted until the following summer. West Yellowstone residents at the time were spooked, and some even skipped town because of fears that they were feeling harbingers of a Yellowstone “supervolcano” eruption.

Although of much interest to scientists such as Smith, earthquake swarms of such a small magnitude are generally inconsequential to infrastructure and human safety. Events smaller than magnitude 2.5 on the Richter scale are usually not perceptible to human beings, and only detected by seismography.

Earthquake swarms are also routine phenomena in the seismically active Greater Yellowstone Ecosystem. In the past two years they have come and gone in Yellowstone, near the Teton Range and in the Caribou Range between Soda Springs, Idaho, and Star Valley. Typically they occur along smaller faults that branch out from major fault lines.

The Maple Creek Swarm, Smith said, is occurring along an eastward extension of the Hebgen Lake fault zone. He also believes the Yellowstone Caldera is playing a role in the activity, even though its boundary lies about 6 miles to the east.

As the caldera rises at a rate of 3 to 4 centimeters a year, lands outside of its boundaries — including where the Maple Creek Swarm is popping off — tend to sag. The push and pull, which is being fed by a slowly growing magma chamber, act something like a pressure cooker, Smith said, and earthquake swarms along the periphery serve as the release valve.

“That’s what I think that’s what this one is,” he said.

Rather than a “worrisome” sign of a supervolcano eruption, geophysicists like Mike Poland and Jamie Farrell see the seismic unrest east of West Yellowstone as a learning opportunity. The duo wrote of that potential in a recent blog post on the USGS’s Yellowstone Volcano Observatory website.

“It is during periods of change when scientists can develop, test and refine their models of how the Yellowstone volcanic system works,” Poland and Farrell wrote. “Past seismic swarms like those of 2004, 2009 and 2010 have led to new insights into the behavior of the caldera system. We hope to expand this knowledge through future analyses of the 2017 and 2018 seismicity.”