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This graph shows the number of earthquakes per day since an earthquake swarm began in Yellowstone National Park. The swarm peaked Jan. 21, but now seems to be subsiding. GRAPH COURTESY UNIVERSITY OF UTAH

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After nearly a month, an earthquake swarm that has rattled Yellowstone National Park with more than 1,800 temblors is diminishing, researchers say.

The swarm, which began Jan. 17 near West Yellowstone, Mont., is the largest since a 1985 swarm that lasted three months and included least one magnitude-4.7 earthquake. That account comes from Dr. Robert Smith, a professor of geophysics at the University of Utah and coordinating scientist for the Yellowstone Volcano Observatory who also owns a cabin in Jackson Hole.

The latest swarm has resulted in nearly a dozen earthquakes that were magnitude 3 or larger.

A similar swarm roughly a year ago rattled the area around Yellowstone Lake with more than 500 earthquakes in 12 days starting Dec. 27, 2008.

This year’s swarm is clustered in an area roughly four to six miles long. The swarm extends over the area of Yellowstone caldera’s western boundary about five miles deep.

The caldera, a 37-mile-by-25-mile volcanic feature at the center of the park, lies above a magma plume that extends hundreds of miles beneath the Earth’s surface and has caused large volcanic eruptions as recently as 640,000 years ago.

 “It’s clearly not a time-progression or space-progression like we had last year,” Smith said. “This is out on the edge of the caldera system.”

“This is a big one,” he said of the swarm. “This is very substantial.”

While Smith said he doesn’t think the earthquakes have stopped altogether – the University of Utah earthquake Web site recorded at least two small-magnitude earthquakes late Tuesday – he said activity has diminished to the point where university researchers are no longer filing daily reports on the swarm.

The university has had researchers analyzing earthquakes 24 hours a day, seven days a week since the swarm began, Smith said. About 900 earthquakes have been analyzed so far.

“There are a lot of data to look at,” he said.

Eventually, Smith said, he hopes to develop a 3-D movie that will allow people to watch the simulated swarm evolve over time.

“It never stops,” Smith said. “Every day of my career, there’s something new at Yellowstone. It’s a fantastic place for new science and new observations. It’s a window into the earth’s interior where you’re seeing these things happen all the time.”

On the U.S. Geological Survey “Did you feel it?” earthquake site, people filed more than 100 reports after feeling the ground shake since the swarm began.

The last Yellowstone event reported on the site was a magnitude-2.3 earthquake Feb. 3.

People who wish to report feeling an earthquake can visit http://earthquake.usgs.gov/earthquakes/dyfi/.