The rumbling appears to have abated, but University of Utah scientists are busy analyzing a "swarm" of 900 earthquakes that have struck Yellowstone National Park since Dec. 26. In the meantime, federal officials say the seismic activity, clustered around the north end of Yellowstone Lake, is no cause for alarm even as a swarm of alarmist warnings shakes cyberspace.

There is no indication the earthquakes presage a major volcanic or seismic event, said park geologist Hank Heasler in response to a fake evacuation order and non-official advisories appearing on the Internet urging people to flee.

"We take the public safety very seriously. There was never any damage. There was nothing that indicated a potential catastrophe," he said.

Earthquake swarms -- small and modest earthquakes close in time and space -- are hardly unusual at the park, which normally experiences up to 20 quakes a day. But this swarm is noteworthy for its intensity and the speed with which it appeared, then dissipated around Tuesday, said U. geologist Robert Smith, who leads seismic research at Yellowstone. Smith and at least eight U. colleagues have toiled almost around the clock, analyzing the quakes once seismographs began registering robust shocks.

"We were working on it because of the potential of how it could lead to a bigger earthquake or generate hydrothermal activity," he said. Thanks to seismic monitoring equipment installed around the park in recent years, scientists quickly determined magmatic movement as a cause, according to Heasler.

But that doesn't answer some intriguing questions as to what the event signifies for the world's most famous volcanic region. The underground movement of superheated water is the suspected cause, but regular old tectonic activity could be playing a role as well.

The cluster included 111 quakes greater than magnitude 2.0. The strongest, measuring magnitude 3.9 on Dec. 28, was felt by park personnel 30 miles from its epicenter at Old Faithful, Heasler said. But many people in Salt Lake City, 300 miles to the south, claimed to have felt it, too.

"It would very unusual for an earthquake in the magnitude 3s to be felt farther away than 30 miles. We had a report that someone felt them 1,300 miles away," Heasler said.

An underground magma chamber, responsible for the park's famous hydrothermal features, comes within five miles of the surface at Yellowstone. The area is one of the most seismically active places in the world, but it has been nearly 50 years since the last major earthquake, and 70,000 years since the last volcanic eruption. The Yellowstone "hotspot" has produced three catastrophic globe-affecting eruptions in the past 2 million years, the last one forming the Yellowstone Caldera 640,000 years ago.

According to Smith's research, the caldera floor has been rising at an accelerated rate of seven centimeters a year since 2004, but he is unsure whether any connection exists between caldera movement and earthquake swarms.

In the recent swarm, the quakes migrated northward from Stevenson Island to Fishing Bridge along a 10-kilometer path. The event is the second most intense swarm in the park's 137-year history and the strongest since a 1985 string of 3,000 quakes hit the western edge of the caldera between Madison Junction and West Yellowstone. That swarm lasted three months, with temblors measuring up to 4.9 magnitude.

Once the weather clears, park officials expect to perform aerial reconnaissance of remote geysers, hot springs and other thermal features, using infrared cameras. Front country features appear to be unaffected.

"We're going to make observations as best we can," Heasler said. "If we see things in the imaging that would warrant on-the-ground field investigation, we'll go out and look. This isn't the easiest time of year for getting around."

bmaffly@sltrib.com

www.sltrib.com/ci_11409304

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