The quakes all were centered in the middle of the plateau, a lava dome at the southern end of the park. Geophysicist Dr. Bob Smith, a University of Utah professor and a Jackson Hole homeowner, said he was working the night the swarm started and found it “interesting and intriguing.”

“I was up working and watching these, saying ‘Whoa, what does this all mean?’” Smith said in a telephone interview from Utah on Thursday. “It kept my interest quite high.”

Earthquakes are most commonly associated with a pattern that has a main shock and after shock, Smith said. There also is a model in which fore shocks precede the main and aftershocks. Swarms of small earthquakes, like those that occurred beneath the Pitchstone Plateau, are found in volcanic regions, like Yellowstone, he said.

The world’s first national park is actually the remains of a super volcano that last saw an eruption ages ago. The quakes occurred on the southern edge of the most recent caldera and beneath the youngest lava flow that makes up the plateau. Lava there stopped flowing 70,000 years ago, Smith said.

“Yellowstone is very much characterized by earthquake swarms,” Smith said. Between 1983 and 2006 there have been as many as 70 swarms of small earthquakes around the park, according to research by one of his students, Smith said.

The professor said he pinpointed the quakes as occurring on a hypothesized extension of the Teton Fault, which runs along the eastern base of the Teton Range and north to the caldera. At the north end of Jackson Hole, the fault has no surface expression because it is buried by the Pitchstone Plateau lava.

The fault creates a zone of weakness that likely allowed the quakes to occur, Smith said.

Photo: Yellowstone geyser

A seismograph station is located within a kilometer of the surface site of the quakes, which allowed for precise measurements, Smith said. The quakes ranged in depth from one to four miles.

Smith called the 2.7 magnitude quake “quite energetic.” But it is unlikely it was felt by anybody, he said.

A Yellowstone ranger at the South Entrance station said he felt no quakes at the time they occurred.

Swarms of minor earthquakes are related to fluids, like magma and hot water, found in volcanic systems, Smith continued. On Pitchstone Plateau, however, there are no hydrothermal features, like there are in Yellowstone’s famous geyser basins, save for Phantom Fumarole, he said.

The quakes were likely caused by a fluid seeping into a fault and causing it to slip, Smith said. The entire area around the quakes is “very fractured” he said of the upper 10 kilometers of the earth’s crust.

Smith recently released the result of research that showed the Yellowstone caldera has been rising 6 cm a year over the last two years. Yet there have been no earthquakes associated with that rise, he said.

A host of information about Yellowstone and Jackson Hole quakes can be found at the university’s seismic Web site www.seis.utah.edu/.

Smith said he doesn’t expect the super volcano to blow, based on the recent shaking. But he did note that he has both earthquake and volcano insurance on his house in Jackson.

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