Hundreds of earthquakes have hit Yellowstone National Park, raising fears of a more powerful volcanic eruption.

The earthquake swarm, the biggest in more than 20 years, is being closely monitored by scientists and emergency authorities.

The series of small quakes included three last Friday which measured stronger than magnitude 3.0. The strongest since this latest swarm of quakes began on December 27 was 3.9.

No damage has yet been reported but scientists say this level of activity - there have been more than 500 tremors in the last week - is highly unusual.

"The earthquake sequence is the most intense in this area for some years," said the Yellowstone Volcano Observatory. Some of the larger earthquakes have been felt by park employees and guests, according to the observatory.

The swarm is occurring beneath the northern part of Yellowstone Lake in the park. Yellowstone sits on the caldera of an ancient supervolcano and continuing geothermal activity can be seen in the picturesque geysers and steam holes, such as Old Faithful.

About 1,000 to 2,000 tremors a year have been recorded since 2004. The most devastating earthquake in recent history in the Yellowstone region occurred on August 17, 1959, when a magnitude 7.1 earthquake hit.

It was centered near Hebgen Lake, Montana and it caused landslides that killed 28 people and caused more than $11 million in damage.

Geysers in Yellowstone National park changed eruption times, and new ones began to erupt. On June 30, 1975, a magnitude 6.4 tremor hit the park.

Professor Robert B. Smith, a geophysicist at the University of Utah and one of the leading experts on earthquake and volcanic activity at Yellowstone, said that the swarm was significant.

"It's not business as usual," he said. "This is a large earthquake swarm, and we've recorded several hundred. We are paying careful attention. This is an important sequence."

The last full-scale explosion of the Yellowstone Supervolcano, the Lava Creek eruption which happened approximately 640,000 years ago, ejected about 240 cubic miles of rock and dust into the sky.

Geologists have been closely monitoring the rise and fall of the Yellowstone Plateau as an indication of changes in magma chamber pressure.

The Yellowstone caldera floor has risen recently - almost 3in per year for the past three years - a rate more than three times greater than ever observed since such measurements began in 1923.

From mid-summer 2004 through to mid-summer 2008, the land surface within the caldera moved upwards as much as 8in at the White Lake GPS station. The last major earthquake swarm was in 1985 and lasted three months.

The observatory said similar swarms have occurred in the past without triggering steam explosions or volcanic activity. However, the observatory said there is some potential for explosions and that earthquakes may continue and increase in intensity.

Joe Moore, director of the Wyoming Office of Homeland Security, said his office is tracking the events at Yellowstone on a minute-by-minute basis. "It's being followed very closely," Mr Moore said.

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