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Scientists watching unusual events near volcano

Series of earthquakes could signal eruption of Mount St. Helens
A series of earthquakes may signal an imminent eruption of Mount St. Helens, shown in this undated photo.
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There are some signs that Mount St. Helens could erupt again, just like it did 24 years ago.
Volcanic eruption
Portland (Oregon)
U.S. Geological Survey

VANCOUVER, Washington (CNN) -- A series of unusual earthquakes near Mount St. Helens in recent days has scientists warning that something more serious could be imminent.

The "hazardous event" the U.S. Geological Survey warns is possible could be an explosion caused by steam building up inside the volcano, or it could be more serious -- involving molten rock and deadly gas.

The quakes are occurring less than a mile below the surface of an 876-foot-tall lava dome within Mount St. Helens' crater. Some of them are of a type that indicates the presence of pressurized fluids or magma, the USGS and the University of Washington Pacific Northwest Seismograph Network in Seattle said Monday in a joint statement.

"We're still thinking it's not likely to be anything real big," said Tom Pierson, a USGS research scientist at the Cascades Volcano Observatory in Vancouver. "We're not real worried. [The seismic activity is] very interesting to us because it's unusual."

A group of very small, shallow earthquakes, called a "swarm" by seismologists, began Thursday morning and peaked about midday Friday, Pierson said, then slowly declined through Sunday morning. Those quakes were all less than magnitude 1.0.

But since then, more than 10 earthquakes with a magnitude between 2.0 and 2.8 have been recorded, according to the notice issued Monday.

"They're all pretty small magnitude," Pierson said. "Maybe that largest one, you might feel if you were pretty close by."

Still, that's the most earthquakes recorded in a 24-hour period since October 1986, when Mount St. Helens had a minor eruption that added to the lava dome that began forming after the catastrophic eruption of May 18, 1980. That eruption blew out the side of the mountain, killing 57 people and deforesting 230 square miles.

In the fall, quakes in the area are common, Pierson said, as rainwater seeps into the ground and turns into steam when it reaches the lava below ground. With no outlet, the steam can build up until a small explosion occurs, he said.

"That, we feel is the most probable thing that could happen, if anything happens," he said. "Could be this thing would just shake for awhile."

Such an explosion could send rocks flying into the air in the area of the crater and the outer flanks of the volcano, Pierson said.

Yet scientists cannot rule out the possibility of a more serious eruption, he said. An aircraft will soon fly over the lava dome to test for the presence of carbon dioxide and sulfur dioxide, signs that magma might be building up.

Similar earthquake swarms were recorded in 1998 and 2001, but no explosion occurred in either year, Pierson said.

The U.S. Forest Service, which manages the park on Mount St. Helens, has closed areas around the crater and the volcano's outer flanks to hikers.

Two small trails across the north side of the mountain have also been closed, and hikers are not being issued permits, Ranger Todd Cullens said. Hikers and climbers are still allowed on trails below 4,800 feet, he said, and tours are continuing.

Mount St. Helens is about 55 miles northeast of Portland, Oregon.

CNN's Kimberly Osias contributed to this report.

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