SEATTLE -- Seismic energy releases from Mount St. Helens have been increasing for days now, cranking up to a level not seen since 1986, when the volcano's last dome-building eruption occurred.

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MOUNT SAINT HELENS CAM Mount St. Helens VolcanoCam

"Since this morning, the energy releases have been slowly but steadily ramping up," said Jeff Wynn, chief scientist at the Cascade Volcano Observatory operated by the U.S. Geological Survey in Vancouver, Wash. -- about 50 miles south of the 8,364-foot peak and 140 miles south of Seattle.

"We're furiously setting up new GPS stations all around the place," Wynn said Monday, adding about six to the dozen or so already there. Using Global Positioning Satellite data, scientists can detect tiny movements by the earth.

Data from the Pacific Northwest Seismograph Network Web site show the small quakes in red and blue.

Swarms of small earthquakes have been recorded at the mountain since Thursday. On Sunday, there were more than 10 larger events, with magnitudes ranging between 2 and 2.8.

By Monday, the quakes were moderating -- with magnitudes ranging between 0.5 and 2-plus -- and occurring at a rate of one or two per minute, Wynn said.

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"Energy is just boiling out," he said.

The quakes are occurring less than a mile below the 925-foot dome of hardened lava inside the volcano's gaping crater -- created when Mount St. Helens erupted May 18, 1980, killing 57 people, leveling hundreds of square miles of forests, spewing mud and debris for miles and spreading volcanic ash across the region.

Early tests of gas samples collected above the volcano by helicopter Monday did not show unusually high levels of carbon dioxide or sulfur.

"This tells us that we are probably not yet seeing magma moving up in the system," said Wynn.

Scientists used a helicopter to collect gas samples and to check for ground deformation.

They want to know whether the quakes are the result of water seeping into the mountain or magma moving under its crater.

In either case, scientists will continue to watch the volcano.

Observatory scientists are working 12-hour days, he said. "This is just a hoot to them. This is what they live and breathe and now it's walking and talking to them."

The USGS issued a notice of volcanic unrest on Sunday, citing "increased likelihood of a hazardous event." U.S. Forest Service officials closed hiking trails above the tree line at 4,800 feet. The visitor's center and most other trails at the Mount St. Helens National Monument remained open.

Hiking above the tree line is by permit only and limited to 100 people a day. This time of year, weekday numbers tend to be low while clear weekends draw near capacity, said U.S. Forest Service spokesman Tom Knappenberger. The more accessible Johnston Ridge Visitors Center draws as many as 110,000 visitors a month in late summer, dropping to about 90,000 in early fall.

In the event of an explosion, survey geologist Willie Scott said concern would be focused within the crater and on the upper flanks of the volcano. A five-mile area, primarily north of the volcano, could receive flows of mud and rock debris.

In October 1980, five months after the volcano's devastating eruption, the lava dome began building in the crater. The last dome-building eruption came six years later, though steam explosions have periodically rocked the dome.

Earthquake swarms in 1998 and 2001 did not result in any surface activity.

On Monday morning, a helicopter lowered geophysicist Michael Lisowski onto the lava dome to replace a failed GPS instrument, Wynn said.

While the chopper was near the dome, the pilot was in constant radio contact with geophysicist Bobbie Myers, the observatory's aircraft manager, who during the 1980 blast learned to detect subtle changes in seismic monitors.

"She's known to be able to predict explosive events up to a couple of minutes ahead of time," said Wynn, who confessed to being a nervous wreck while the device was being swapped out.