SEATTLE – Small earthquakes rattled Mount St. Helens at the rate of one or two a minute Monday, and seismologists were working to determine the significance of some of the most intense seismic activity in nearly 20 years.
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 Jeff Wynn, chief scientist with the U.S. Geological Survey (search) in Vancouver, Wash. He noted additional tests are necessary to better define whether there's magma moving under the mountain's crater.
Scientists are trying to figure out what is going on beneath the 925-foot-high dome of hardened lava within the mountain's gaping crater. They want to know whether the quakes are the result of water seeping into the mountain or magma.
In either case, they'll continue to watch the volcano — which erupted to devastating effect in 1980 — from the Cascade Volcano Observatory (search) in Vancouver, about 50 miles away. Additional flights are also planned but not yet scheduled, Wynn said.
A helicopter carried scientists and instruments over the crater Monday to assess the gases and ground deformation that would indicate pressure building below the dome.
Measurements of ground movement "will tell us whether there's any new magma coming into the system," said Seth Moran, a seismologist at the observatory. That data will not be immediately available.
Swarms of small earthquakes began Thursday and increased in frequency and magnitude until Sunday, when there were more than 10 events with a magnitude of 2 to 2.8. The quakes are at depths less than one mile below the lava dome.
By Monday, the pace was unchanged and the magnitude range had narrowed to between 0.5 and 2-plus, Wynn said.
"Since this morning, the energy releases have been slowly but steadily ramping up," he said. "We don't know what that means. ... That kind of energy hasn't been seen since 1986," when the mountain's last lava-dome-building eruption occurred.
Moran said there was potential for explosions within the crater that could throw rocks as far as the rim.
The USGS issued a notice of volcanic unrest on Sunday, citing "an increased likelihood of a hazardous event." U.S. Forest Service (search) officials closed hiking trails above the tree line at 4,800 feet on the 8,364-foot mountain, though the visitor's center and most other trails at the Mount St. Helens National Monument (search) remained open.
St. Helens' May 18, 1980, eruption killed 57 people, leveled hundreds of square miles of forests and dumped volcanic ash across the Northwest.
Sunday's activity was the most in a 24-hour period since the 1986 eruption, said survey geologist Willie Scott. Earthquake swarms in 1998 and 2001 did not result in any surface activity.
If there is an explosion, 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.
On Monday, a helicopter lowered a geophysicist onto the lava dome to replace a failed instrument used to measure tiny movements that indicate whether the dome is swelling, Wynn said.
While the chopper was near the dome, the pilot was in radio contact with Bobbie Myers, another geophysicist 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," Wynn said.