Scientists were not sure what was going on beneath the southwest Washington volcano and planned a full day of investigations.
Seth Moran, a seismologist with the U.S. Geological Survey at the Volcano Observatory in Vancouver, said that while a major blast was unlikely, there was potential for explosions within the crater that could throw rocks as far as the rim.
The earthquake swarms began Thursday and on Sunday the Geological Survey issued a notice of volcanic unrest, saying there was "an increased likelihood of a hazardous event."
U.S. Forest Service officials closed hiking trails above the treeline at 4,800 feet on the mountain. The visitor's center and most other trails at the Mount St. Helens National Monument remained open, but officials did not want climbers exposed to a sudden gas or steam explosion while peering over the crater rim.
"Standing on the rim, from what geologists tell us, would not be a good idea," said Peter Frenzen, monument scientist.
Scientists planned to fly over the 8,364-foot mountain on Monday to test for carbon dioxide and sulfur gases, which could indicate whether magma is moving beneath the crater. They also planned to set up additional seismic sensors and global positioning devices to measure activity.
The May 18, 1980, eruption killed 57 people, leveled hundreds of square miles of forests and dumped volcanic ash across the Northwest.
In October 1980, a lava dome began building in the crater, eventually growing to 925 feet from the crater floor. The last dome-building eruption was in October 1986, but steam explosions have periodically rocked the dome.
Previous earthquake swarms three years ago did not result in any activity on the surface.
"The key issue is a small explosion without warning. That would be the major event that we're worried about right now," said Willie Scott, a geologist with the USGS office in Vancouver.
The hundreds of tiny earthquakes that began Thursday morning slowly declined through Saturday. But on Sunday, the swarms of small quakes were punctuated by more than 10 quakes of magnitude 2.0 to 2.8, the most in a 24-hour period since the 1986 dome-building eruption, Scott said.
The swarms continued Monday. The quakes were too small to be felt, even at the monument, said Tom Knappenberger, spokesman for the Gifford Pinchot National Forest.
Similar swarms of quakes in 1998 and 2001 did not result in eruptions.
The current quakes have occurred at depths less than one mile below the lava dome within the mountain's crater. Some of the earthquakes suggest the involvement of pressurized fluids, such as water or steam, and perhaps magma.
In the event of an explosion, Scott said the concern would be focused on the area within the crater and the upper flanks of the volcano. It's possible that a five-mile area primarily north of the volcano could receive flows of mud and rock debris.
"We haven't had a swarm of earthquakes at Mount St. Helens since 2001," state seismologist Tony Qamar said. "Clearly something new is happening."
Qamar said if an eruption did occur it would possibly involve ash and steam, called phreatic eruptions.
"There's been no explosions, there's no outward sign that anything is occurring. This is all based on the pattern of earthquake activity that is occurring below the dome," said Scott.
Experts believe there is "an increased probability of explosions from the lava dome if the level of current unrest continues or escalates," USGS and the University of Washington Pacific Northwest Seismograph Network in Seattle said in a joint statement Sunday.
For More Information:
St. Helens Info -- www.pnsn.org