MOUNT ST. HELENS NATIONAL MONUMENT, Wash. -- Mount St. Helens stewed in volcanic gases amid low-level earthquakes yesterday, with crowds of eager tourists hoping to glimpse an eruption that scientists said could occur immediately or within a few weeks.
A second long tremor early yesterday and an increase in volcanic gases strongly suggest that magma is moving inside, researchers from the US Geological Survey said. The mountain's alert was raised to Level 3, the highest possible, after a volcanic tremor was detected Saturday for the first time since before the mountain's 1980 eruption.
''I don't think anyone now thinks this will stop with steam explosions," geologist Willie Scott said yesterday at the Geological Survey's Cascades Volcano Observatory in Vancouver, Wash.
But Scott said scientists discussed lowering their alert from a Level 3 ''volcano advisory," which indicates eruption is imminent, to Level 2 ''volcanic unrest," which indicates an eruption is possible. They needed more data before making any change, he said.
''What we haven't gotten back today yet is a lot of field measurements -- there's a gas flight going on, a flight to use thermal imaging to look at the [lava] dome, [Global Positioning System] data need to be downloaded," Scott said yesterday. ''There's a lot of work that needs to be done. That will occur overnight and tomorrow morning."
Scientists said they do not expect anything close to the devastation of the explosion in May 18, 1980, that killed 57 people and coated much of the Northwest with ash.
''Of course, the volcano reserves the right to change its mind," said monument scientist Peter Frenzen with the US Forest Service, which operates the park.
Some specialists had said Saturday that an explosion would probably occur within 24 hours. But as the hours passed, others cautioned that the timing is hard to predict.
''No one is predicting it as a sure thing," said Bill Steele at the University of Washington's seismology lab in Seattle. ''This could be going on for weeks."
Crowds gathered along a park road at what was said to be a safe distance, about 8.5 miles from the mountain, to see what would happen next. Barbecues were fired up and entrepreneurs sold hot dogs and coffee to people camped along the side of the road in lawn chairs and pickup beds.
''It'd be neat if it spews something over and out," said Chris Sawyer, 40, of Dundee, Ore., who had a large camera set up on a tripod at the Coldwater Ridge Visitors Center.
Hundreds of people were cleared from a popular observatory closer to the peak Saturday after a tremor and brief release of steam.
The mountain was outwardly quiet at midday. Clouds of dust rose occasionally, caused by rockfall from the towering canyon walls. But earthquakes were occurring ''multiple times per minute," Steele said, peaking every few minutes at magnitudes as high as 3.
Seismic activity became more sporadic over the day, said seismologist Tony Qamar at the University of Washington's seismic lab in Seattle.
Scientists were unsure how explosive the eruption may be; depending on the gas content of the magma and conditions, it could range from a passive emission to an explosion that throws up a column of ash, Scott said.
Besides lava flows, ash, and rock-throwing, an eruption could cause melting of the volcano's 600-foot-deep glacier and trigger debris flows to the barren pumice plain at the foot of the mountain.
The 1980 blast obliterated the top 1,300 feet of the volcano, devastated miles of forest, and buried the North Fork of the Toutle River in debris and ash as much as 600 feet deep.
This time, scientists expected populated areas to get little ash if the light west-northwest wind holds. The closest community is Toutle, 30 miles west near the entrance to the park in the Gifford Pinchot National Forest about 100 miles south of Seattle.
The main concern was a significant ash plume carrying gritty pulverized rock and silica that could damage aircraft engines and the surfaces of cars and homes.
Steele said the mountain took scientists on a ''roller coaster ride" early yesterday when instruments detected the second extended volcanic vibration in two days -- 25 minutes long compared with the 50-minute vibration Saturday.
''It died off and quickly became a non-issue. But had it been as long as the one following that little steam burst yesterday, we could be moving to an eruption pretty quickly," Steele said.
Scientists also detected elevated levels of carbon dioxide and other volcanic gases, including the rotten egg smell of hydrogen sulfide, that reflect changes in the volume of magma rising within the mountain.