Volcano still shaking 2 days after 'hiccup'
Mount St. Helens eruption could be imminent, scientists warn
The morning sun shines Sunday on Mount St. Helens in Washington.
Scientists expect a small eruption from Mount St. Helens.
Mount St. Helens rumbles to life in a spectacular seismic display.
Observatory near Mount St. Helens is evacuated.
VANCOUVER, Washington (CNN) -- Seismologists were watching Mount St. Helens closely late Sunday after unusual seismic activity during the weekend led them to predict an eruption was imminent.
A small tremor shook the volcano early Sunday, less than a day after it spewed a cloud of steam. Saturday's event prompted geologists to raise the volcano alert to Level 3, indicating another eruption could occur within 24 hours.
"Something could happen at any time, basically," U.S. Geological Survey seismologist Willy Scott said Sunday evening. "We think the probability of such an event is high."
The tremor began about 3 a.m. (6 a.m. ET) Sunday and lasted about 25 minutes, said Peter Frenzen, a scientist at the Mount St. Helens National Volcanic Monument. The volcano is about 50 miles northeast of Portland, Oregon.
After the tremor, seismic activity dropped, but then rose to a level similar to that before the tremor occurred, the U.S. Geological Survey said.
The USGS detected "harmonic tremors" -- continuous, rhythmic quakes that indicate molten rock is rising to the surface and often precede eruptions -- at noon Saturday, about two hours before they raised the alert level. With the alert level up, the U.S. Forest Service and the USGS evacuated Johnson Ridge Observatory, the observation point nearest the volcano.
Scientists on the volcano's flanks are measuring the likelihood of an eruption through thermal imaging, precise measurements of rock movement and gas analysis.
Signs of fresh magma rising are literally in the air. Scientists have noted increased concentrations of carbon dioxide and hydrogen sulfide in the air, said Jim Vallance, research scientist with the USGS.
The volcano had what scientists called "a hiccup" Friday, spewing a harmless plume of steam and ash into the air for about 24 minutes, and a small steam emission on Saturday not long after the harmonic tremors, which lasted for just less than an hour.
Earthquakes are occurring at a rate of up to four per minute, with maximum magnitudes of about 3.0, Vallance said Sunday.
According to USGS geologist Tom Pierson, the volcano has released more seismic energy since earthquakes began September 23 than it has at any point since its devastating eruption of May 18, 1980, which killed 57 people and scattered ash over hundreds of miles. The eruption blew away more than 1,000 feet of the top of the mountain.
After the 1980 explosion, small eruptions continued at Mount St. Helens until 1986, when the volcano went quiet. Friday's eruption was comparable to the minor eruptions seen during that period.
Other small eruptions took place in 1998 and 2001. Lava that oozed out of the volcano during the smaller eruptions built up a 920-foot rocky dome inside the crater left by the 1980 eruption.
"At no time since the pre-1980 buildup to the eruption on May 18 have we seen earthquakes like this," said Bill Steele, a seismologist at the University of Washington's seismic laboratory in Seattle.
"All of the dome-building that went on after the 1980 big eruption didn't have earthquake activity the way we're seeing it now."
Geologists don't expect any eruption in the near future to be as strong as the 1980 blast.
The activity has left the scientists excited and "also a little bit tired," Vallance said.
"This is stretching out capacity a bit," he said. "It's a hectic time, but it's also really an exciting one."
Geologists are working double shifts, often 12-18 hours per day, sometimes 24. But their hope is that after the stress of watching and waiting will come an opportunity for learning.
"Mount St. Helens certainly reminds us of the power of nature, and we can certainly see that in the evidence of the 1980 eruption that's all around us," Frenzen said. "And here we just have an opportunity to see sort of another chapter in its history and to understand the forces that lie beneath our feet."