Mount St. Helens evacuated; substantial eruption "imminent"
In a sharp reversal of earlier predictions, scientists say they now believe the volcano is capable of a substantial explosion that could create an ash cloud rising tens of thousands of feet and a lava flow from new magma entering the volcano.
Northwesterly winds potentially could carry an ash plume to Chehalis and Kelso.
Because of these new developments, the U.S. Geological Survey (USGS) yesterday raised the volcanic alert to Level 3, indicating an eruption is imminent with a potential for injury to life and property.
In quick response, the U.S. Forest Service, which manages the Mount St. Helens National Volcanic Monument visitor centers, closed down the Johnston Ridge Observatory five miles north of the crater, evacuating about 2,500 visitors downhill to safer locations.
While scientists say an eruption, expected within the next few days to a week, is unlikely to eclipse the May 18, 1980, event, they do believe the volcano could hurl ash and rocks more than five miles — which would still be in the uninhabited area around the mountain. Last week's activity at the mountain is the most powerful since the 1980 blast that killed 57 people.
Based on the Volcanic Explosivity Index, which measures the power behind an eruption, scientists yesterday predicted a 50 percent chance of a new eruption reaching Level 2 on the scale from 1 to 10, and a 30 percent chance of a Level 3, according to a preliminary estimate from Thomas Pierson, a USGS hydrologist.
By comparison, Friday's combined steam and ash cloud measured less than 1 on the index, while the 1980 eruption measured 5.
Pierson underscored that predicting what will happen with the volcano remains a difficult task and that there remains a substantial "error band" in the new forecast.
"There's a possibility of a new bigger eruption," Pierson said. "We're not sure how big."
Shallow earthquakes measuring more than magnitude 3 ramped up in volume yesterday morning until a short steam release occurred within the crater near noon.
Silence followed but was quickly replaced by an hourlong harmonic tremor — steady, rumbling seismic activity — signaling to scientists the upward movement of fresh magma into the mountain. Monitors more than 70 miles away registered the low, steady rumble, prompting the increased warning and evacuation.
Earlier last week, scientists were predicting nothing more than a minor eruption based on the belief that magma from a mid-1980s push was moving again. The absence of sulfur fumes and other gases accompanying fresh magma was further evidence, Pierson said.
But yesterday's events led scientists to suspect that magma gases were being absorbed by heavy moisture resulting from area rains.
"Usually with these things, where [the magma and energy] need to go is out," said Steve Malone, a University of Washington seismologist. "I can't think of anything but rare cases where you have this type of seismicity and tremors and nothing comes out."
Yet, Mount St. Helens is the exception, Malone pointed out, adding that a deep tremor consistent with yesterday's event occurred more than a month before the 1980 eruption.
"Still, because there's no top this time and it's less stable, it's not going to last that long or be that big," he said, acknowledging that seismic activity in 1980 was registering more than magnitude 5.
The word of evacuation spread quickly after 1 p.m.
"They said everyone had to get out of here right now. They were pretty abrupt," said Dan Cherry of Snohomish, who had been at the observatory several hours before the evacuation. "Some of the tourists couldn't get out fast enough, but most wanted to stay as long as they could."
Forest Service officials were in no mood for people to tarry, Cherry said. "One of them said if I'm not seeing your back, you're headed the wrong way," he said.
The cars, motorcycles and trucks snaked west down Highway 504 for more than an hour after the evacuation order. Some visitors tried to pull off at a wayside near a bridge over Coldwater Creek, but they were eventually shooed away by Forest Service officials trailing the caravan.
"How far do we have to go?" asked one motorist. "As far as we tell you to go," retorted a frustrated Forest Service employee.
The evacuation order also took hikers by surprise, with several emerging from monument trails, only to have to hitchhike to their cars. Most trails around the mountain now have been closed.
By late afternoon, many found a new viewing spot at the Coldwater Ridge Visitor Center, farther down the mountain. The mountain was quiet except for what appeared to be small plumes of dust kicked up by falling rocks.
"We're staying for as long as we can," said Paul McKerracher, who'd come down from Redmond and was ousted from Johnston Ridge. "If it doesn't happen today, we'll try to come back tomorrow."
Hal Bernton: 206-464-2581 or hbernton@seattletimes.com
Christopher Schwarzen: 425-783-0577 or cschwarzen@seattletimes.com