VANCOUVER, Wash. (AP) - Mount St. Helens appeared to be settling down early today after a startling blast that shot gritty volcanic ash to about 36,000 feet above sea level.

A fine dusting was reported as far away as 125 miles to the east-northeast in southern Grant County by the time ashfall stopped late Tuesday night, the National Weather Service reported. An ashfall advisory for some areas east of the Cascade Range was canceled at midnight.

"It looks like it's gone back to roughly the same type of (earthquake) signal that we were seeing before." University of Washington seismologist Steve Malone told The Seattle Times late Tuesday.

Scientists said it was the most powerful blast from St. Helens since the latest round of volcanic activity began last fall.

The 30-minute outpouring began with practically no warning around 5:25 p.m. Tuesday, about an hour after a 2.0-magnitude quake on the east side of the 8,364-foot volcano, the most active in the 48 contiguous states, said Bill Steele, coordinator of the Pacific Northwest Seismograph Network at the university in Seattle.

In the preceding hours there had been a subtle increase in quake activity, Malone said.

"We've had this relatively placid dome-building eruption going on, but we've been saying all along that could change at any time," Carolyn Driedger, a spokeswoman for the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash., told The Times.

"It is the kind of eruption that we have been talking about the possibility of all along," Driedger told the Seattle Post-Intelligencer.

Seismologists said the blast explosion destroyed three sensors in the crater but other instruments around the rim of the crater remained intact.

Roger Cloutier, a senior weather service forecaster, said very fine ash was reported in much of Yakima and Kittitas counties, including Yakima, Ellensburg and Toppenish.

"It's a very light dusting," Cloutier said. "You probably could only see it on cars."

Aviation officials said commercial flights at Seattle-Tacoma and Portland international airports were not affected.

Scientists said the explosion did not appear to indicate a higher risk of a more dangerous blast, noting that high levels of the kind of gases that often signal an eruption had not been detected in recent flights over the crater.

"We don't expect another explosion," said Peggy Johnson, a seismologist at the lab.

Steele said the latest blast may have been triggered by partial collapse of a lava dome which began growing in the crater beside an older dome in October.

"Until we get a better view in the crater, we won't know," Steele said.

The newer dome had been crumbling slightly in the past week, releasing small puffs of ash and steam, Malone said.

Geologists have said there is little chance of anything like the massive explosion that removed the top 1,243 feet of the mountain on May 18, 1980, killing 57 people and covering the region with gritty ash.

Even so, Scott Miller and William Nicoll, both 19, college roommates visiting the Mount St. Helens National Volcanic Monument, were stunned.

"The first thing that went through my mind was, 'Is this 1980 again?'" Miller told The Times.

Miller snapped a couple of photographs over Nicoll's protests, after which they leaped into their car and drove west, yelling at other motorists to turn back until they had gone about a mile and felt safe again.

"It was a pretty big adrenaline rush," Nicoll said.

Mount St. Helens, located about 50 miles north-northeast of Vancouver, has been spewing ash and steam since last fall.

Starting Sept. 23, swarms of earthquakes peaked above magnitude 3 as magma broke through solid rock as it rose through the mountain, reaching the surface on Oct. 11. Since then the emerging magma has resumed dome-building after a 19-year hiatus.