Seattle ? Government scientists raised the alert level Saturday for Mount St. Helens after its second steam eruption in two days was followed by a powerful tremor. They said the next eruption was imminent or in progress, and could threaten life and property in the remote area near the volcano.

Hundreds of visitors at the building closest to the volcano — Johnston Ridge Observatory five miles away — were asked to leave. They went quickly to their cars and drove away, with some relocating several miles north to Coldwater Ridge Visitors Center, which officials said was safe.

The volcano alert of Mount St. Helens was raised to Level 3, which “indicates we feel an eruption is imminent, or is in progress,” said U.S. Geological Survey geologist Tom Pierson from the observatory. He said Saturday afternoon that an explosion probably would happen within the next 24 hours.

Pierson said the volcano had released more energy in about a week of seismic activity than it had at any point since its devastating May 18, 1980, eruption, which killed 57 people and coated much of the Northwest with ash. But scientists expect the impending eruption to be much smaller than the 1980 blast.

The growing consensus among scientists is that new magma is probably entering the volcano’s upper levels, possibly bringing with it volatile gases that could lead to eruptions, said Bill Steele at the University of Washington’s seismic laboratory in Seattle.

Explosions from the crater could occur without warning, possibly throwing rock onto the flanks of the volcano, the USGS said in a news release. Still, scientists said the evacuation of the observatory was primarily a precaution in case of heavy ash discharge, which could make it difficult to drive.

“We still feel the risk is confined to this area,” Pierson said.

No communities are near Mount St. Helens; the closest, Toutle, is 30 miles west.

Visitors watch as a large rock slide takes place in the main crater of Mount St. Helens near the Johnson Ridge Observatory in Washington state. Within an hour, the observatory was evacuated Saturday as scientists feared a magma-producing eruption was imminent.

A day after the volcano spewed a plume of steam and ash thousands of feet into the air, there was a very brief steam release Saturday — a puff of white cloud, followed by a dust-raising landslide in the crater. A volcanic tremor signal that came next was what prompted the heightened alert level.

The signal “was far stronger after today’s steam eruption” than the tremor that followed Friday’s blast, Steele said. “We were picking it up throughout western Washington and into central Oregon. Yesterday we had a very weak tremor signal.”

A tremor — a steady vibration — indicates movement of gases or fluid within the volcano,” Steele said, while individual earthquakes indicate “a pounding and breaking of rock.”

Saturday’s tremor lasted about an hour and was followed by a series of earthquakes — one or two a minute, some greater than magnitude 2, said Tom Yelin, a USGS seismologist in Seattle.

More steam explosions are likely, and possibly an extrusion of lava.

“This is the most intense seismic activity we’ve seen since the May 18th eruption,” said geologist Dan Dzurisin at the U.S. Geological Survey’s Cascades Volcanic Observatory in Vancouver, Wash., about 50 miles south.

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.