Scientists warn that a small or moderate blast could spew ash and rock as far as three miles from the crater around Mount St. Helens in the next few days.
The volcano began rumbling more intensely Wednesday, with earthquakes ranging from magnitude 2 to 2.8 coming about four times a minute and possibly weakening the lava dome in the crater of the 8,364-foot mountain, the U.S. Geological Survey said.
Scientists did not expect anything like the mountain's devastating eruption in 1980, which killed 57 people and coated towns 250 miles away with ash.
Few people live near the mountain, which is surrounded by a national forest about 100 miles south of Seattle. The closest structure is the Johnston Ridge Observatory, about five miles from the crater.
The Geological Survey raised the mountain's eruption advisory from Level 2 to Level 3 out of a possible 4 on Wednesday, prompting officials to begin notifying various state and federal agencies of a possible eruption. The USGS also has asked the National Weather Service to be ready to track an ash plume with its radar system.
In addition, scientists called off a plan to have two researchers study water rushing from the crater's north face for signs of magma. A plane was still able to fly over the crater Wednesday to collect gas samples. Negligible amounts of volcanic gas were found.
"An aircraft can move the hell out of the way fast," said Jeff Wynn, the chief scientist at the survey's Cascade Volcano Observatory. "We don't want anyone in there on foot."
The USGS has been monitoring St. Helens closely since last Thursday, when swarms of tiny earthquakes were first recorded. On Sunday, scientists issued a notice of volcanic unrest, closing the crater and upper flanks of the volcano to hikers and climbers.
Scientists said they believe the seismic activity is being caused by pressure from a reservoir of molten rock a little more than a mile below the crater. That magma apparently rose from a depth of about six miles in 1998, but never reached the surface, Wynn said.
The mountain's eruption on May 18, 1980, blasted away its top 1,300 feet, spawned mudflows that choked the Columbia River shipping channel, leveled hundreds of square miles of forests and paralyzed towns and cities more than 250 miles to the east with volcanic ash.