The U.S. Geological Survey issued a notice of volcanic unrest in response to the swarm of hundreds of earthquakes that began Thursday.
"The key issue is a small explosion without warning. That would be the major event that we're worried about right now," said Willie Scott, a geologist with the USGS office in Vancouver.
The quakes were tiny at first, but on Saturday and Sunday there were more than 10 temblors of magnitude 2.0 to 2.8, the most in a 24-hour period since the last dome-building eruption in October 1986, Scott said.
In the event of an explosion, Scott said the concern would be focused on the area within the crater and the flanks of the volcano. It is possible that a five-mile area primarily north of the volcano could receive flows of mud and rock debris.
Mount St. Helens is about 55 miles northeast of Portland, Ore.
That portion of the mountain blew out during the 1980 eruption that left 57 people dead, devastating hundreds of square miles around the peak and spewing ash over much of the Northwest.
The quakes have occurred at depths less than one mile below the lava dome within the mountain's crater. Some of the earthquakes suggest the involvement of pressurized fluids, such as water or steam, and perhaps magma.
The cause and outcome of the swarm were uncertain Sunday evening. A group of scientists will be on the mountain Monday, collecting additional data on the cause and probable outcome of the swarm of quakes.
"There's been no explosions, there's no outward sign that anything is occurring. (The notice) is all based on the pattern of earthquake activity that is occurring below the dome," said Scott.
Experts believe there is "an increased probability of explosions from the lava dome if the level of current unrest continues or escalates," USGS and the University of Washington Pacific Northwest Seismograph Network in Seattle said in a joint statement.
By Melanthia Mitchell