Seismologists believe there's an increased likelihood of a hazardous event at Mount St. Helens (search) due to a strengthening series of earthquakes at the volcano.

"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 United States Geological Survey (search) office in Vancouver.

Initially, hundreds of tiny earthquakes that began Thursday morning had slowly declined through Saturday. By Sunday, however, there had been 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.

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.

Mount St. Helens is about 55 miles northeast of Portland, Ore.

A group of scientists planned to visit the mountain Monday and conduct a flyover to test for carbon dioxide and sulfur gases, which could suggest the involvement of magma. They'll also erect additional seismic sensors and sophisticated global positioning devices to measure activity.

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's possible that a five-mile area primarily north of the volcano could receive flows of mud and rock debris.

That portion of the mountain blew out during the May 18, 1980, eruption that left 57 people dead, devastating hundreds of square miles around the peak and spewing ash over much of the Northwest.

"We haven't had a swarm of earthquakes at Mount St. Helens since 2001," state seismologist Tony Qamar said. "Clearly something new is happening."

Qamar said if an eruption did occur it would possibly involve ash and steam, called phreatic eruptions.

The cause and outcome of the swarm were uncertain Sunday evening.

"There's been no explosions, there's no outward sign that anything is occurring. This 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 (search) in Seattle said in a joint statement.

A similar swarm of quakes in November 2001 and another in the summer of 1998 did not result in an eruption. However, the quakes could increase the likelihood of small rock slides from the 876-foot-tall lava dome within the mountain's crater.

In the 1986 eruption, magma reached the surface and added to the pile of lava on the crater floor.