St Helens showing volcanic unrest

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Sunday, September 26, 2004
Last updated 7:23 p.m. PT

Scientists issue notice of volcanic unrest in Mount St. Helens crater

SEATTLE -- Seismologists believe there's an increased likelihood of a hazardous event at Mount St. Helens due to recent changes in the mountain's seismic activity, and on Sunday the U.S. Geological Survey issued a notice of volcanic unrest.

"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.

Initially, hundreds of tiny earthquakes that began Thursday morning had slowly declined through Saturday. By Sunday, however, the swarm had changed to include more than 10 larger earthquakes 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.

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. A group of scientists planned to visit the mountain Monday to collect data.

"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 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.

I just heard on the radio that the mountain is pretty much off limits below 4800'.<br><br>Where's the best place to gape at the lava dome given that constraint?

The volcanoe observatory has a webcam so one may gape virtually at the lava dome almost anywhere.<br>www.fs.fed.us/gpnf/mshnvm/volcanocam/

Thanks astrand.<br><br>This morning the link gave me a re-direct to <br><br> www.fs.fed.us/gpnf/volcanocams/msh/