Quakes beneath Mount St. Helens concern seismologists

By Seattle Times staff and news services

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Seismologists think there's an increased likelihood of a hazardous event because of recent changes in the mountain's seismic activity.

"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, Wash.

Officials with the Mount St. Helens National Volcanic Monument yesterday closed the mountain to climbing and closed three hiking trails north of the volcanic crater.

Initially, hundreds of tiny earthquakes that began Thursday morning had slowly declined through Saturday. By yesterday, the swarm had changed to include more than 10 larger earthquakes of magnitude 2 to 2.8, the most in a 24-hour period since the last dome-building eruption in October 1986, Scott said.

Some of the earthquakes suggest the involvement of pressurized fluids, such as water or steam, and perhaps magma.

The quakes have occurred at depths less than one mile below the lava dome within the mountain's crater.

The cause and outcome of the swarm was uncertain last night. A group of scientists planned to visit the mountain today 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 System devices to measure activity.

"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," Scott said.

Experts think 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.

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 1980 eruption that left 57 people dead, devastating hundreds of square miles around the peak and spewing ash over much of the Northwest.
 
Since debris could fly outside the crater, monument officials decided to close the mountain to climbers above 4,800 feet, which is a rare step, said Peter Frenzen, monument scientist. Current climbing permits have been canceled, and no more permits will be issued, he said.

Below the crater to the north, parts of the Loowit and Truman trails and the entire Willow Trail have been closed. An explosion could send water and rock sliding out of the crater, and the three trails are in the path of the two streams that already flow out of the mountain, Frenzen said.

The trails, which make up less than five miles of a 200-mile trail system, are closed about once a year when rainfall is particularly heavy. No roads or other trails are closed, and the popular visitors center remains open.

As a precaution, Anthony Qamar, the Washington state seismologist, canceled his yearly research trip into the crater Saturday.

But Qamar said he was more interested in the swarm of earthquakes than worried by them. If there is an explosion, he said, it is likely to be "tiny" and without lava flow. It also could be what he called a quiet eruption, with molten rock oozing up to the surface and spilling out.

"For the first time in many, many years, there may actually be some molten rock pushing up on the dome," Qamar said.

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.

Seattle Times staff reporters Ashley Bach and Cara Solomon contributed to this report.