Mount St. Helens earthquakes may signal eruption

08:30 AM PDT on Monday, September 27, 2004

From KING5.com Staff and Wire Reports

USGS The deep crater of Mount St. Helens.

LONGVIEW, Wash. - 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.

A notice of volcanic unrest was issued Sunday afternoon by the U.S. Geological Survey and the University of Washington.

Hundreds of tiny earthquakes began late last week and slowly declined through Saturday morning. However, the swarm has since changed to include more than 10 larger earthquakes of magnitude 2 to 2.8.

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

The quakes were occuring at depths less than one mile below the lava dome formed in the the mountain's crater.

It's the greatest number of quakes in a 24-hour period since an eruption on October 1986, which was the last dome-building eruption, in which magma reached the surface and added to the pile of lava on the crater floor.

The USGS warning said several causes of the quakes are possible, but most point toward an increased probability of explosions from the lava dome if the level of current unrest continues or escalates.

"If an eruption does occur, the main hazards would be either things being thrown up into the air or debris coming out of the crater, like a mud flow. There's an awful lot of water locked up inside that crater because ice has been accumulating, in fact there are two new glaciers that are forming around the dome," Qamar told KING 5 News on Sunday.

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

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 meantime, recreational climbing and trail in that area have been closed until they can be deemed safe.

Mount St. Helens erupted on May 18, 1980, and left 57 people dead. It devastated hundreds of square miles around the peak and spewed ash over much of the Northwest.

US seismologist Steve Malone said an event the size of the 1980 eruption is "not in the cards."

"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 Willie Scott, a geologist with the USGS office in Vancouver.

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.

The Associated Press contributed to this report.