Quake gives Mount Rainier a shake

SEATTLE – Mount Rainier shook during a magnitude 3.2 earthquake under the volcanic crater, but scientists said Tuesday that the quake is not related to activity at Mount St. Helens and does not signal an eruption soon.

The small quake at 11:23 a.m. Sunday was centered one mile below the surface of Rainier’s volcanic crater, said Bill Steele, coordinator of the Pacific Northwest Seismograph Network at the University of Washington.

“Directly under the volcano, that’s a significant size,” Steele said.

One other 3.2 magnitude quake has been recorded at the mountain in the past 30 years: on Feb. 19, 2002.

Sunday’s quake occurred within a cluster of 17 to 18 shallow temblors, most of which occurred over several hours, Steele said.

“This doesn’t mean there’s an increased chance of eruption, but we will watch it carefully,” he said.

Steele said quake activity at Rainier has increased over normal levels in recent weeks. Five quakes greater than magnitude 2.0 were recorded Oct. 25-31, he said.

But Mount Rainier calmed down Monday and has been “blessedly quiet” since, he said.

Scientists are watching for continuing earthquakes near the volcano’s surface and a particular type of seismic activity that results from fluid moving through rocks. Those clues might signal an eruption, Steele said.

“Mount Rainier has been quiet for a long time, and it would take a lot of pounding away from magma (molten rock) to work its way up toward the summit,” he said.

Mount Rainier is considered the most hazardous of all Cascade Range volcanoes because debris flows could affect thousands of people living in nearby communities, he said.

“We hope to have weeks to months of warning, and perhaps even a year, but volcanoes can move pretty quickly and so we’re always keeping an eye out,” Steele said.

Mount Rainier last erupted about 150 years ago and scientists say it’s likely to erupt again at some point.

The quakes are not related to activity at Mount St. Helens, 50 miles south of Rainier, said seismologist Seth Moran at the U.S. Geological Survey’s Cascades Volcano Observatory in Vancouver, Wash.

Mount St. Helens continued Tuesday to build its lava dome, with molten rock reaching the surface at the rate of seven to eight cubic meters – about one large dump-truck load – per second.

“It’s certainly tempting to think there might be connection,” Moran told the Seattle Post-Intelligencer. “But there’s no evidence of any link between the two. … They have completely separate plumbing systems.”

The magma originates in the same place, deep below the surface.

West of the Cascade volcanoes, the Juan de Fuca plate is sliding southeast and underneath the North America continental plate. As one plate dives beneath the edge of another, friction and pressure melt rock along the crumbly edges, called subduction zones. Sticky, gassy molten material gets superheated to 1,600 degrees Fahrenheit and tries to shoot up through cracks in the crust.

Mount St. Helens rumbled back to life Sept. 23, with shuddering seismic activity that peaked above magnitude 3 as hot magma broke through rocks in its path. Molten rock reached the surface Oct. 11, marking resumption of dome-building activity that had stopped in 1986.

A more explosive eruption, possibly dropping ash within a 10-mile radius of the crater, is possible at any time, scientists have said.