August 1, 2006
By Erik Robinson, Columbian staff writer
The Columbian - Vancouver, WA
For the third time in two weeks, Mount St. Helens trembled with a 3.6-magnitude earthquake.
Photo: New growth on Mount St. Helens' lava dome as seen from the south on July 26; 2006. Mount Rainier; another Northwest volcano; is in the distance. (USGS Photo)
Scientists with the U.S. Geological Survey in Vancouver said the quakes probably do not represent a change in eruption style, although they are the largest since the earliest days of the eruption in the fall of 2004.
"They are consistent with what we've seen before," said Seth Moran, a seismologist with the USGS David A. Johnston Cascades Volcano Observatory.
The volcano quaked with a 3.6-magnitude temblor at 2:34 a.m. Monday, which follows two other 3.6-magnitude quakes and a 3.5-magnitude quake over the past two weeks. Scientists suspect that the movement represents a lurching upward of the massive new lava spine evolving on the crater floor.
"We've been seeing these kinds of events throughout," Moran said. "The only difference is these are slightly bigger."
Several earthquakes of low- to mid-3 magnitude have punctuated the regular drumbeat of tiny earthquakes that have occurred every few minutes for the past several months.
Scientists are focused on two potential causes: Each quake either represents a lurch or surge of the massive spine coming out of the conduit, or it could represent fracturing of rock around the conduit pumping lava onto the crater surface. A third scenario raises the possibility of a slump or fracture within the lava dome after lava is extruded.
The volcano continues to extrude lava at a rate of about a pickup truck load every few seconds.
UPDATE
Previously: Mount St. Helens reawakened with a flurry of thousands of tiny earthquakes beginning in the early morning of Sept. 23, 2004. On Oct. 11, 2004, lava emerged for the first time since 1986 on the crater created by the catastrophic eruption of May 18, 1980.
What's new: Over the past two weeks, the volcano has rumbled with its biggest earthquakes since the early days of the current eruption.
What's next: Scientists are continuing to monitor the volcano to determine whether the eruption may move out of its relatively placid pace into something more violent.
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