Earthquake swarm near Mount St. Helens rattles region
By Andre Stepankowsky / The Daily News | Posted: Monday, February 14, 2011 5:30 pm
A series of aftershocks that rattled the area northwest of Mount St. Helens for about an hour late Monday morning started with an initial jolt that geologists upgraded to a magnitude 4.3.
"It's popping. There are lots of ... aftershocks. Just keeping up with it is a little difficult," said Bill Steele, a seismologist at the University of Washington Geophysics Lab in Seattle.
The initial quake, recorded at 10:35 a.m. six miles northwest of the volcano, was felt over a broad area as far west as Astoria, north to Puyallup and Port Orchard, south to Lake Oswego and east to Lyle, Wash., Steele said. It was first estimated at magnitude 3.3.
The seismology lab received 680 reports from people who felt the shock.
There were no immediate reports of damage or injuries from the initial jolt or the eight aftershocks, which measured up to magnitude 2.8 and were occurring about 3 miles below the surface. A magnitude 3 quake is 10 times less powerful than a magnitude 4.
Despite the widespread area affected, the level of shaking was relatively weak, measuring light to moderate even at the epicenter, according to the U.S. Geological Survey.
Linda Tucker, who lives at the end of Headquarters Road north of Kelso, said the quake woke her up. A native of Southern California, Tucker has felt bigger quakes, but Monday's temblor startled her.
"It was a good jolt," she said.
The cluster of earthquakes is not believed to be directly related to any volcanic activity, Steele said.
The seismic activity occurred in what is known as the Mount St. Helens seismic zone, a fault that starts northwest of the volcano and heads up in a northwesterly direction into Lewis County. The fault cuts through a remote, unpopulated area and was discovered after a 5.5 magnitude earthquake occurred on the fault, coincidentally enough, exactly 30 years ago on Valentine's Day 1981.
"It is not an area we are surprised to see an earthquake swarm. (But) this is awfully vigorous," Steele said.
Although the earthquakes do not appear to be related to any flow of molten rock within Mount St. Helens, the two geologic features likely are linked in some ways, he said.
For example, the fault is a weak spot in the earth's crust that molten rock initially may have exploited to make its way to the surface and gave rise to the volcano in the first place. Mount St. Helens is only estimated to be about 40,000 years old, and most of its cone is only a few thousands years old, scientists say.
The last significant quake to hit the fault — the 5.5 in 1981 — followed the eruption of a huge amount of molten rock in 1980-81. It's possible the eruption changed stresses within the earth's crust, triggering that earthquake. Likewise, the volcano's last eruptive period — from 2004-2008 — could have caused similar changes in the crust that may have contributed to Monday's earthquake swarm.
It's still too early to tell, but the seismic zone is too close to the volcano to rule out some connection between the two, Steele said.
He noted that a magnitude 2.6 quake hit in the same area Jan. 29.
"Right now it looks like a typical Mount St. Helens seismic zone event with vigorous aftershocks."