Earthquake News

4.3-magnitude earthquake near Mount St. Helens is biggest in 30 years

The 4.3-magnitude earthquake at Mount St. Helens is by far the most powerful in 30 years.

Monday, February 14, 2011 at 6:04 p.m.

A magnitude 4.3 earthquake struck a fault line six miles north of Mount St. Helen's Monday morning, the second strongest since the volcano erupted.
Astoria, Lake Oswego, and even Bremerton, WA near Seattle, were all affected.

The last one, which happened to be on Valentine's Day 30 years ago, was a magnitude 5.5 quake.

After magma seeped up through the fissure and blew the mountain's top on May 18, 1980, the earth's crust appeared to re-adjust, causing the 1981 earthquake. 

coverage in the past

The 30th anniversary of the May 1980 eruption was commemorated with images and reportage.

Seismologist Seth Moran of the Cascades Volcano Observatory in Vancouver said Monday's quake was of the "strike-slip sort."

The Juan de Fuca plate, a huge tectonic plate, is submerging beneath the North American plate.
The plates become caught together at times.
When the plates glide past each other, energy is released, causing an earthquake, he explained.

More than 900 people reported feeling the earthquake at 10:35 a.m., and their replies were posted on the USGS's "Did you feel it?" page on its website.
There was no damage recorded. 

"I felt it, and I'm sure a lot of folks here did as well," Moran added.
"It was widely felt, and it was a decent-sized earthquake for a 4.3, which is reasonable."

Since the May 1980 explosion, Mount St. Helens has rumbled, belched, and quaked virtually every day, averaging one to two earthquakes each day, according to Moran.
A swarm of earthquakes, known as the Mount St. Helen's Seismic Zone, was reported in late January in the same approximate location as Monday's quake.

The zone stretches 30 miles north from Mount St. Helens to Morton, Washington.
According to him, the greatest quake in the January swarm had a magnitude of 2.6.
The quake on Monday, like the others before it, occurred at a shallow depth of roughly 3.1 miles. 

The quake does not portend a new eruption or a new cycle of dome-building inside the crater, as happened after a smaller magma outburst in fall 2004, according to Moran.
The fault was merely the result of tectonic plates colliding beneath the earth's surface.

Nonetheless, he stated: "Because 4.3 earthquakes aren't often in our area, you pay attention whenever one occurs.
Realistically, we don't know what will happen, but it's a safe bet that this will be the largest event we've ever seen, based on the fact that there has only been one other event of comparable size in the last 30 years." 


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