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

February 16, 2011

By Stuart Tomlinson, The Oregonian


Fault line, won’t you be my Valentine?

The second largest earthquake since Mount St. Helens erupted — a magnitude 4.3 shaker — rocked a fault line six miles north of the volcano Monday morning. People felt it as far away as Astoria, Lake Oswego, Hood River and even Bremerton, Wash., near Seattle.

The last one, as it happens, was 30 years ago also on Valentine’s Day, a magnitude 5.5 temblor.

That 1981 earthquake appeared to be the result of the earth’s crust readjusting after magma oozed up through the fault and blew the mountain’s top on May 18, 1980.

Monday’s quake was of the “strike-slip variety,” said seismologist Seth Moran of the Cascades Volcano Observatory in Vancouver.

The large tectonic Juan de Fuca plate is diving beneath the North American plate. At places, the plates get stuck together. An earthquake occurs when the plates slip past each other, releasing energy, he said.

More than 900 people reported feeling the 10:35 a.m. earthquake, posting their responses on the U.S. Geologic Survey’s “Did you feel it?” page on its website.  No damage was reported.

“I felt it, and certainly a lot of people here felt it,” Moran said. “It was broadly felt, and, for a 4.3, that’s appropriate, it was a decent-sized earthquake.”

Largest earthquake in 30 years rocks area north of Mount St. Helens Largest earthquake in 30 years rocks area north of Mount St. Helens A 4.3 earthquake rumbled beneath the earth about six miles north of Mount St. Helens Monday morning. The quake, which was felt by hundreds of people from as far away as Bremerton, Wash. and Astoria, occurred along a well-known fault line and is not a precursor to another eruption or more dome-building inside Mount St. Helens’ crater, seismologists said. Watch video

Mount St. Helens has rumbled and belched and quaked nearly every day since the May 1980 eruption, averaging one to two earthquakes a day, Moran said. In late January, a swarm of earthquakes was detected in the same general area as Monday’s quake, known as the Mount St. Helen’s Seismic Zone.

The zone runs from Mount St. Helens 30 miles north to Morton, Wash. The largest quake in that January swarm was a modest magnitude 2.6, he said. Monday’s quake, like those previous ones, occurred at a relatively shallow depth of about 3.1 miles.

The quake doesn’t presage another eruption, or another round of dome-building inside the crater like the action that followed a smaller magma eruption in fall 2004, Moran said. This one was simply a result of tectonic plates clashing beneath the earth’s surface along the fault.

Still, he said: “Anytime a 4.3 earthquake happens in this area, you pay attention because they’re not that common. Realistically, we don’t know exactly what’s going to happen, but there’s a reasonable guess that this was the largest event that we’re going to see, and that’s based on the fact that we’ve only had one event that’s larger in the past 30 years.”

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