By Laura L. Myers
SEATTLE (Reuters) - An earthquake measured at a magnitude of 4.3 struck near Mount St. Helens in Washington state on Monday, shaking an area extending north to Puget Sound and south across the Oregon border, geologists said.
15 February 2011 01:32 GMT
It was the biggest tremor to hit the geologically active seismic zone around Mount St. Helens since 1981, a year after the volcano erupted in a catastrophic blast that decimated over 200 square miles of forest, said Carolyn Driedger, a hydrologist for the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Washington.
There were no reports of damage or injuries, but the main tremor was felt over a distance of roughly 120 miles from north to south, said Bill Steele, a spokesman for the University of Washington's Pacific Northwest Seismic Network.
The quake, recorded at 10:35 a.m. local time, was centered about 5 1/2 miles north of the Mount St. Helens crater at a depth roughly 3 miles underground.
It was followed by three aftershocks measuring between 2.0 and 3.0 in magnitude and more than 100 smaller tremors, Steele told Reuters.
He said the quake was likely noticed by thousands of people, and that at least 870 people reported feeling the tremor by registering their experience on the network's website.
Visitors to the Mount St. Helens national Volcanic Monument, about 40 miles from the quake's epicenter, felt several seconds of ground motion, said Peter Frenzen, a scientist at the park.
"One of our visitors told me it felt like a truck hit the building," he said.
Although the 1980 eruption of Mount St. Helens began with a 5.2 earthquake, and a jolt of similar size hit the area the following year, geologists do not believe the latest tremor was triggered by changes within the volcano itself, Steele said.
Mount St. Helens remains the most active volcano in the Cascades range, though no major volcanic activity has been detected there since 2008.