Minor earthquake rattles Eureka area

The Daily Inter Lake

and The Associated Press

EUREKA - A minor earthquake shook parts of Northwest Montana on Wednesday morning but apparently caused no damage.

"It really rattled the house pretty good," said Gary Delorenzo, who lives north of Rexford. "It didn't last too long. It shook the house with a great bang, like an explosion."

Chrystal Stacy, a Eureka-area emergency services dispatcher, said people reported pictures falling off walls. There were no injuries.

"It was just so unbelievable when it happened, because it's something you don't expect up here," Stacy said.

Karmen McKinney, office manager at the North Lincoln County Annex, said she felt and heard the earthquake.

"The whole building shook, and I could hear it rumble," she said. "There's no damage that I know of."

The U.S. Geological Survey said the quake had a magnitude of 3.9 and hit at 10:24 a.m. It was centered about 12 miles west of Eureka, on the north side of Sullivan Creek.

Law-enforcement officers said the quake was felt in Libby, Eureka and the Yaak Valley.

In Rexford, Postmaster Tillie Butts noticed the heating ducts rattling overhead.

"It was rather obvious we had an earthquake," Butts said. "There was the shaking, some visual movement."

She called area postmasters and learned the earthquake was felt in Olney and Fortine, too. Then she went online to the U.S. Geological Survey and found out that residents from nine ZIP codes had reported feeling the tremor.

Wednesday's event was the second small earthquake in the Eureka area recently. A 3.7-magnitude tremor occurred March 14 north of Eureka.

Almost all earthquakes in Montana occur along the Intermountain Seismic Belt, a 1,000-mile-long series of faults that's rooted in California, near the southern end of the San Andreas fault system.

The belt runs east across Nevada, then north along Utah's Wasatch Front into Yellowstone and up the Continental Divide.

Northwest Montana is at the far end of the belt.

Although earthquakes helped shape much of western Montana's landscape, the type of forces involved have changed considerably over time.

"That's one of the fascinating things about Montana's geology," said Michael Stickney, director of the Montana Bureau of Mines and Geology Earthquake Studies Office, during a 2004 interview.

Most mountain ranges in the state were formed millions of years ago by compressional forces, when the western edge of the North America tectonic plate collided with microcontinents and island chains carried by the Pacific plate and caused huge blocks of crust to buckle and shift from west to east.

"After that, the exact opposite occurred," Stickney said. "There were extensional forces - the area was slowly stretched and pulled apart, generally from east to west."

These extensional forces are responsible for most modern earthquakes in Montana, he said.

Many geologists think the extension is related to stress that's being generated in southern California, where the North American and Pacific plates slowly are brushing past each other.

Most of that stress is accommodated by movement along the San Andreas Fault, Stickney said. That's why California has so many earthquakes.

However, a portion of it - maybe 10 percent - is being transferred farther east, into the Basin and Range region of Nevada and along the Intermountain Seismic Belt.

"The extensional forces we're seeing here today probably stem from the widely distributed shear resulting from the differential motion of the North American and Pacific plates," Stickney said.

Why these forces are distributed so widely, across more than a thousand miles, is a topic of active research.

"Western North America is one of the few places in the world where we see this," Stickney said.

Wednesday's quake, though small, was the largest event recorded in Montana since a magnitude 4.6 earthquake hit the Centennial Valley on Feb. 5, according to the Earthquake Studies Office Web site, http://mbmgquake.mtech.edu.