MSU geologist warns Montanans to be ready for earthquakes

By WALT WILLIAMS, Chronicle Staff Writer

Geologist Dave Lageson was at his home Monday night when the house started to shake. At first he thought it was the wind, but then he realized it wasn't storming.

It was, in fact, something he has studied all his life.

"I realized immediately with tremendous joy that it was an earthquake," he joked with a standing room only crowd at Montana State University's Student Union Building Thursday.

Monday's quake has so far turned out to be a gentle reminder that people in western Montana live in one of the most earthquake-prone areas of the nation.

It could have been worse. The Hebgen Lake earthquake of 1959 killed 28 people and caused $11 million in damage. It's epicenter was just outside West Yellowstone.

But Lageson has found that not many locals are aware of the danger.

"I'm shocked over and over again by how many folks in Montana who have lived here for years and years do not think of this as earthquake country," he said.

Lageson, an MSU professor of geology, summed up what was known so far about Monday's quake during a lunchtime lecture on campus.

The quake measured 5.6 on the Richter scale; its epicenter was about 13 miles northeast of Dillon near a mountain known as the Hogback.

Western Montana is lined with a spiderweb of faults. They're not like the famous San Andreas Fault of California, which has formed where two tectonic plates are slowly grinding against each other.

Instead, the Montana faults are the result of the land being stretched and pulled, resulting in uplifting. It is same process that created Montana's mountains and wide valleys.

Bozeman is literally boxed in by faults in all directions, including faults along the slopes of the Bridger and Gallatin ranges. Paradise Valley has a long fault running the length of the valley.

Monday's earthquake occurred along the McCartney Fault Zone in an area near the Big Hole River. So far it has followed a normal pattern of one large quake followed by a decreasing number of smaller aftershocks, meaning it probably isn't a precursor to anything bigger, Lageson said.

But larger quakes can strike. Western Montana was rattled with a series of large quakes in the first half of the last century, including one that struck midway between Three Forks and Manhattan in the 1930s, damaging both towns.

Montana's faults probably won't produce earthquakes stronger than anything in the 7.0 range on the Richter scale because they're not big enough, Lageson said.

That's enough to do some serious damage to buildings, particularly buildings with unreinforced masonry like those found in downtown Bozeman.

Don't go running out of a building should an earthquake strike, particularly if you're downtown, Lageson warned.

"If you're down there in one of those nice little restaurants and there's an earthquake, do not run out onto the sidewalk," he said. "They're not very high buildings, but, you know, it doesn't take many bricks to hurt."

He also said it was a good idea to keep emergency supplies around the house, remembering to be prepared for all types of weather.

MSU Extension has a Web site about disaster preparedness at http://www.montanahelp.org .

Walt Williams is at wwilliams@dailychronicle.com

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