Quake was caused by Montana being ‘stretched apart'

BOZEMAN — Western Montana is earthquake country, but the frequent quakes in the area are usually moderately sized, like the July 25 quake that was centered 13 miles north of Dillon, says a Montana State University geologist.

"That's why we have young, rugged mountains. They are lifted up one quake at a time," David Lageson told a room full of people at MSU last week.

Lageson, head of MSU's Earth Sciences Department and an expert in tectonics and rock-faulting, said western Montana has "a whole lot of little faults rather than one big one like the San Andreas fault." Because of that, Montana will tend to have many moderately sized earthquakes.

"Little poodles can only bark so loud. We're going to have poodles and cow dogs for earthquakes, not huskies," Lageson said.

"Montana is being stretched apart," he said, adding that Yellowstone National Park's thermal features are the result of that expansion and that "the very bottom of the Yellowstone hot spot is 200 to 300 kilometers under Dillon." However, the recent quake is not the harbinger of a volcanic eruption.

"We would have huge background activity for years before any noticeable volcanic activity." Any earthquake you can feel still deserves your respect, however, and Lageson suggested people take precautions in their homes. He suggested that the most important precaution is to secure gas hot water heaters to the wall.

If not secured, they can shift during a quake, rupture the gas line and cause an explosion. In addition, heavy objects that might fall should be secured to the walls, things such as tall bookcases and grandfather clocks.

Another precaution, he suggested, is to not run outside to the sidewalk if you are near tall structures, especially those with exteriors of bricks and masonry.

The largest Montana quakes have been on the order of the 7.5 magnitude quake that formed Hebgen Lake in 1959, killing 28 people. Last week's quake had a magnitude of 5.6. Lageson said it was felt strongly because it was at the relatively shallow depth of 3.1 miles below the surface.

The Hebgen quake's epicenter was about 14 miles underground.

The July 25 quake was felt as far away as western Idaho and Billings, but was recorded by seismographs around the world. It took about five minutes for Monday's quake to register in south Texas, and about 20 minutes for its rolling waves to travel through the earth to the South Pacific, about the same length of time it took for the Indonesian quake in December 2004 to register on MSU and Lewis and Clark Cavern seismographs.

Lageson can monitor all of those seismographs from his office computer.

Though he joked that seismologists could "become very lethargic in the future, because you can do it all by computer," he added that MSU geology undergraduates are in the field mapping the Dillon area every year, because it is such a well known fault area.

Lageson said attributing any earthquake to surface features is chancy, but because last week's quake was so shallow, he would guess that it was caused by the McCartney Fault, which runs roughly from the Wise River on the northwest to Melrose on the southeast.

Lageson's map of known fault lines was so complex it looked like a street map, but he said just as many or more fault lines are unknown, and perhaps unknowable, until they shift.

He said earthquakes cannot be predicted with accuracy, and less-accurate forecasting is of limited use. For instance, not enough is known about Montana fault systems to make either forecasts or predictions.

In California, where millions of dollars of earthquake research has been done, geologists can only say that there is a 65 percent chance of a major earthquake in the next 30 years.

More useful than forecasts, said Lageson, would be for Montanans to acknowledge the risk of earthquakes and do what they can to minimize the risks in their own homes.

Montana has about a thousand earthquakes a year, and one that does some damage every few years, he said. Helena is one of the most active areas.

Lageson's research program in structural geology at MSU focuses on the tectonics of the intermountain area and Yellowstone hotspot, with the goal of contributing to better seismic risk assessments in the region.