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Seismologist believes area due within next 300 years

Missoula had better get ready to rumble, because the Mission fault is due for a major earthquake, a seismologist told his colleagues in the Geological Society of America on Tuesday.

"This is earthquake country," said Michael Stickney, director of the Office of Earthquake Studies at the Montana Bureau of Mines and Geology in Butte.

And the largest, most active fault in western Montana runs along the base of the Mission Mountains from St. Ignatius to Pablo.

Scientists from the U.S. Bureau of Reclamation discovered the fault in 1990, while assessing the safety of earth-filled reservoirs in the Mission Mountains.

During three summers of field work, they found the 45-kilometer-long scarp that marks the fault and is evidence of its last failure - a 7.5 magnitude earthquake that shook western Montana 7,700 years ago.

Stickney believes the fault will produce a similarly large earthquake every 5,000 to 8,000 years.

"So we are within 300 years of the maximum length of time between earthquakes," Stickney said during a presentation at the yearly meeting of the Rocky Mountain Section of the Geological Society of America. "It's time."

Missoula does not, however, think of itself as earthquake-prone, Stickney said. "There's a perception that earthquakes don't happen here. But the seismic data suggests that may not be an accurate perception."

Since the 1920s, 43 earthquakes have rattled Missoula - albeit none big enough to cause damage. The Mission fault, though, will likely fail in a big way - with another 7.5 magnitude or better earthquake, by Stickney's estimation.

"These big faults may sit quietly for thousands of years, then suddenly out of the blue, there's a big earthquake," he said. It happened in Three Forks in 1925 (magnitude 6.8), in the Helena Valley in 1935 (quakes of magnitude 6.2 and 6, spaced two weeks apart), and at Hebgen Lake in 1959 (magnitude 7.5).

"Now we have a whole generation of people who have grown up in Montana without experiencing a major earthquake," Stickney said. "So it takes people by surprise when we get a little rumble."

Growing up in Missoula, Stickney said he too was lulled by the area's relative quiet. "I barely felt the 1959 earthquake," he said. "My bed rattled against the wall. Earthquakes were things that happened somewhere else."

If the Mission Valley fault does fail again, and at a magnitude 7.5, buildings in Missoula will be damaged, Stickney said. Chimneys will topple. Windows will break. "There will be shaking, the likes of which has not been seen here in recorded history."

In the Mission Valley, of course, damage will be even more widespread, Stickney said. Roads will crumble. Pipelines will rupture. Water will slosh out of reservoirs. "A 7.5 earthquake is very large. It would be significant."

Stickney told scientists and teachers at Tuesday afternoon's presentations that he is an advocate of better earthquake preparedness and education throughout the intermountain seismic belt, which runs from southern Nevada through western Montana. The entire region, he said, is earthquake country.

Montana doesn't have the money it needs for seismic monitoring, Stickney said. He supports legislation pending in Congress to provide $38 million a year for five years to upgrade and modernize equipment used to monitor earthquake activity. In Montana, that equipment is 25 years old, and uses 40-year-old technology.

Only by learning more about the region's seismic activity can government prevent the levels of damage seen in California in recent years, Stickney said.

And there will be earthquakes in Montana, he said. "The only reason we haven't experienced a major earthquake in Missoula is because we've been here such a short time. That happy coincidence won't likely hold up."

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