Quakes Beneath Our Feet

By Beacon Staff

The Jan 13. earthquake centered near Seeley Lake was relatively small. Registering a magnitude of 3.9 on the Richter Scale, it occurred at 4:17 a.m. about 23 miles northeast of Seeley Lake, and west of Augusta in the Swan Range. While some Seeley residents said they couldn’t even feel it, others reported mild shaking, though no one was injured, nor was any property damaged.

But the quake occurred one day after the worst earthquake to hit Haiti in 200 years caused widespread death and devastation in the small island nation. And in the week that followed, ongoing “swarms” of small quakes clustered around Yellowstone National Park occurred. All of which raises the question: Is there any relation between all these quakes?

“No, I don’t see any connection,” Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology, said. “I think the timing was pure coincidence.”

Haiti’s quake resulted from activity along the southern edge of the North American tectonic plate, he added. “We’re more related to what’s going on on the edge of the Pacific plate,” to the west.

The Seeley quake occurred along a fault known as the South Fork Flathead Fault, which runs from southwest of Augusta toward the Hungry Horse reservoir. The location of the Jan. 13 quake’s epicenter indicates it was the result of the east-west expansion of the earth’s crust, as it is being pulled apart slightly through ongoing geologic activity, according to Stickney, who added that such occurrences are also quite normal.

“Having an earthquake in the southern Swan Mountains, there, really is no surprise,” he said. “It certainly has plenty of precedence.”

The Bureau of Mines’ seismic equipment registers, on average, four earthquakes a day, nearly all of which are of small magnitude.

“We get a whole lot of ‘ones,’” Stickney said, referring to the Richter scale, which measures the strength of earthquakes along orders of magnitude. That means the Haiti quake registering a 7.0 was actually hundreds of times bigger than the Seeley quake that registered 3.9.

In Montana, the big ones are few and far between.

“Rarely, fortunately, the big destructive earthquakes happen,” Stickney said.

According to the U.S. Geological Survey, Montana is one of the most seismically active states in the union, with most of the activity concentrated in the mountainous western third. The biggest in Montana’s history was the magnitude 7.3 earthquake of Hebgen Lake on Aug. 17, 1959. The massive quake triggered a landslide in the Madison River Canyon that buried 26 people. Two other campers were killed by a rolling boulder, and the quake caused more than $11 million in damage.

Though nothing on the scale of Hebgen Lake has struck close to the Flathead Valley, northwest Montana has seen its share of shakes. A large quake caused minor damage along the eastern shore of Flathead Lake in March of 1952. A magnitude 4.7 earthquake occurred along the southern half of Flathead Lake in April of 1969, causing damage in Big Arm, Proctor and Dayton and causing some wells near Polson to go dry. Over the following years, 325 aftershocks were reported in the area around Polson and Big Arm.

The quake activity in Western Montana is due to its location along what is known as the Intermountain Seismic Belt, a corridor of geologic activity extending from the Flathead Valley southeast through Helena and toward the Yellowstone region. From there, it continues along the Wyoming-Idaho border, through Utah and into Nevada.

“It’s kind of a broad zone with a shotgun pattern of epicenters that occur along it,” Stickney said.

According to the Earthquake Studies office at the Montana Bureau of Mines and Geology, another branch called the Centennial Tectonic Belt also extends west from Yellowstone into Idaho. It is along the Centennial Belt that the two biggest quakes, the Hebgen Lake quake and another near Borah Peak in Idaho in 1983, occurred.

Unlike California, where quake epicenters correspond closely to mapped faults, the epicenters of Montana’s quakes are usually near, but not directly on, these known faults.

“It’s just the opposite here,” Stickney said. “The small earthquakes kind of avoid the faults we know about.”

There are 45 known faults in Western Montana, he said, most of which have not been studied in great detail. Stickney speculated that the epicenter of the recent Seeley quake, which he described as a common, “background seismic event,” was deeper than the South Fork Flathead fault. Seismic activity directly on those mapped faults is more rare.

“It may be that these faults only move during the major earthquakes,” he said. “These 45 faults may sit there quietly for thousands, perhaps tens of thousands, of years.”