Tick, tock, aftershock: Mission Valley fault line one of the region’s largest
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With recent catastrophic damage due to Japan’s massive earthquake on March 13, residents in the Mission Valley should be aware of a nearby fault line that could cause damage. Along the western edge of the mountains, thousands of fault lines make up the larger Mission Valley fault.
According to the director of Earthquake Studies in Butte, Michael Stickney, the Mission Valley fault line is one of the two largest in northwest Montana.
“There are 45 faults in western Montana with potential to cause geologic upheavals,” Stickney said, “and the Mission Valley fault is one of those.”
The Mission Valley could experience an earthquake with a magnitude up to 7.5, Stickney said. An earthquake magnitude over seven is considered a major quake and could cause serious damage.
According to Stickney, the fact that the mountain range runs directly in a north-south path is a classic sign of a fault line lying along the edge of the range.
The fault is divided into northern and southern segments. The northern part of the fault runs approximately from Big Fork to Polson, and is thought to be the less active section. The southern segment is from Polson to Arlee, and is where a major quake could most likely occur.
Although the northern segment is less likely to be cause for concern, Stickney says an earthquake along Flathead Lake could cause significant damage.
“The north is not as active, but if it ruptured, there is potential for a wave to destroy buildings,” Stickney said.
The giant wave, called a seismic seiche, occurs when an earthquake passes through an area with a lake. The shake from the earthquake (if large enough) could cause the lake to create a standing wave from the lake sloshing around.
Stickney admits that though an event such as a seiche on Flathead lake is possible, it is highly unlikely.
According to Stickney, there is no way of knowing when the next major rumble from the fault will occur.
“There has never been a historic quake along the fault,” he said. “The last time the quake went off was around 7,700 years ago.”
If one looks closely, there are bits and pieces of geological evidence hidden along the fault from its previous quake. South of Lake McDonald Dam there are fault scarp marks in the earth’s crust. A fault scarp is the feature on the surface of the earth that looks like a step caused by slip on the fault, which is when one plate slips underneath or over another.
At the northwestern edge of the lake large vertical cliffs provide more evidence of the historic event, where land along the base of the mountain rose hundreds of feet, while land near the lake sank downward.
According to Stickney, there is very little, if any, warning prior to an earthquake, and residents in the Mission Valley should be prepared for a seismic event.
“The best thing to do is to take precaution,” he said.
He says that all heavy objects should be on low shelves, if near a place one often spends time. Also, all homes in the Mission Valley are close enough to the fault line to sustain damage in a quake, and should have their hot water heaters anchored by strapping them to a wall to prevent the heater from tipping over. Homeowners should also have their homes bolted to the foundation.
