MISSOULA — The earthquake south of Lincoln that shook people from Spokane to Billings occurred along a fault not previously mapped by seismologists, which is not surprising in a region less studied than the seismically active West Coast.
"We know many faults around here from their signature on the landscape or because they have had little or big earthquakes on them, but there are very likely many more we don't know about because they've been quiet like this one," said Rebecca Bendick, an associate professor at the University of Montana who studies earthquakes, the formation of mountains, and natural disasters worldwide.
"Going forward, there will probably be a dashed line on fault maps somewhere near Lincoln."
Montana's quake risk is unique, although the exact causes of it remain in debate, said Mike Stickney, seismologist at the Earthquake Studies Office of the Montana Bureau of Mines and Geology on the Montana Tech campus in Butte.
"If you look globally, it is interesting and unusual to have a seismic belt such as we have form far from an active plate tectonic boundary," he said.
What is an earthquake?
Many fault lines in Montana remain unmapped. Yellowstone National Park has more quakes but weaker ones than those like the one centered near Lincoln. To understand why, it helps to review what an earthquake is and how the state's iconic peaks formed.
"Even though for us the surface of the Earth feels pretty firm, it's always moving around super duper slowly," Bendick said.
Earth's crust is not solid but made from several tectonic plates that are pushed under, over, or alongside each other where they collide.
The San Andreas and Cascadia faults are classic examples. In California, a plate that forms part of the floor of the Pacific Ocean grinds along the side of another plate, moving several centimeters each year. Along the Northwest coast, an ocean plate slides underneath another that forms the western edge of the continent.
The faults in Montana are smaller and far from the edge of a tectonic plate. They move just millimeters or fractions of a millimeter each year, Stickney said. They are, in effect, cracks formed by extra pressures in the middle of the larger plate.
The basic physics of all earthquakes are the same. Bendick compared them to shooting a rubber band at a friend.
When you pull back a rubber band, you gradually load it with potential energy. Letting go of that rubber band releases the energy suddenly, shooting it forward. Faults store and release energy similarly.
Fault lines are "sticky," Bendick said, so as forces are applied to the crust, the earth bends and stretches, loading it with energy. These tectonic pressures are unnoticeable to humans without scientific monitoring tools. But when they're released, they can shake walls or topple buildings.
"When that bending finally is too much, the fault slips all of a sudden," she said. The earth lurches and shifts as it releases the stored energy. "That's an earthquake."
Types of earthquakes
There are three kinds of faults that shape the Earth in different ways.
"Strike-slip" faults, like the one near Lincoln and the San Andreas, are places with vertical fractures where rocks slide past each other horizontally with little upward or downward motion.
"Normal" faults occur when the crust is stretched to a breaking point, pulling apart to form a valley.
"Reverse," or "thrust," faults are places where one piece of the crust slides over the top of another, such as the Cascadia fault.
Each type creates unique seismic readings, which is how Montana seismologists were quickly able to determine the Lincoln earthquake was caused by a strike-slip fault even though they had not mapped or studied it before.
Bendick compared those seismic signatures to musical genres.
"You can hear a song you've never heard before and know it's pop or it's classical, but that doesn't mean you know the artist or the name of the song," she said.
Yellowstone and the Intermountain Seismic Belt
The planet's crust is thinner in the region near Yellowstone National Park, which sits on top of a collapsed super volcano. The Yellowstone hotspot bakes the surface and softens it. That's why minor earthquakes are more frequent and a very big earthquake is less likely in the immediate region. The crust is weaker, so it can't store up as much energy before being released, just like a thinner string will snap before a thicker one.
Activity in the Yellowstone region generally is unrelated to movement elsewhere in the Intermountain Seismic Belt, Stickney and Bendick said.
The Intermountain web of fault lines stretches from the Kalispell area through Lincoln and Helena toward Yellowstone. It then splits into two forks. The main one reaches the Wasatch Mountains and Salt Lake City in Utah. The smaller one branches into southwest Montana and central Idaho.
Stickney said the quake that originated about nine miles underground near Lincoln appears to be connected to "an ancient zone of faults that runs basically from Helena through Missoula all the way to Spokane." That area, dubbed the Lewis and Clark Zone, is composed of 12 major faults.
He said many of Montana's modern earthquakes appear to be related to that old fault zone, which essentially is a network of weak points vulnerable to shift under modern tectonic stresses.
The state's known faults were identified one of two ways: with geographic clues of the landscape or because of earthquakes.
Bendick said one way to identify a strike-slip fault like the newly discovered one near Lincoln is to look for "creeks and rivers that have a little jog in them," where the earth sheared sideways and redirected the water flow.
Many of western Montana's iconic mountain ranges also were formed by sudden seismic activity along a fault. That includes the Madison, Centennial, Tendoy, Red Rock, Big Belt, and Mission ranges. When the tension at a fault was released, portions of the earth's crust shot upward or cracked and folded to form peaks. Often, the valleys below lie along the fault line.
"Most, if not all, of those obvious range fronts were made by faulting over the last few million years," Bendick said. "Which of those are active now and whether or not there are any little faults being created, we honestly don't really know. It's uncharted territory."
Scientists have spent more time studying bigger, faster faults, like those on the West Coast. They have larger teams, more monitoring equipment, and more residents who submit citizen reports to flesh out their data collections. New technologies like GPS are making it easier to study the geologic forces at play in more remote areas like Montana, Bendick said.
"The most sensible thing for people to do is to assume that moderate earthquakes like we had last night could happen basically anywhere in Western Montana, anywhere west of the Rocky Mountain Front, and to plan accordingly," she said. "We expect those earthquakes to be infrequent, but they can be big and damaging."