Quake Talk: seismologist discusses recent Lincoln-area earthquakes

The earthquakes that have rattled the Lincoln area intermittently since last July may not have caused any major damage or ripped open cracks in the earth, but Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology, said it's nevertheless been a "pretty spectacular earthquake sequence."

The latest notable tremors from that sequence hit just this week, with a magnitude 3.7 striking the area Monday morning at 5:11 a.m., followed just two minutes later by a magnitude 3.5.

The most recent quakes were centered in the same area near Stemple Pass and roughly the same intensity as the magnitude 3.8 that struck April 17 and prompted Lincoln teacher Annette Gardner to invite Stickney to town for a presentation on the seismic activity in the area.

"I was certainly interested in learning more about it myself, then it never stopped. We kept having more and more earthquakes," Gardner said.

During his May 2 visit to Lincoln, Stickney explained that the temblors Lincoln keeps experiencing are still aftershocks from the 5.8 magnitude temblor that struck just after midnight on July 6 last year.

"We have not gone a 24-hour period yet, since last summer, without having at least one aftershock," Stickney said. "We're still averaging two to 43 per day in the magnitude one or smaller range."

During Stickney's presentation, it was clear that the July 6 earthquake here was notable for several reasons. Foremost, it was the largest earthquake in Montana in 58 years, the largest earthquake in the lower 48 states for the entire year of 2017 and the eighth largest in Montana history.

It also struck an area of Montana where the probability of a large quake wasn't considered to be very high.

"A lot of times when I talk about earthquakes, people think 'well, we don't have earthquakes in Montana; this is a special circumstance,'" Stickney said.

The reality is that western Montana is more active seismically than most people think.

The Montana Regional Seismic Network, which began recording earthquakes in 1982, has 43 stations around the state, as of last year has recorded about 476,000 quakes, Stickney said.

Most of them have been too small to feel, but the reason for so much activity can be traced to the intermountain seismic belt, which starts in southern Utah and splits near Yellowstone National Park, with one branch running to the west and the other heading northwest from Bozeman, through Butte, Helena and Lincoln and on into northwestern Montana.

Stickney said there are 79 "potentially active" faults in Montana, but none of them are in the Lincoln area. In fact, a national seismic hazard map showed the two areas with the highest probability for a significant quake were the Flathead region to the northwest and the Butte-Helena area to the south. Lincoln was believed to be a comparatively calm area between the two.

Two ancient faults - the St. Mary's- Helena Valley fault to the north and the Bald Butte fault to the south - northwest-southeast near the area, but they're considered inactive and the recent earthquake sequence doesn't align with them.

"We think the fault that's down here at depth slipped during the main shock is s a north-south trending fault," Stickney said. "Data indicates strike-slip movement, where one side moves horizontally past the other. It wasn't up-down motion, it was side to side."

He said they believe the west side of the fault slid south while the north side moved north. Although the quake occurred within the seismic network run by the Montana Bureau of Mines and Geology, none of the sensors were near the epicenter. After the quake, the US Geologic Survey installed three portable sensors in the area, which allowed them to locate many of the smaller aftershocks.

Through the end of March, they saw about 1400 earthquakes in the area, forming a band running about four miles north-south and a mile-and-a-half to two miles east-west, he said.

Although there is an elevated zone of seismic activity to the southeast, near Canyon Creek and to the west toward Kleinschmidt Flats, Stickney said they're not associated with the fault that has been rocking the Stemple Pass area.

Although the big quake here occurred not long after an earthquake swarm in the West Yellowstone area, and prompted speculation that it heralded an eruption of the Yellowstone super volcano, Stickney said there's no evidence of any connection between the two, nor is there evidence an eruption is imminent.

"They think what happens is that those swarms result from magmatic fluids, mostly water, moving up from deeper into the earth, firing its way up through cracks in the rocks, causing them to slip in small quakes."

Stickney said aftershocks gradually taper away with time, but since they've seen continuous activity since last July and still consider the recent tremors to be aftershocks, as opposed to earthquakes in their own right.

"I think what we're seeing right now for this aftershock sequence so far - nine months into it - seems to be fairly typical, but its been punctuated by a couple of larger events like last months event that has its own mini aftershock sequence."