Ancient fault line likely responsible for DeKalb County earthquake, NIU expert says

SOMONAUK – The earthquake Monday in southern DeKalb County near Somonauk likely was caused by an ancient fault line that runs through northern Illinois, a Northern Illinois University professor said.

A 3.4-magnitude earthquake struck at 2:53 a.m. Monday about a mile northwest of Somonauk at the border of DeKalb and La Salle counties, according the U.S. Geological Survey.

Mark Frank, chair of NIU’s department of earth, atmosphere and environment, said he wasn’t surprised to discover an earthquake had occurred in that area, even though earthquakes aren’t particularly common in northern Illinois.

“The earthquakes we get here in northern Illinois are associated with ancient cracks in the earth’s crust,” Frank said. “So, this one is plotting very close to a known fault called the Sandwich fault, which kind of runs from Dixon through and then south of DeKalb through DeKalb County.”

Frank hypothesized that the Sandwich fault, which runs northwest from Will County through Kendall, La Salle, DeKalb, Lee and Ogle counties, was the culprit behind the shaking that many northern Illinois residents reported feeling Monday morning.

“So that ancient crack in the earth – 400-ish million years old – that is a weak point,” Frank said. “And so, whenever there is a little bit of motion in the earth along that fault, along that crack in the earth, it’ll build up stress between those two sort of pieces that are moving in a slightly different way. Then, when the stress reaches the point that the fault can no longer hold, it breaks. There is offset of motion along that fault, and then that releases the seismic waves, or the energy then goes out that everyone feels.”

The earthquakes we get here in northern Illinois are associated with ancient cracks in the earth’s crust. So, this one is plotting very close to a known fault called the Sandwich fault, which kind of runs from Dixon through and then south of DeKalb through DeKalb County.”

—  Mark Frank, chair of NIU's department of earth, atmosphere and environment

U.S. Geological Survey research geophysicist Oliver Boyd said he wasn’t aware of the Sandwich fault line, but it’s not uncommon for earthquakes in the central and eastern U.S. to occur along old, often largely dormant fault lines.

“Most of the faults in the central and eastern U.S. are pretty old, and sometimes they get reactivated. Like in the New Madrid seismic zone, there are a series of faults associated with old rifting that had been reactivated and generated pretty large earthquakes,” Boyd said. “So, it’s certainly possible that an older fault had been reactivated.”

Between Dec. 16, 1811, and Feb. 7, 1812, a set of three earthquakes with magnitudes occurred along the Mississippi River near Missouri, Illinois, Tennessee, Kentucky and Arkansas, according to the Missouri Department of Natural Resources.

“That earthquake was so powerful that it actually caused the Mississippi River to reverse flow, and it caused a whole bunch of damage,” Frank said.

Compared with the New Madrid seismic zone in southern Illinois, Frank described the rocks below the feet of northern Illinois residents as “benign.”

“When I say benign, there’s a lot of energy, but there usually isn’t much damage from northern Illinois earthquakes,” Frank said.

Boyd said strong earthquakes are exceptionally rare in northern Illinois. Earthquakes with a magnitude of 3 are the average and a once-in-10-years event. Still, an earthquake about the same magnitude as what occurred Monday happened in Standard, Illinois, in November 2023.

“You might have them every few months, or a couple in a year, and then go 20 or 30 years without an earthquake, so it can be highly variable,” Boyd said.

If another earthquake does occur, Boyd said Illinois residents should get under a sturdy object and cover their head until the shaking subsides. No injuries were reported from Monday’s earthquake, Somonauk and DeKalb County authorities said.

Somonauk Police Chief Bill King said no significant property damage was reported either.

Frank wasn’t surprised to hear that Somonauk avoided catastrophe. He said Illinoisans shouldn’t been too concerned by the possibility of future earthquakes.

“I wouldn’t be too worried about it, and the building codes are good enough that with the amount of shaking we’re going to get from an earthquake, we shouldn’t see substantial damage,” Frank said. “Even if you’re very, very close to the focus or the epicenter of the earthquake, it shouldn’t be a big deal.”

Frank said he thinks Illinois residents are at greater risk of being in an earthquake while traveling than they are at home.

“The only thing I would caution people about is don’t get lulled into a false sense of security knowing the way earthquakes are here,” he said. “If you go to other parts of the country or other parts of the world, those earthquakes that can occur can be significantly more massive. So the only way I’d be worried if I lived in this area is if I [visited] someplace else.”