Parts of Illinois are due for a "felt" earthquake. Those are magnitude 3 or 4 earthquakes -- big enough that you can feel them. That's according to Philip Carpenter. He's a professor with the Department of Geology and Environmental Geosciences at Northern Illinois University and a registered professional geologist in the State of Illinois.
"If you look at the historical record, we get one of those about every 10 years or so," he said. "Now we did have a couple of very small quakes -- one in 2012 and one in 2015 -- but those weren't like the Lily Lake Quake of 2010."
The Lily Lake Quake was a magnitude 3.8 earthquake. It occured at 3:59 a.m. on February 10, 2010, waking and startling thousands of Illinoisans.
"They usually catch everybody very much off guard," said Carpenter.
Though northern Illinoisans can feel the tremors, Carpenter said northern parts of the state aren't at risk for a seriously damaging earthquake.
"By California standards, they are very small," he said. "But the situation in southern Illinois is completely different."
There are two zones in southern Illinois that are of concern. One is in the Wabash Valley where there have been magnitude 5 to magnitude 6 eathquakes. And in Cairo and the New Madrid area by Missouri, they have had historically very large earthquakes in that region.
"If you live in that part of the state, it's important to be 'earthquake prepared.'" He said.
That means if you are in southern Illinois, you should tie down your bookcases and water heaters -- and make sure your chimney is in good shape.
"Usually that's the first damage you'll see -- along chimneys," he said. "That's because it's kind of an inverted pendulum. It's got a lot of weight on top."
Earthquakes are responses to long-term stresses in Earth's crust. They occur along faults -- breaks in the rock -- generally several miles below the surface.
"There are two major faults in northern Illinois," said Carpenter.
But because of the soil cover across most of northern Illinois, we cannot see the rocks.
"We just don't know where they are," he said. "So until an earthquake occurs, like Lily Lake for instance, we had no idea there was a fault there because everything is covered up."
Two major faults have been identified but most of the earthquakes don't occur along these faults. They occur along unmapped "hidden" faults.
"There [are] a lot of mysteries about what's going on at that depth," Carpenter said. "We're trying to explore that with some of the research we're doing."
If you are interested in learning more about earthquakes, there is a seismograph in Davis Hall on the NIU campus. Carpenter said you will be able to view the display in the north foyer of Davis Hall once classes resume. It's shut down now because of COVID.
"But," he said, "I think we're hoping to have that up running next week sometime."
Carpenter has another station running in his office "in case one of the other stations fail." He said he also has access to the national seismograph network operated by the U.S. Geological Survey and other institutions.
The strongest earthquake that originated in northern Illinois occured on May 26, 1909.