Earthquakes occur almost every day in Illinois, but they're so small that most of the time people don't notice them.
A "felt" earthquake has a magnitude of at least 3.0 and, as the name suggests, is felt by many people. This type of earthquake occurs every few months in the Wabash Valley and New Madrid areas of southern and southeastern Illinois. That's where the state's two major seismic zones are located. Dr. Philip Carpenter is a professor of geology and environmental geosciences at Northern Illinois University. He said, "We have actually felt some of the quakes that have occured there up here in DeKalb."
For as active southern Illinois is, central Illinois is not. "Down around Springfield," Carpenter said, "there's been only a few earthquakes that have occured in that area."
Earthquakes are monitored throughout the state at various seismic stations. Each station has a seisomograph. "It can be in a basement or a separate unit called a vault," explained Carpenter. "Some of these are placed in farm fields and caves and they record earthquakes -- they record ground vibrations." That means that if there's an earthquake, then several stations throughout the state will record it and, "You can very quickly determine where the earthquake occured and what the magnitude looks like."
The seismic stations are usually located away from highways and human types of vibrations. NIU, however, is home to a seismic station in Davis Hall. You can't go there now because of the coronavirus, but once things open up again, the public can watch the vibrations as they get recorded.
"Our station is actually in a quiet location in the basement, but it's less than half a mile from the railroad tracks," said Carpenter. "If you stand there for a few minutes, you'll see a train signal on the seismograph."
Nowadays, seismographs implement digital technology and can differentiate earthquake vibrations from train vibrations.
The seismic stations located in southern Illinois detect small (less than 3.0 magnitude) earthquakes nearly every day, but as you head toward Wisconsin, felt earthquakes are rare. But, Carpenter said, "We are due for another magnitude three to four anytime."
Carpenter said northern Illinois will have a felt earthquake "maybe every 10 years" and spoke to the recent history of earthquakes in northern Illinois. He said the February 10, 2010 3.8 magnitude earthquake was felt by millions of Illinoisans but added there have been a couple smaller quakes since then. "In 2015, there was one in Lake of the Hills and then McHenry County had one in 2012." Those were felt by a few hundred people.
Carpenter talked about his experience with the February 10, 2010 earthquake. "It was 3:59 in the morning. I was still up working on a paper and I felt the sudden jolt and said, 'Yeah, that's an earthquake,' and then 15 minutes later my phone started ringing."
Besides being a scientist, Carpenter also grew up in California and experienced earthquakes up to magnitude 5.9. That is another reason he was immediately able to recognize jolt as an earthquake.
Though the professor said we are due, he also said there is nothing to fear. "There's really been no damage in northern Illinois caused by any of these earthquakes," he said. "They'll give you a "rumble or a jolt or a swaying sensation."
Though he cannot predict exactly when and where the next felt earthquake will occur in northern Illinois, Carpenter said he has noticed a pattern.
"The most recent earthquakes tended to occur in the winter or the spring," he said. "After wet years where we've had a lot of rain in August or September, they tend to precede earthquakes in February or March." He said that's one hypothesis that he and his colleagues are considering. "We are looking at it statistically to see whether there's a really valid correlation between those things." He added, "It may be right, it might be wrong. We test it by looking at the data."
Carpenter stated that it's almost impossible to predict where the next felt earthquake will occur in northern Illinois. "It could be anywhere," he said. "And people have been talking about earthquake prediction for 50 years and they've been notoriously poor at predicting earthquakes."
Though prediction isn't in the cards for Carpenter, he explained that earthquakes are not random and that he and his colleagues have looked at the distribution of earthquakes in northern Illinois.
"They tend to cluster in time for some reason," he said. "There was a very active period in the early 1900s where we had several felt earthquakes between 1909 and 1920. Then things kind of shut off until the 1940s where we then then had another set of earthquakes -- and then things shut down until most recently, the 1990s to the 2000s."
An earthquake occurs when there is a broken zone in rock called a fault. Carpenter said, "And you suddenly have a slip at some point on that broken zone and the fault basically breaks and slips, and that's what causes an earthquake."
He said that fracking can, indirectly, cause a slip.
"Fracking produces a lot of wastewater and you have to do something with that wastewater," he explained. "What's done typically is it's injected down way, way, way below the water table where you only have saltwater present. If you have a fault that is about ready to slip, you add that water pressure, it sort of lubricates the fault, causing it to slip."
There is a lot of information about the link between earthquakes and fracking and we'll explore that in our next environmental story.
In the meantime, to learn more about earthquakes, Dr. Carpenter recommended visiting the National Earthquake Information Center and the Illinois State Geological Survey.
So what should you do if there is an earthquake?
If you are inside, get under a sturdy table or a piece of sturdy furniture. If those aren't available, Carpenter recommended "a doorframe or any structural element that can protect you from falling debris."
If you are outside, stay away from buildings. "Typically what happens is things like fire escapes and parapets -- anything overhanging on the outside of a building -- will fall off if the earthquake is large enough," he said.
In terms of preparedness, Carpenter warned that interior bookcases, water heaters, and anything top heavy could topple over or break with a magnitude 4.0 earthquake and to make sure those items are anchored to the wall. But, he said, "that's more important in southern Illinois where they do have damaging earthquakes."
Though we don't think about earthquakes in Illinois as much as we do in California, Carpenter reminded, "The earth is always moving and things are always moving below the ground."