Anyone who felt the 5.2-magnitude earthquake that shook cities from Chicago to Louisville and Cincinnati on April 18 of this year knows that even a modest temblor can be an unnerving experience. The ground beneath our feet is supposed to be solid and unchanging, and usually it is – at least on the surface.
But deep beneath the earth, things are moving … changing … reforming and realigning themselves according to geologic pressures and principles that not even scientists claim to fully understand. And, although Chicago and Northwest Indiana are situated in the heart of the relatively flat and quiet Midwest, conditions here also exist that may trigger infrequent but nonetheless substantial earthquakes, ones comparable to the April 18 event that was centered near Mount Carmel, Ill.
“A lot of people think we don’t get earthquakes of that magnitude in our area, but we do,” said seismic researcher and Chicago resident Kris Huysken, Ph.D., who is associate professor of geosciences at Indiana University Northwest in Gary, Ind. “The largest earthquake that ever occurred – that we know of – in (northern) Illinois, was in 1909. It was estimated to be a 5.1. The big one that happened down in Mount Carmel was a 5.2. So this was comparable to that, almost right on the money.”
Huysken co-authored a recent study on the 99-year-old quake, titled “A Re-evaluation of the Aurora, Illinois Earthquake of 1909,” with Michigan State University Professor of Geological Sciences Kazuya Fujita, Ph.D. In it, the researchers suggest that the quake’s center of highest intensity -- which previous investigations have concluded was anywhere from Kane, Ogle and Winnebago Counties to the current “official” epicenter of Aurora, in Will County – was actually in the La Salle County community of Morris, Ill. The May 26, 1909 temblor – for which no seismographic data exists -- was felt in parts of five states, including Iowa, Michigan and Indiana, as far south as Bloomington, Ill., and even as far west as Platteville, Wis., former site of the Chicago Bears’ summer training camp. But the absence of hard numbers about this event has made the quake’s epicenter the focus of much fascination among seismologists, leading to multiple studies and myriad conclusions about the quake’s origin.
“We think we’ve got a better handle on it than has been (the case) in the past,” Huysken said. “What we’ve been able to do is link it to a particular geologic structure that occurs there.”
That structure, which Huysken called the Peru Monocline, is a layer of sedimentary bedrock in which the rock has flexed or folded downward at one end. This geologic irregularity, Huysken said, appears to have been linked to several other area quakes during the past 36 years, including those occurring in 1972, 1999 and 2004. The city of Morris is situated on the edge of that monocline, and local newspaper reports detailing the 1909 quake indicated significant tremor activity in Morris during that event.
“We’re not saying Morris is the epicenter, or where the fault-cracking occurred, but we’re saying Morris is where there was the greatest shaking,” Huysken explained. “We actually found a local paper reporting that 50 chimneys were down. And they give addresses. Plus, there are reports of wells running dry and then shooting up like geysers, and the Illinois River being agitated and boiling … intense stuff going on.
“They’ve got some other interesting things going on in Morris,” she added. “They’re right along the flood plain of the Illinois River. So you have sediment that is water-saturated, and when you shake it, it can amplify the energy released by an earthquake.”
Huysken and Fujita believe they’ve made a good case for Morris as the real center of seismic activity for the “Aurora earthquake.” One reason they believe previous estimates of the quake’s epicenter have been off the mark is that those conclusions were based partly on 1909 newspaper reports that, according to Huysken and Fujita’s research, greatly exaggerated tremor damage in various communities throughout the quake zone.
“We know why the location of this earthquake is so poorly understood or so poorly located,” Huysken said. “It’s because there was a study that came out that had a weird pattern of high intensity (activity) with these really high outliers in Bloomington and Kenosha and Platteville. The newspaper articles, the initial reports, were really exaggerated. They get picked up by the wire service and get widely reported all over the Midwest. Later, in the local paper, there would be a retraction of the initial report. That did not get picked up.”
Huysken cited conflicting news reports from Platteville as one example. In the May 26, 1909 evening edition of the "Fond du Lac Daily Reporter," it was reported that the State Mining School in Platteville had been “wrecked” by the earthquake and would have to be rebuilt. However, in the local Platteville newspaper, the "Grant County News," accounts held only that the south wall of the school’s Rountree Hall had been cracked by the quake from the ground floor to the third story. That story did not suggest any major structural damage of the type reported in the Fond du Lac paper.
Such widely reported misinformation, Huysken argued, appears to have skewed the original research on the 1909 quake that was done by noted geologist J.A. Udden. Huysken and Fujita reviewed Udden’s original notes and the original newspaper clippings he referenced when determining shock intensities throughout the quake zone. They also sought out additional local news reports from 170 communities throughout the area. The search yielded similar conflicting accounts of damage in other communities, including Bloomington, where the local paper, the "Pantagraph," reported only minor, repairable damage to the foundation of the local jail building, despite reports by the "St. Paul Dispatch" that the jail would have to be rebuilt.
“What we were able to do was essentially go through, one by one, and show that certain areas that were reported as high intensity didn’t have high intensity,” Huysken said. “Then what we did was to go through and find where the real high intensity was. And we found it.”
Now that Huysken and Fujita have identified Morris as the probable point of highest intensity, they are one step closer to determining the actual epicenter of the 1909 quake. Nor do they mean to stop with this one earthquake. The researchers hope to investigate other historical earthquakes in the region and analyze the accuracy of their reported zones of intensity. By verifying or modifying the presumed epicenters of area quakes, Huysken said, they can learn more about what geologic structures in northern Illinois have the potential to trigger earthquakes like the magnitude 5.2 event that hit downstate on April 18.
“Ultimately, we want to understand where the earthquakes are likely to occur in northern Illinois,” Huysken said. “This is one step in that direction. We think it’s an important step to be able to link the largest earthquake in northern Illinois history to the location of three other recorded earthquakes, and being able to link that to a known bedrock structure in the area.”
Because sizable earthquakes are relatively uncommon in the Midwest, Huysken believes it’s crucial to learn as much as possible about those that do occur.
“It’s not like California, or even Reno, where you get a lot more earthquakes,” she said. “Because our situation, geologically, is a little bit different, and we get fewer earthquakes, in some ways it’s even more important to get an exact location, because you don’t get many opportunities.”