Local Expert Explains Regional Seismic Activity and Recent Earthquakes in Illinois

Dr. Scott Michael Olsen, Professor in the Department of Earth Science & Environmental Change, University of Illinois Urbana-Champaign answers questions about the occurrence of earthquakes in Illinois

Photo byScreen Capture/Youtube Video [Creator]

A minor earthquake was detected early Saturday near O’Fallon, Illinois, according to the United States Geological Survey. The 2.4 magnitude quake occurred at 1:50 a.m., approximately one kilometer south-southwest of O’Fallon, with an epicenter depth of 0.3 kilometers.

Several residents in the vicinity, including those in Festus and Godfrey, Illinois, reported feeling the tremor to the USGS. This follows another recent earthquake in Illinois, a 3.4 magnitude quake, which struck near Somonauk at around 3 a.m. on July 15th.

Although aware of the numerous earthquakes that occur in California, many people in Illinois wonder at the occurence of earthquakes in this area. I asked a local expert on the topic, Dr. Scott Michael Olsen, who is a Professor in the Department of Earth Science & Environmental Change at the University of Illinois Urbana-Champaign some questions about seismic activity in Illinois and the recent earthquakes.

NF: We had two earthquakes in the past few weeks in Illinois. Is that unusual and does it indicate that more are on the way? Are they bigger than what are usually found in this area?

SO: The two earthquakes in the past 2 weeks (one in northern Illinois and one in SW Illinois) are purely coincidental, as earthquakes of this small magnitude (M3 and smaller) can happen just about anywhere in the world, and Illinois is no exception. And, no, these earthquakes are not bigger than one would expect to happen anywhere at any time.

NF: When was the last significant earthquake in Illinois and what kind of damage did it cause?

SO: The most recent moderate-sized earthquake in Illinois (that I can recall) was the M5.2 Mt. Carmel earthquake. I actually drove down to the area to investigate damage from that event. (Most of the damage was relatively minor, and included fallen chimneys and the near collapse of a large stained-glass window in an area church—the glass fortunately got stuck in the brick window frame as it was beginning to fall.)

NF: We're said to be on two major seismic zones which extend into Southern Illinois. The New Madrid Fault Line is said to produce some of the biggest earthquakes in US.history. Yet the strongest earthquake in IL is said to be the one in 1968, when a 5.3-magnitude quake was recorded near Norris City. If the New Madrid Fault Line produces such strong earthquakes, even supposedly reversing the Mississippi at one point, why don't we experience earthquakes in Illinois anywhere close to the big ones we see out west?

SO: Illinois is close to two important seismic zones: (1) the New Madrid seismic zone (NMSZ) that extends close to Cairo, Illinois, right at the southern tip of Illinois; and (2) the Wabash Valley seismic zone (WVSZ) that runs roughly along the Wabash River between Illinois and Indiana. The really large earthquakes (approximately M7 to M8) that you're referring to happened in the winter of 1811-12 in the NMSZ, which actually consists of a number of large faults. In contrast, the 2008 Illinois earthquake likely was related to a fault in the WVSZ.

However, these are not the only faults in Illinois; there are a number of other faults and fault systems throughout Illinois, including the Sandwich fault system with which the July 15 earthquake likely was related. However, we don't see frequent earthquakes in Illinois, or in the central U.S. in general, because this is region is part of the stable North American continental plate. While crustal movements (which cause earthquakes) do occur in the interior of this plate, the movements are relatively small and require long periods of time (hundreds to thousands of years) to build up stress and trigger larger earthquakes.

In contrast, California, Oregon, and Washington sit on several continental plate boundaries (both strike-slip and subduction boundaries) where significantly more movement occurs. As a result, stresses on these boundaries build more quickly and larger earthquakes occur more frequently (tens to hundreds of years).

When these large earthquakes do occur in the central U.S., there is a significant risk for damage and injuries, particularly in places like St. Louis and Memphis (for NMSZ earthquakes), because of the building stock that includes a significant number of unreinforced masonry buildings. While many owners have seismically retrofit their structures, there are still many of these structures in these larger cities.

I'd like to thank Dr. Scott Olsen for his willingness to take the time to explain these issues for the purpose of this article.