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Would it surprise you to know there’s a slope that rivals the eastern front of the Rocky Mountains here in the Midwest?
It begins at the bottom of a geological structure called the Illinois Basin, four miles below sea level underneath southern Illinois. It tops out on a structure in Missouri called the Ozark Dome, half a mile above sea level.
Unfortunately, this impressive Prairie State slope isn’t open for climbing. That’s because the Illinois Basin isn’t empty, but rather, filled with sedimentary layers that have accumulated during the past 500 million years as the floor of the basin has sunk. In effect, the surface of the ground today is the top of the stuff that fills up the bowl.
What are the contours of this formation? How did it come to be? These are questions U of I geology professor Stephen Marshak, Tim Larson of the Illinois State Geological Survey, and colleagues at Purdue and Indiana University are keenly interested to answer. They hope to do so with information gathered by a sophisticated earthquake monitoring system they began installing this summer.
In June and July, teams of students and technicians traversed the farmlands and hills of southern Illinois and adjacent Missouri to find suitable spots to place instruments—quite spaces without traffic noise or swaying trees. One of the biggest challenges came in finding landowners willing to have a three-foot hole dug in their property to place an instrument, all in the name of science.
When complete, the system will include 120 stations arrayed over a 200-mile long, 60-mile wide swath that stretches from central Missouri across southern Illinois and into Kentucky.
This intensive monitoring effort, which should be fully in place by next summer, is scheduled to run for two years. It’s part of a much broader project funded by the National Science Foundation called EarthScope, which involves a less-dense array of earthquake monitoring equipment that is gradually being moved across the entire continental U.S.
At each of the monitoring stations, the researchers are installing a seismometer, an instrument that measures movement in the ground from earthquake waves—not just the ones from once-in-a-great-while earthquakes people can feel, but also the ones from very small events that occur many times a year, even in the Midwest. According to Marshak, a better understanding of these minor earthquakes might help detect small, local faults in the Earth’s crust that have been overlooked before. [Photo by Michael Hamburger. Seismologists Gary Pavlis of Indiana University (left) and Hersh Gilbert of Purdue University use irrigation tile to create a temporary seismometer vault at an Earthscope site in southern Illinois.]
The seismometers involved are also sensitive enough to record seismic waves from larger earthquakes that happen elsewhere around the world—from events in Japan or South America, for example—even though those waves lose much of their energy as they pass through the Earth. That’s exciting because measurements of vibrations that pass through the Earth’s interior can be used to create a more refined three-dimensional image of the planet than those now available. (In an unexpected twist, the first waves from a significant quake came from Virginia, not the west coast.)
Marshak likens the anticipated product of this monitoring to a CAT scan of the Earth’s crust beneath the Midwest. Just as a medical CAT scan can detect unusual features inside a human body, a seismic CAT scan can detect unusual features underground, such as regions of light or dense rock. Recognition of such features may explain why the Illinois Basin and the Ozark Plateau exist, and may even help to explain how our continent formed in the first place, over a billion years ago.
