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5.2 Midwest Earthquake | April 18, 2008
At 4:37 am central time a magnitude 5.2 earthquake rattled southeastern Illinois near the border with Indiana. Located near the larger towns of Terra Haute and Evansville, IN, the earthquake was felt throughout the Midwest from Michigan and Wisconsin south to Georgia. A large 4.6 magnitude aftershock was also widely felt throughout the area.
So far the damage reports have been minimal in this mostly rural farming area with some house and chimney damage reported. Earthquakes of this magnitude are felt by most people, but damage generally occurs only to poorly-built structures.
Why did an earthquake occur in Illinois?The earthquake of April 18th along the Illinois/Indiana border is not the first to occur in this region. Several similar sized and smaller earthquakes have occurred in the area over the last 100 years. Scientists have identified these earthquakes as occurring on the Wabash fault zone, the Wabash fault zone lies along what is called a “failed rift”. Failed rifts are portions of the continent that were stretched at some point in the past, causing the crust to break (fault). Now that the continent is no longer stretching and is in fact under compression, these faults are being reactivated. Basically, the continent is like a log raft that has been assembled over billions of years, some of the lashings between the logs are loose and when the continent (or raft) gets pushed, the places where it is loose move.
Why was it felt over such a wide area?While an earthquake of this magnitude in California would generally only be felt on a smaller, several county scale, the smooth, relatively unbroken crust of the continental interior transmits earthquake waves over a much larger area. The analogy is that California is broken by so many faults that when an earthquake hits it rings like a bell with lots of cracks (not so well) while the continental interior rings like a solidly built bell with some very small cracks (rings very loudly).
Will it cause an earthquake here?
There is no evidence suggesting that the relatively small earthquake that occurred in Illinois will cause seismic activity here.
Will an earthquake like this occur here?
While we currently do not have information on the recurrence interval of magnitude 5 earthquakes for South Carolina, data suggests that large, magnitude ˜7.2 earthquakes occur here approximately every 400-500 years. This suggests that we may be more susceptible to smaller earthquakes of a magnitude 5 or 6 on smaller time scales.
Simulations run by Dr. Norm Levine from the College of Charleston’s Dept.
of Geology and Environmental Geosciences suggest that a magnitude 5 earthquake in the Charleston area would result in more damage than we are
seeing associated with the Illinois earthquake. This is primarily due to the density and type of population in the Charleston region. We have
a densely populated commercial city while southern Illinois is mostly farmlands. We also have large areas of “fill” in Charleston. Because of
our location on the coast, much of the land we have built on is marsh/mud that has been “filled” in with more dirt and rubbish. The underlying
wet ground that makes up the base for the “fill” is inherently less stable than rock and sediments naturally emplaced. Therefore, buildings on
fill are more likely to suffer damage. This was seen dramatically in the 1886 Charleston earthquake, where the great majority of buildings
that suffered catastrophic damage in downtown Charleston were those built on fill.Where can I go for more information?
The SCEEP website has a section on the 1886 earthquake [http://www.cofc.edu/~scearthquakes/SCEQ/SCEQ1886.html], as well as extensive information on South Carolina earthquakes in general and links to various educational and scientific resources. The U.S. Geologic Survey has real-time data on earthquakes and extensive maps and information on the Illinois earthquakes.
Curator: hallcr@cofc.edu
