April 2008 Illinois M5.2 Earthquake
On April 18, 2008 at 4:36am central daylight time, a magnitude 5.2 earthquake shook southern Illinois, near Mount Carmel, Illinois. The main shock originated, just 7 kilometers from Bellmont, Illinois and the epicenter was 38.450°N, 87.890°W and its hypocenter was 11.6 kilometers deep. The earthquake occurred in the Illinois basin-Ozark dome region within the Wabash Valley Seismic zone which is a system of multiple north-northeast trending normal faults, as seen in figure 1. This normal fault system has steep dips and covers an area 97 kilometers and 48 kilometers wide.

A Seismologist working with USGS’s National Earthquake Information Center, Jessica Sigala, notes that the main shock lasted between 30-40 seconds before ceasing. Because the earths’ crust in the Midwest is made of more solid, older and less fractured material than areas of the west like California, which has softer, energy absorbing materials, the effects of an earthquake in the area are felt throughout the region and not just in the immediate surrounding area. This earthquake was felt all around the epicenter, approximately 120,000 square miles. It was experienced in areas of at least sixteen different states and was even felt in Niceville, Florida, 891 miles from the epicenter. The USGS website collected 38,595 reports of having felt the earthquake from 2382 different zip codes.
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Figure 1: The normal faulting of the Wabash system can be seen here in a map from a USGS reflection seismic study during 1988–90.

Aftershocks
Over a span of 10 days after the main shock seismographs at Olney and Mount Carmel detected 29 aftershocks (see figure 2). Eleven, magnitude 1.5 or greater, of which occurred on the same day as the main shock (see figure 3). The largest of the aftershocks occurred only hours after the main shock at 10:14 am central daylight time with a magnitude of 4.6 near West Salem; and, three days after the second largest aftershock hit at a 4.0 magnitude northwest of the main shock’s epicenter at 12:38 am central daylight time.
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Figure 1
Figure 2: The graph shows the magnitude of the aftershocks recorded by seismographs at Olney and Mount Carmel, Illinois.

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Figure 3: The graph shows the time and magnitude of the aftershocks on the initial day of the 5.2 earthquake.
Damage
Consequences and damage from the earthquake was minimal. “No one was killed, and only minor injuries were reported. (Source 1)” One such injury was a cut by a fallen crystal figurine to a woman in Princeton, Indiana. Precautionary evacuations occurred at Vincennes University dormitories and a coal mine in Gibson County, Indiana. However the earthquake did not create any damage and the students and miners were able to return. Emergency lines received many calls, however they were mostly to find out what was happening or to report trembling furniture. Many residents were awakened by the earthquake, their widows rattled, items fell of shelves and furniture shook. Residents in East and Northern Middle Tennessee reported floor buzzing and shaking of: houses, furniture, windows, beds, holiday china, and lamps. Skyscrapers in Indianapolis and Chicago shook but did not fall apart. Bricks fell off a structure in Louisville, Kentucky and an apartment building near the epicenter, in Mount Carmel, was evacuated due to falling bricks. Also in Mount Carmel, a house’s porch fell apart and on Fourth Street a mobile home and its foundation were separated. In Louisville, Kentucky also experienced the façade of an older building falling. Air traffic at the Indianapolis International Airport was stopped, but only for an hour. Harvey Fenton, the Police Chief of West Salem, noted only slight damage of school walls cracking and his chimney collapsing. Additionally, he had plenty of time for answering media phone calls which turned out to be the most active he was all morning due to the area's damage being so minimal. Caesars Indiana, a riverboat casino, in Harrison County Indiana experienced a power outage due to the earthquake and multiple streets were closed in south-side communities of St. Louis, Missouri due to minor damage and pieces of scattered concrete.

Who Knew?
This earthquake came as a shock to most of the general public, because noticeable earthquakes do not occur often in the region and as such served as a reminder that the Midwest is part of active seismic zones, both the Wabash fault zone and the New Madrid fault zone. The probability of an earthquake occurring around the April 2008 timeframe was not announced but history shows scientists that periodically, earthquakes are to be expected in the Midwest. St. Louis University’s director of the Center for Environmental Studies, Timothy Kusky remarks that predicting the arrival of earthquakes is especially difficult in the Midwest, because the faults are not at plate boundaries. This difference in fault locations also makes it difficult to apply what is learned about earthquakes on the West Coast to those in the Midwest. Due to the infrequency of major earthquakes in the region, not much is known about the fault systems in the Midwest. While some scientists note that the Wabash faults are a northern extension of the New Madrid faults, some disagree, keeping the two fault zones as separate systems. The Wabash faults are also difficult to learn from, know which faults are active, predict future earthquakes and pinpoint which exact fault an earthquake came from because they are not expressed at the surface but rather underneath. Nevertheless, connected or not both have a history of earthquakes and thus reason to have expected the April 18th earthquake and future ones, as seen in figure 5.

Kusky shares, “This [the Wabash fault zone] fault line can give 7.0 magnitude earthquakes about once every 1,000 years” (Source 4). Noticeable, earthquakes of smaller magnitudes generally occur in the area once or twice a year. In 2006, a 3.6-magnitude earthquake shook this fault zone and in 2002 the region had a 4.6-5.1 (depending on the source) magnitude earthquake. This fault system also released a 3.9 magnitude earthquake in 2000 and a 5.2 magnitude earthquake in 1987. Won-Young Kim, a seismologist at Columbia University, adds that in 1968, southern Illinois experienced a magnitude 5.3 (5.4 according to USGS) earthquake and that there is evidence of stronger earthquakes, magnitude 7.0 and higher occurring 4,000 - 6,000 years ago.

The New Madrid fault zone releases one 7.5-8.0 earthquake every 400 - 500 years. The New Madrid Fault system has not had 6.0 magnitude or higher earthquake in over 100 years. (Whether the faults are no longer active or if they are building up stress that is bound to be released in a high magnitude earthquake is unknown.) The New Madrid fault zone had a series, around 3 – 5, 7.0-8.5 magnitude earthquakes from December of 1811 - February of 1812. These earth“quakes changed the course of the Mississippi River and were felt in New England. (Source 17)”
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Figure 4: The image shows rubble, one of the effects of the earthquake in Louisville, Kentucky.
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Figure 5: The map plots the earthquakes that occurred from 1974 - 1991 in central United States. The Yellow "epicenter symbol sizes are scaled into four magnitude categories: magnitudes 1.5 - 2.0, magnitudes 2.0 - 3.0, magnitudes 3.0 - 4.0, and magnitudes greater than 4.0. (Source 18)"






Sources
1. http://www.nytimes.com/2008/04/19/us/19quake.html?_r=1&scp=2&sq=5.2%20earthquake&st=cse&oref=slogin
2. http://earthquake.usgs.gov/eqcenter/eqinthenews/2008/us2008qza6/#summary
3. http://www.cnn.com/2008/US/04/18/illinois.earthquake/index.html?eref=rss_topstories
4. http://www.pbs.org/newshour/bb/weather/jan-june08/earthquake_04-18.html
5. http://www.courierpress.com/news/2008/apr/18/earthquake-rattles-tri-state/
6. http://www.geotimes.org/apr08/article.html?id=WebExtra041808.html
7. http://www.physorg.com/news128270651.html
8. http://www.isgs.uiuc.edu/research/earthquake-hazards/pdf-files/aftershocks-f-gra.pdf
9. http://www.iht.com/articles/ap/2008/04/18/america/NA-GEN-US-Midwest-Earthquake.php
10. http://thelede.blogs.nytimes.com/2008/04/18/an-earthquake-rattles-the-midwest/?hp
11. http://www.thetimesonline.com/articles/2008/04/18/updates/breaking_news/doc480880d0ce467279946352.txt
12. http://www.knoxnews.com/news/2008/apr/18/54-earthquake-rocks-illinois-also-felt-indiana/
13. http://www.nationmaster.com/encyclopedia/2008-Illinois-earthquake
14. http://www.isgs.uiuc.edu/research/earthquake-hazards/pdf-files/ashock-bar-graph.pdf
15. http://blog.mlive.com/chronicle/2008/04/quake_felt_in_west_michigan.html
16. http://abcnews.go.com/Technology/Weather/Story?id=4682373&page=1
17. http://www.sfgate.com/cgi-bin/article.cgi?f=/n/a/2008/04/18/national/a025745D02.DTL
18. http://education.usgs.gov/common/resources/mapcatalog/image.html
19. http://education.usgs.gov/common/resources/mapcatalog/images/image/earthquake_central_region_11x14.pdf
20. http://igs.indiana.edu/Geology/structure/seismicProfiling/index.cfm