The early morning earthquake that jolted many in the Central United States on Friday, April 18, is a reminder that seismic events do occur in areas not normally thought of as "earthquake country." It is also a lesson that earthquakes east of the Mississippi River are felt more widely than those in the West. This event was felt as far west as Kansas, as far north as Upper Michigan, and as far south as Georgia.
"Earthquakes of comparable size are felt over greater distances in the East than those occurring in the West," said Harley Benz, seismologist for the U.S. Geological Survey (USGS). "Earthquakes in the Central [United States] are infrequent, but not unexpected."
The preliminary magnitude 5.2 earthquake occurred at 4:37 a.m. Central Daylight Time and was centered about 38 miles north-northwest of Evansville, Ind., or 128 miles east of St. Louis. It occurred in an area known seismically as the Wabash Valley Seismic Zone. The event is the strongest earthquake in Southern Illinois since Nov. 1968, when a 5.4 earthquake occurred.
On Monday, April 21, the USGS issued updated earthquake hazard assessment maps for the entire United States. The information on these maps is used to update building codes.
Classified as "moderate," Friday’s event caused some damage and was followed by aftershocks, the largest being a M4.6 that occurred at 10:15 a.m. Central Daylight Time. Of much greater concern, however, is the potential for the adjacent New Madrid seismic zone to generate severe earthquakes. During the winter of 1811-1812, a series of three very large earthquakes—the strongest earthquakes to strike the lower 48 states during historic times—devastated the area and were felt throughout most of the nation. Occurring only a few weeks apart on Dec. 16, Jan. 13, and Feb. 7, they generated hundreds of aftershocks, some severely damaging by themselves, which continued for years.
Building codes used in the region incorporate a significant degree of risk from earthquakes, but many buildings constructed before these codes were in place or updated have not been adequately retrofitted.
USGS research into ground shaking is used by building officials to update building codes based on the most up-to-date information. As new buildings in the United States replace older, more dangerous structures, death tolls from earthquakes have been significantly reduced.
Did you feel this earthquake? You can report your experiences at http://earthquake.usgs.gov/eqcenter/dyfi/.
More information on this event and the history of the region is found at http://earthquake.usgs.gov/eqcenter/recenteqsus/Quakes/us2008qza6.php.