U.S. Geological Survey seismologist Harley Benz earthquakes east of the Mississippi River are actually felt more widely than those in the West.

"Earthquakes of comparable size are felt over greater distances in the East than those occurring in the West," Benz said in a statement. "Earthquakes in the central (United States) are infrequent, but not unexpected."

The magnitude 5.2 earthquake -- centered 38 miles northwest of Evansville, Ind., in the Wabash Valley Seismic Zone -- is the strongest earthquake in southern Illinois since November 1968, when a 5.4 earthquake occurred.

While the earthquake and subsequent magnitude 4.6 aftershock were classified as moderate, the USGS said it is more concerned about the potential for the adjacent New Madrid seismic zone to generate severe earthquakes. A series of three very large earthquakes devastated the area between 1811 and 1812.