cbs2chicago.com - 5.4 Quake Strikes Southern Illinois
Bedrock, not the North American Craton, but old, is cool and hard, carries the waves broadly. Lots of sediment over it can shake.
Didn't feel it up here, but reports from Chicago to Georgia
A bit later: more info
Magnitude 5.2 - ILLINOIS:
"Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi)."