Southern Illinois sits upon the verging point of two major fault systems , the New Madrid Seismic Zone and the Wabash Valley Seismic Zone . In the 1970s after the 5.4 Richter magnitude scale 1968 Illinois earthquake , scientists realized that there was an unknown fault under Saline County , just north of Eldorado , Illinois . This fault is called the Cottage Grove Fault , a small tear in the Earth 's rock , in the Southern Illinois Basin . Seismographic mapping completed by geologists reveal that monoclines , anticlines , and synclines are present within the region ; these signs suggest deformation during the Paleozoic , coincident to strike-slip faulting nearby
1968 Illinois earthquake
1968 Illinois earthquake
Southern Illinois