After the 5.5 Richter Scale magnitude 1968 Illinois earthquake , scientists realized that there was a previously unknown fault under Saline County , just north of Eldorado , Illinois near Harrisburg . This fault is called the Cottage Grove Fault , a small tear in the Earth 's rock running west – east , in the Southern Illinois Basin . The fault is connected to the north – south trending Wabash Valley Fault System at its eastern end . Seismographic mapping completed by geologists reveal that monoclines , anticlines , and synclines are present within the region ; these signs suggest deformation during the Paleozoic era coincident to strike-slip faulting nearby
1968 Illinois earthquake
1968 Illinois earthquake
Harrisburg, Illinois