Clastic dikes filled with sand and gravel, interpreted to be of seismic liquefaction origin, occur throughout much of the southern halves of Indiana and Illinois. Nearly all of the liquefaction features originated from earthquakes centered in the southern halves of Indiana and Illinois, rather than further south in the nearby source region for the great 1811–12 New Madrid earthquakes. At least eight earthquakes strong enough to induce liquefaction have occurred during the past 20,000 years. At present, the best dated paleoliquefaction evidence lies in Indiana. More paleoliquefaction features have been found in Illinois, but studies there have not yet narrowly bracketed the age of the liquefaction evidence at most sites.
Estimated magnitudes of at least two prehistoric events are higher than M 7, which greatly exceeds the largest historic earthquake of M 5.5 that has originated in Indiana and Illinois. The strongest prehistoric earthquakes have been centered in the general vicinity of strongest historic seismicity, in and near the Wabash Valley seismic zone in the lower Wabash Valley of Indiana and Illinois. However, in several cases prehistoric events greater than M 6 have struck far outside the Wabash Valley seismic zone, in areas where there has been little or no historic seismicity. Numerous other strong paleoearthquakes could have struck elsewhere in the southern halves of Indiana and Illinois but not be recognized because of the lack of liquefiable deposits in many areas.