Liquefaction evidence for strong earthquakes of Holocene and latest Pleistocene ages in the states of Indiana and Illinois, USA

Sand- and gravel-filled clastic dikes of seismic liquefaction origin occur throughout much of southern Indiana and Illinois. Nearly all of these dikes originated from prehistoric earthquakes centered in the study area. In this area at least seven and probably eight strong prehistoric earthquakes have been documented as occurring during the Holocene, and at least one during the latest Pleistocene. The recognition of different earthquakes has been based mainly on timing of liquefaction in combination with the regional pattern of liquefaction effects, but some have been recognized only by geotechnical testing at sites of liquefaction.

Most paleo-earthquakes presently recognized lie in Indiana, but equally as many may have occurred in Illinois. Studies in Illinois have not yet narrowly bracketed the age of clastic dikes at many sites, which sometimes causes uncertainty in defining the causative earthquake, but even in Illinois the largest paleo-earthquakes probably have been identified.

Prehistoric magnitudes were probably as high as about moment magnitude M 7.5. This greatly exceeds the largest historic earthquake of M 5.5 centered in Indiana or Illinois. The strongest paleo-earthquakes struck in the vicinity of the concentration of strongest historic seismicity. Elsewhere, paleo-earthquakes on the order of M 6–7 have occurred even where there has been little or no historic seismicity.

Both geologic and geotechnical methods of analysis have been essential for verification of seismic origin for the dikes and for back-calculating prehistoric magnitudes. Methods developed largely as part of this study should be of great value in unraveling the paleoseismic record elsewhere.