LIQUEFACTION EVIDENCE FOR STRONG EARTHQUAKES OF HOLOCENE AND LATEST PLEISTOCENE AGES IN THE STATES OF INDIANA AND ILLINOIS, USA

Sand- and gravel-filled elastic dikes of seismic liquefaction origin o ccur throughout much of southern Indiana and Illinois. Nearly all of t hese dikes originated from prehistoric earthquakes centered in the stu dy area. In this area at least seven and probably eight strong prehist oric earthquakes have been documented as occurring during the Holocene , and at least one during the latest Pleistocene. The recognition of d ifferent earthquakes has been based mainly on timing of liquefaction i n combination with the regional pattern of liquefaction effects, but s ome have been recognized only by geotechnical testing at sites of liqu efaction. 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 elastic dikes at many site s, which sometimes causes uncertainty in defining the causative earthq uake, but even in Illinois the largest paleo-earthquakes probably have been identified. Prehistoric magnitudes were probably as high as abou t moment magnitude M 7.5. This greatly exceeds the largest historic ea rthquake 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 seismi city. Both geologic and geotechnical methods of analysis have been ess ential for verification of seismic origin for the dikes and for back-c alculating prehistoric magnitudes. Methods developed largely as part o f this study should be of great value in unraveling the paleoseismic r ecord elsewhere. (C) 1998 Elsevier Science B.V. All rights reserved.