Liquefaction evidence for at least two strong Holocene paleo-earthquakes in central and southwestern Illinois, USA

Two strong mid-Holocene earthquakes in Illinois have been documented by pal eoliquefaction features such as elastic dikes, sills, and detachments of fi ne-grained sediment that sunk into liquefied sand. At least one paleo-earth quake occurred in central Illinois about 35 km NE of Springfield, probably sometime between 5,900 and 7,400 yr BP, Dike widths are as much as 0.4 m ne ar the energy center of the earthquake, Outward from this center, dike widt hs attenuate and ultimately disappear at about 35 km, More than one paleo-e arthquake is probably represented by liquefaction features near Springfield , Another paleo-earthquake that appears to have been centered about 65 km E SE of St. Louis, Missouri, occurred near 5,700 yr BP, The energy center is inferred as being in Illinois, and most likely near lowermost Shoal Creek w here the meizoseismal region is defined by dikes as wide as 0.5 m and by a regional abundance of dikes. Dikes from this earthquake probably extend at least as far as 35 km from its inferred energy center. The earthquake near Shoal Creek and one earthquake near Springfield almost certainly exceeded M 6, The paleomagnitudes can be more closely bracketed by geotechnical testi ng and analysis, when used in conjunction with existing data.