We have discovered hundreds of planar, nearly vertical sand-and gravel-filled dikes that we interpret to have been caused by earthquake-induced liquefaction in the Wabash Valley of southern Indiana and Illinois. These dikes range in width from a few centimeters to as much as 2.5 meters. The largest dikes are centered about the general area of Vincennes, Ind.; they decrease in size and abundance to the north and south of this area. Preliminary studies indicate that it is highly possible that many, if not all, of the dikes were formed by a single large earthquake that took place in the Vincennes area sometime between 1,500 and 7,500 years ago. The severity of ground shaking required to have formed the observed dikes far exceeds the strongest level of shaking of any earthquake in the Central United States since the 1811-12 New Madrid earthquakes. Our engineering-seismologic analysis, based on comparison of liquefaction effects with those of historic earthquakes in the Central and Eastern United States, indicates that the moment magnitude of the prehistoric earthquake was on the order of 7.5.

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Earthquake-induced liquefaction features in Holocene sediments provide evidence of strong prehistoric shaking, magnitude m(b) 6.2 to 6.7, in the Wabash Valley bordering Indiana and Illinois. The source of the one or more earthquakes responsible was almost certainly in or near the Wabash Valley. The largest event is interpreted to have occurred between 7500 and 1500 years ago on the basis of archeological, pedological, and stratigraphic relations.

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