Abstract

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 Indiana- Illinois. These dikes range in width from a few cm to as much as 2.5 m. The largest dikes are centered about the general area of Vincennes, Indiana; they decrease in size and abundance to the north and south of this area. Preliminary studies indicate the high possibility that many, if not all, of the dikes were formed by a single large earthquake that took place in the Vincennes area sometime between 2,500 and 7,500 years ago. The severity of ground shaking required to have formed the dikes far exceeds the strongest level of shaking of any earthquake in the central United States since the 1811–12 New Madrid earthquakes.

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