The northern part of the map area and large adjacent regions havebeen severely shaken by one or more moderate to large prehistoricearthquakes. The evidence of these takes the form ofpaleoliquefaction features, including shaking-induced sandblows anddikes that fed them.Over several summer field seasons from 1990 through 1994,several workers searched ditches and river banks for evidenceof liquefaction. They found numerous dikes and sandblows.This database includes the locations and descriptions of dikes theyobserved and measured.The most widespread paleoliquefaction is attributed to amid-Holocene earthquake near what is now Vicennes, IN, with anestimated moment- magnitude of about 7.5. Stratigraphic,geomorphological, pedological, archealogical, and geochronologicevidence indicates that probably some of the paleoliquefaction wascaused by several additional, smaller earthquakes during theHolocene and late Pleistocene.
