The northern part of the map area and large adjacent regions have been severely shaken by one or more moderate to large prehistoric earthquakes. The evidence of these takes the form of paleoliquefaction features, including shaking-induced sandblows and dikes that fed them. Over several summer field seasons between 1990 and 1994, Cheryl and Pat Munson, along with several of their students, searched ditches and river banks for evidence of liquefaction. They found numerous banks with shaking-induced sandblows and the dikes that fed them. This database includes the location of dikes that the Munsons and those with them observed and measured. Dike thickness was divided into 5 classes, 1 being the smallest at 0-4 cm, 2 = 5-14 cm, 3 = 15-29 cm, 4 = 30-59 cm, and 5 = over 60 cm. The most widespread paleoliquefaction is attributed to a mid-Holocene earthquake near what is now Vicennes, IN, with an estimated moment- magnitude of about 7.5. Stratigraphic, geomorphological, pedological, archealogical, and geochronologic evidence indicates that perobably some of the paleoliquefaction was caused by several additional, smaller earthquakes during the Holocene and late Pleistocene. (excerpted from map I-2583-A)
