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 of paleoliquefactionfeatures, including shaking-induced sandblows and dikes that fed them.Over several summer field seasons between 1990 and 1994, Cheryl andPat Munson, along with several of their students, searched ditchesand river banks for evidence of liquefaction. They found numerousbanks with shaking-induced sandblows and the dikes that fed them.This database includes the location of dikes thatthe Munsons and those with them observed and measured. Dike thicknesswas 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-Holoceneearthquake 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 someof the paleoliquefaction was caused by several additional, smallerearthquakes during the Holocene and late Pleistocene.(excerpted from map I-2583-A)
