Paleoseismology study of the Cache River Valley, southern Illinois, and New Madrid seismic zone, southeast Missouri and northeast Kansas

Abstract

Our understanding of earthquake hazard in the mid-continent region of the United States has benefited greatly from paleoseismology studies. Numerous earthquake-induced features have been documented throughout the region, though the timing and location of prehistoric earthquakes needs to be better constrained. Some areas of the mid-continent, such as the Cache River Valley (CRV) of southern Illinois, have not been studied in the detail of the New Madrid seismic zone, thus the earthquake hazard is poorly understood. l have conducted a paleoseismology study of the CRV and the New Madrid seismic zone to add to the earthquake chronology of the mid-continent region. Field reconnaissance in the CRV during the summer and fall of 1998 yielded no new evidence of recent faulting or liquefaction. It is important to note, however, that this time period was characterized by abnormally high river levels that significantly reduced cutback and ditch exposures. One previously discovered liquefaction feature, that was exposed, was excavated and logged in detail. Organic matter from this site indicates that the maximum age of this feature is 3421-2861 BC. The lack of soil development and weathering in the dike, however, suggests that it is probably less than 2000 years old. Given the probable age of this feature and the current paleoearthquake chronology, the source for this feature is most likely the New Madrid Seismic Zone. Additional work, concentrating along the balks of the lower Cache River and its larger tributaries is needed during low water periods to further determine the distribution and age of earthquake-induced deformation of this region. Data from several trenches at the Sigman site, northeast Arkansas, suggest that the liquefaction at this site likely occurred during the 1811-1812 New Madrid earthquake sequence. Organic matter from several stratigraphic horizons associated with two stacked sand blows at the Braggadocio site, southeastern Missouri, yielded modern dates. In contrast, the degree of soil development on the sand blows suggests that these features are several hundred years old. This discrepancy suggests that the dated samples were actually modern roots growing through the section. As a result, additional trenching and dating are needed at this site.