Inherent in the new U.S. Geological Survey National Seismic Hazard Maps is that every 1,000 years the New Madrid seismic experiences magnitude 8 earthquakes, similar to those that occurred in 1811 and 1812. There are several lines of thinking that makes this a reasonable estimate to use. Primary among these is evidence from paleoseismology, which is the study of the geological record of prehistoric earthquakes. In the New Madrid seismic zone, the most dramatic record comes from earthquake-induced soil liquefaction. During large earthquakes, saturated sand liquefies and is erupted through fissures onto the ground surface forming sand blows. Sand blows from the 1811-1812 and earlier earthquakes are commonly preserved on the ground surface or buried by river deposits. We can estimate the ages of these sand blows if we can determine the ages of the materials above and below them. We do this mainly through radiocarbon dating of organic materials and with Native American artifacts. We have found that, for the past few thousand years, widespread liquefaction has occurred in the seismic zone every few hundred years, and that within a hundred years of AD 900, an earthquake caused liquefaction as intense and widespread as in 1811-1812. The rate of historical and instrumentally measured seismicity also suggests that 1,000 years between magnitude 8 earthquakes is appropriate.

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