Sand blows and fissures that cover >10500 km2 in northeastern Arkansas and southeastern Missouri attest to the severity of the 1811-1812 earthquake series in the New Madrid seismic zone. However, except for one occurrence near New Madrid, Missouri, the region has been devoid of any evidence of other major shocks for at least 1.3 ka prior to 1811 and possibly for >9 ka. Stratigraphic relations and radiocarbon dating at a recently excavated archaeological site near East Prairie, Missouri, have revealed liquefaction phenomena attributable to a shock dated to within about 100 yr prior to A.D. 539 and a probable second one dated between about A.D. 539 and 991.

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