<P> The zone had four of the largest North American earthquakes in recorded history, with moment magnitudes estimated to be as large as 7.0 or greater, all occurring within a three - month period between December 1811 and February 1812 . Many of the published accounts describe the cumulative effects of all the earthquakes (known as the New Madrid Sequence); thus finding the individual effects of each quake can be difficult . Magnitude estimates and epicenters are based on interpretations of historical accounts and may vary . </P> <P> Because uplift rates associated with large New Madrid earthquakes could not have occurred continuously over geological timescales without dramatically altering the local topography, studies have concluded that the seismic activity there cannot have gone on for longer than 64,000 years, making the New Madrid Seismic Zone (NMSZ) a young feature, or that earthquakes and the associated uplift migrate around the area over time, or that the NMSZ has short periods of activity interspersed with long periods of quiet . Archaeological studies have found from studies of sand blows and soil horizons that previous series of very large earthquakes have occurred in the NMSZ in recent prehistory . Based on artifacts found buried by sand blow deposits and from carbon - 14 studies, previous large earthquakes like those of 1811--1812 appear to have happened around AD 1450 and around AD 900, as well as approximately AD 300 . Evidence has also been found for an apparent series of large earthquakes around 2350 BC . About 80 km southwest of the presently - defined NMSZ but close enough to be associated with the Reelfoot Rift, near Marianna, Arkansas, two sets of liquefaction features indicative of large earthquakes have been tentatively identified and dated to 3500 BC and 4800 BC . These features were interpreted to have been caused by groups of large earthquakes timed closely together . </P> <P> Dendrochronology (tree ring) studies conducted on the oldest bald cypress trees growing in Reelfoot Lake found evidence of the 1811--1812 series in the form of fractures followed by rapid growth after their inundation, whereas cores taken from old bald cypress trees in the St. Francis sunklands showed slowed growth in the half century that followed 1812 . These were interpreted as clear signals of the 1811--1812 earthquake series in tree rings . Because the tree ring record in Reelfoot Lake and the St. Francis sunk lands extend back to AD 1682 and AD 1321, respectively, Van Arsdale et al. interpreted the lack of similar signals elsewhere in the chronology as evidence against large New Madrid earthquakes between those years and 1811 . </P> <P> The first known written record of an earthquake felt in the NMSZ was from a French missionary traveling up the Mississippi with a party of explorers . At 1 p.m., on Christmas Day 1699, at a site near the present - day location of Memphis, the party was startled by a short period of ground shaking . </P>

When was the last time the new madrid fault went off