Earthquake chronology for New Madrid seismic zone from dating and correlation of liquefaction features at sites (listed at top) along NE-SW transect. Some sites show age estimates for more than one feature related to different events (e.g., Eaker 2 and L2). Inferred timing of events is shown with colored bands.

In addition, the size and spatial distributions of historic and sand blows that formed about 1450 A.D. and 900 A.D. were determined to be strikingly similar to each other, suggesting that the prehistoric earthquakes had similar locations and magnitudes to the 1811-1812 earthquakes. Furthermore, sand blows attributed to the 1450 A.D., 900 A.D., and 2350 B.C. earthquakes are composed of multiple, fining upward layers similar in thickness to those that formed in 1811-1812. These observations support the interpretation that the prehistoric events were similar in location and magnitude to the 1811-1812 earthquakes and also suggests that they too were earthquake sequences. Paleoseismic studies concluded that the New Madrid seismic zone generated magnitude 7 to 8 earthquakes about every 500 years during the past 1,200 years.

Photograph of sand blow along Obion River near Dyersburg, Tennessee, composed of three fining upward sedimentary layers. Each layer probably formed as result of individual large shock in earthquake sequence. Radiocarbon dating indicates that sand blow formed in 1811-1812. For scale, hoe is 1 m in length. Photograph by Martitia Tuttle.

(A) Time line of New Madrid events during past 2,000 years. Another event has been identified in 2350 B.C. + 200 yr but earthquake chronology is incomplete for period between 2350 B.C. and 800 A.D.; (B) Uncertainties in age estimates of New Madrid events lead to variability in recurrence times. Average recurrence time for past two earthquake cycles is 500 years.