The New Madrid seismic zone had at least three M>7 earthquakes during the winter of 1811–1812, but its faults are poorly mapped. Interpretation of 140, 91.4 m (300 ft) deep well logs west of New Madrid, Missouri, reveals a 9.7 km by 6.4 km subsurface high with 30 m of relief on Pleistocene‐age Mississippi River gravels. Seismic soundings also reveal an average of 55 m of relief on the Paleocene Wilcox Group, 56 m on the top of the Cretaceous, and 83 m on the top of the Paleozoic section. We interpret this as predominantly structural uplift, herein called the Lilbourn uplift. The uplift appears to be a northwestern extension of the Reelfoot North fault and its hanging wall deformation (the Tiptonville dome).
It is not known whether the full length of the Reelfoot fault (70 km) or just the Reelfoot North fault (32 km) ruptured during the 7 February 1812 earthquake. However, an extension of 14.5 km on the Reelfoot North fault would increase its length to 47.5 km and thus permit an M∼7 earthquake, whereas a 14.5 km increase on the combined Reelfoot North and Reelfoot South faults would increase its length to 84.5 km and thus permit an M∼7.2 earthquake.