Of paramount importance to our ability to assess seismic hazards in the New Madrid Seismic Zone is determining accurate estimates of the magnitudes of the 1811-1812 events. Based on intensity determinations, moment magnitudes for the three largest 1811-1812 earthquakes are estimated to be between M 7.8 and 8.1. Earthquakes of these large magnitudes suggest coseismic fault rupture lengths in excess of 100 km for each event, yet the only clear case of coseismic surface deformation occurred during the February 7, M 8.0 earthquake when uplift occurred over a distance of 32 km along the Reelfoot Fault scarp.
The Lake County uplift and the Reelfoot Fault scarp are within the northwest-trending central arm of the New Madrid Seismic Zone; however, contemporary seismicity in this part of the seismic zone extends beyond the Lake County uplift from New Madrid, Missouri, to near Dyersburg, Tennessee, a total distance of approximately 70 km. Although no fault scarp is present southeast of Reelfoot Lake, we believe that the Reelfoot Fault ruptured this entire length during the February 7 earthquake. A Mini-Sosie seismic-reflection line at Lane, Tennessee, shows evidence of a faulted asymmetric anticline along the projection of the Reelfoot Fault Zone. We believe that prehistoric and 1811-1812 coseismic uplift in the hanging wall of the Reelfoot Fault has caused subtle surface warping from Reelfoot Lake to Dyersburg. Evidence for this warping includes (1) a structural high on top of the Eocene strata in the projected hanging wall of the Reelfoot Fault, (2) a nearly continuous drainage divide on the hanging wall of the Reelfoot Fault that extends from the Mississippi River floodplain onto the uplands to near Dyersburg, (3) drainage-basin asymmetry suggesting a fault block margin along the southeastern projection of the Reelfoot Fault, (4) narrowing of the Obion River floodplain where the river crosses the fault, (5) possible warping of Obion River terraces and floodplain sediments across the fault, and (6) the formation of a temporary Lake Obion in the Obion River Valley on the footwall of the projected Reelfoot Fault.