We mapped 221 large (more than 200 ft across) landslides of three morphologically distinct types on the bluffs bordering the Mississippi alluvial plain in western Tennessee and Kentucky Old coherent slides (146 landslides, or 66 percent of the total) include translational block slides and single and multiple-block rotational slumps, all of which are covered by mature vegetation and have eroded features; no active analogs exist in the area. Earth flows (51 landslides, or 23 percent of the total) are also largely revegetated and eroded, though a few active earth flows are present on bluffs that have been cleared of vegetation. Young rotational slumps (24 landslides, or 11 percent of the total) form solely along actively eroding near-river bluffs and are the only active or recently active landslides in the area.
Two investigations conducted around 1900 indicate that the old coherent slides, in at least part of the area, formed during the 1811-12 earthquakes. The present investigation uses dendrochronology, geomorphology, historic topographic maps, local historical accounts, and comparisons with landslides triggered by other earthquakes to show that most or all of the old coherent slides and earth flows formed during the 1811-12 New Madrid earthquakes. Evidence clearly indicates that the only large, aseismic landslide activity in the area results from fluvial undercutting of near-river bluffs. This erosion of the base of the bluffs triggers slumps that are morphologically distinct from the old slumps on bluffs away from the river.
Our conclusions are consistent with the findings of other recent investigations of the same landslides that indicate extensive seismic triggering of coherent slides and earth flows during the 1811-12 New Madrid earthquakes.