They are among the Great earthquakes of known history, affecting the topography more than any other earthquake on the North American continent. Judging from their effects, they were of a magnitude of 8.0 or higher on the Richter Scale. They were felt over the entire United States outside of the Pacific coast. Large areas sank into the earth, new lakes were formed, the course of the Mississippi River was changed, and forests were destroyed over an area of 150,000 acres. Many houses at New Madrid were thrown down. "Houses, gardens, and fields were swallowed up" one source notes. But fatalities and damage were low, because the area was sparsely settled then.
The magnitudes of the quakes were estimated from the descriptions of their effects. Some of these effects were:
- The perception of the shock, even the ringing of church bells, at great distances from the quake. The December 16th quake rang church bells in Pennsylvania and in South Carolina. The February 7th quake was said to have been felt strong enough to rattle windows in Montreal, Quebec, over a thousand miles away.
- The modification of the channel of the Mississippi River.
- The rising of some sections of land, the falling of other sections. Six foot falls were created in the Mississippi River. At some points there were reports that the Mississippi ran backwards.
- The creation of new ten new lakes, the largest of which was Reelfoot Lake in northwestern Tennessee. In other places the land rose and lakes disappeared.
- Some islands in the rivers disappeared.
- Trees broke loudly from the violent shaking.
- The rising of dead trees from river and lake bottoms.
- The creation of crevasses in the earth as much as ten feet wide. Sometimes the crevasses opened and then closed spurting water and sand into the air.
- The creation of water spouts rising as high as fifteen feet into the air.
- The liquification of ground and the subsequent sinking of structures.
- The toppling of brickwork, particularly chimneys.
- People being thrown out of their beds.
The zone remains active today. In recent decades minor earthquakes have continued. New forecasts estimate a 7 to 10 percent chance, in the next 50 years, of a repeat of a major earthquake like those that occurred in 1811-1812, which likely had magnitudes of between 7.5 and 8.0. There is a 25 to 40 percent chance, in a 50-year time span, of a magnitude 6.0 or greater earthquake.