The New Madrid earthquakes of 1811 and 1812 were the most powerful to hit contiguous United States in its history. The intraplate quakes of 1811 and 1812 were accompanied by numerous aftershocks and both the main shocks and those that followed were felt over most of the continental United States.
The first of the 1811 and 1812 New Madrid earthquakes occurred on the sixteenth of December 1811. Its magnitude was 7.2. The second quake hit the same area on the fifth of February 1812. Its magnitude was 7.4. In between there were numerous aftershocks. Both earthquakes were centered on a part of the Mississippi embayment close to where the states of Missouri, Arkansas, Kentucky, and Tennessee meet. The ensuing destruction was similar and widespread for both events; buildings collapsed, trees toppled, and the Mississippi River changed course. What could be called minitsunamis appeared on the river as fissures opened and closed below the surface. The shock waves rang church bells in Washington, D.C., and they were felt from Indiana to Massachusetts. Fortunately, there were few people living in the area at the time so, in spite of the great intensity of the earthquakes, the loss of life was very small.
On the basis of the size of the area damaged and the extent to which awareness of the events was felt across the continent, the New Madrid earthquakes can be considered the most powerful to have hit the United States since Europeans first settled here. An area of more than 200,000 square miles showed evidence of significant damage by these earthquakes and one million square miles experienced shaking that was strong enough to alarm the general population. This last-mentioned area can be compared with the effects of more recent events. It was more than twice the size of the area affected by the 1964 Alaska earthquake and ten times larger than that of the San Francisco earthquake of 1906. The complex physiographic changes that occurred on the Mississippi River within the earthquake areas were extensive. An uplift of land thirty miles long and fifteen miles wide raised the river’s valley by as much as thirty feet.
The first effects of the earthquakes on those who lived in and around New Madrid were the sounds of timbers groaning, creaking, and cracking, of furniture being thrown around, and of chimneys crashing down. People got out of their homes as quickly as they could in order to avoid the falling debris. The log cabin was the most common type of building in the area, a structure that is well able to withstand earthquakes, yet one that did not stand up in this instance because of the extensive ground movements.
Earth waves similar to those experienced in water kept moving across the surface, bending trees and opening up deep cracks in the ground. Landslides, one after another, swept down from the steeper bluffs and hillsides and, simultaneously, large areas of land were uplifted. Water emerged from below through the cracks. On the river huge waves overwhelmed a number of boats. Others were thrown on to land high above the level of the water and the returning waves took back with them trees and other debris, rather like the actions of a tsunami. Whole islands in the river disappeared.
Aftershocks for both events were numerous and unusual when compared with those of other earthquakes. After the December event, these lesser shocks were almost continuous but with lesser intensities over time, then came two weeks of quiescence followed by several days in which the ground was in a state of constant tremor. Records of the aftershocks were kept locally and it seems that they continued in patterns similar to those of 1811 and early 1812 for about two years. In spite of the great intensity and widespread damage associated with these earthquakes, few lives were lost because the density of population at that time, in that area, was very low. One life was lost in New Madrid through falling buildings. Several drowned when they were thrown into the river as a result of landslides, and several boatmen drowned when their boats sank. A number of canoes had also been abandoned and it was concluded that their owners had drowned.
The most seriously affected areas were characterized by raised and sunken land with the former marked by fissures, sand blows, and landslides. These areas extended from Cairo in Illinois to Memphis in Tennessee, a distance of 150 miles, and southeastward from Crowleys Ridge, past Memphis for a distance of fifty miles. The extent to which the quakes were felt has already been identified in terms of area. It helps to put place names to that statistic: the shocks were felt from Canada all the way to New Orleans and from Montana to Boston. People in Washington, D.C., were frightened badly because the shock in that part of the Appalachians was more acute than in places closer to the coast. This difference was later related to a connection between the Appalachian system and the epicenter of the New Madrid earthquakes.
Precise locations of the epicenters of the New Madrid earthquakes of 1811 and 1812 are now known. The fault zone is one hundred twenty miles long and the epicenters show a northeast-southwest trend. Another series of events like those in the nineteenth century would be felt from Denver to New York City and damage buildings in eight states. Tennessee and the other three states bordering this earthquake zone would be devastated. Today there are more than fourteen million people living within the area that was devastated by the New Madrid earthquakes. Some comparisons can be made with the San Francisco quake of 1906, about which we know much more and the total population affected there was four million. Furthermore, because of the nature of the earth’s crust in the central parts of the United States, the physical size of the area affected by any earthquake is much bigger. Geologists are always working on assessing the frequency rate of large earthquakes; that is to say, the average number of years between each big event. When they succeed preparations can be made to cope with the next occurrence.
The pressures that cause the massive tectonic plates to move will also cause them to rip apart because of the spherical shape of the earth’s surface and therefore the differential rates of motion of the ocean crust as it moves away from the spreading ridges. The North American continent is therefore impacted on its east coast at different rates and so internal stresses are created. Studies of the New Madrid zone show that earthquakes like those in 1811 and 1812 did occur in the past. Exactly when and how consistently are the questions that researchers want to answer. Sand deposits and liquefaction are often useful in dating the past, for example, these two remnants of past earthquakes can be located quite easily today to show that powerful earthquakes hit New Madrid in 1811 and 1812. Charles Lyell, the British scientist who pioneered new developments in the field of geology and was Charles Darwin’s mentor, visited New Madrid shortly after the nineteenth-century earthquakes and discovered that the native Indians had valuable information in their legends about past events.
Other geologists have followed up on the possibility of dating past events by examining the sites of ancient Indian settlements. It is known that native Indians used sandy areas for cooking, leaving on these places after they moved elsewhere fragments of their cooking utensils and charcoal residues. Archeologists can date items of pottery by their form and size; they can also date carbon or charcoal by the radiocarbon method. From evidence of this kind we now know that a major earthquake occurred in the New Madrid area sometime between 1180 and 1400. It was powerful enough to cause liquefaction. Evidence of much earlier earthquakes was also found, one of them being dated several thousand years ago. Large gaps of this kind between successive events are of little value for predicting future earthquakes so the search goes on. One alternative method employed by geologists involved examining earth fissures from the past, the same types of cracks in the surface that were seen in 1812, and looking for deposits of different material in them. By carbon dating these alien materials in the cracks they were able to locate one big earthquake somewhere between 780 and 1000 and another between 1260 and 1650. By interpolating from these it is now possible to come up with a rate of occurrence of 400–500 years.
If the 400–500 frequency rate were to be true for the future, Memphis, the nearest big city to New Madrid, need not expect a huge earthquake for another two hundred years. However, there are other considerations that illustrate the difficulty of being precise when it comes to predicting earthquakes. For one thing, the New Madrid area has been hit with more than twenty-four earthquakes since 1812, all of them events that did substantial damage. On national maps of the United States, New Madrid is shown as having a greater chance of being hit with an earthquake than any other place east of the Rocky Mountains. There is, additionally, one other variable affecting the time of the next earthquake. It has been found in several countries where the average rates of recurrence of earthquakes are known that sometimes the sequence is interrupted by a cluster of earthquakes. That was experienced in Australia when a series of five major earthquakes hit an interior area within a period of twenty years.