The New Madrid Earthquakes of 1811 and 1812

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.