In a three-month period from December 1811 through February 1812, the area around New Madrid, Missouri was rocked by the largest group of earthquakes in U. S. history. Nearly 2,000 earthquakes occurred during this period, including four of the top ten earthquakes ever recorded in the contiguous United States. These earthquakes were felt as far away as New England, ringing church bells in Boston. How can such a large series of earthquakes take place in the center of North America, far from any tectonically active plate boundaries? And, more importantly, could it happen again?
The New Madrid Seismic Zone
The New Madrid Seismic Zone is the most active earthquake region in the United States east of the Rocky Mountains. Located in the "Bootheel" region of Missouri and extending into Arkansas, Kentucky, and Tennessee, the NMSZ is clearly visible on this map of earthquake epicenters.
More than 600 million years ago, molten rock from the mantle began to rise toward the surface in the New Madrid area. Lifted and heated from beneath, the crust began to pull apart, creating a structure known as the Reelfoot Rift. Had this process continued, what is now the North American continent would have been split into two pieces separated by an ocean basin, similar to the Atlantic Ocean today. However, the rifting process stopped before this point was reached, and the rift valley has since been buried under a thick layer of sediments.
Although it is no longer geologically active, the Reelfoot Rift represents a "weak spot" in the North American plate. The cracks in the crust, or faults, resulting from the rifting process remain today. In addition, bodies of igneous rock that intruded from the mantle create additional weaknesses. As seafloor spreading in the Atlantic pushes the North American plate to the west, the underlying rocks in the interior of the plate are compressed. In a weak area such as the New Madrid Seismic Zone, this compressional stress occasionally causes the fractured pieces of the crust to shift position relative to one another, which we feel as earthquakes. The great quakes of 1811-12 are an example of this sort of movement.
Unlike the tectonically fractured rock of California, which damps out seismic vibrations relatively quickly, the rock underlying the eastern pert of North America is much more solid, allowing vibrations from an earthquake to be felt over a much wider area. For example, here is a comparison between the areas affected by two earthquakes of similar magnitudes. A large earthquake in this area today could potentially devastate a very large area.
Earthquake Frequency
What is commonly known as the great New Madrid earthquake of
1811-12 was actually a series of quakes that took place over a three-month
period. The main shocks came from three earthquakes estimated to be
greater than 8.0 in magnitude. Aftershocks included two more events
around magnitude 8.0, five more at magnitude 7.7, ten more at magnitude
5.3, and eighty-nine at an estimated magnitude of 4.3. This seismic
release was the largest ever recorded in the continental United States. Smaller earthquakes are actually quite common in this area. The New Madrid fault is currently active and averages more than 200 seismic events per year with magnitude greater than 1.0. The vast majority of these quakes are too small to even be felt by humans, but damaging ones do occur. The expected intervals for these events are summed up in the following table:
Magnitude Expected Rate (yr)
4.0 14 months
5.0 10-12 yr
6.0 70-90 yr
7.0 254-500 yr
8.0 550-1200 yr
"The big one" will strike this area, but when it will happen is
hard to tell. The probability that it will occur within 15 years is only
.3% - 1.0%. The probability that it will occur within 50 years is 2.7% -
4.0%. These figures sound encouraging, but the possibility of extensive
damage certainly exists. An earthquake of magnitude 7.0 or greater has a
19% - 29% chance of happening within the next fifty years. An earthquake
of this size would be felt throughout much of the central and eastern
United States and would cause several billion dollars in damages. An
earthquake of magnitude 6.0 or greater has a probability of 40% - 63%
within the next fifteen years, and 86% - 97% within the next one hundred
and fifty years. An event of this size would cause serious damage to many
older buildings in the region.
Impact of the New Madrid Quakes
To describe the nature of earthquake hazards in the area, one must examine the largest recorded earthquakes to hit the Mew Madrid fault, which are the quakes of 1811-12. Several minor earthquakes occur at this fault each year, but these are negligible in comparison to the 1811-12 quakes. These monumental earthquakes of magnitude 8.0+ affected the topography of North America more than any other quake in history. The power of these three earthquakes was felt from coast to coast. The ground around the epicenter of the quakes rolled in waves due to the tremendous amount of pressure being released. Large areas of land sunk down or rose up like columns. Fissures were created all over, and they shot out sand and other debris. All of this violent activity of the land changed the course of the Mississippi River, created Reelfoot Lake, and destroyed entire forests. For a more detailed description, read this eyewitness account.
The earthquakes of 1811-12 were tremendously powerful, but luckily the Bootheel area was not very settled at the time. The houses that did exist in the area at the time were destroyed, and more damage to buildings was reported as far away as Washington D.C. and Charleston, SC. Today, the cities of St. Louis and Memphis lie about equal distances from the New Madrid fault. Memphis is a mere 30 miles from the epicenters of the great quakes of 1811-12, and a similar earthquake today could wreck utter destruction. Geologists predict that if an 8.0+ magnitude earthquake struck today around 60% of Memphis would be destroyed. This destruction would cause thousands of deaths and about $50 billion in damages. These numbers are for Memphis alone, and similar numbers can be expected for St. Louis and other nearby urban areas. One reason leading to such a high level of devastation is that the buildings in the New Madrid area are not being built to withstand shaking. The people living in the area are not used to earthquakes like Californians, because the New Madrid fault has not had as many high-intensity earthquakes. Thus, the fear of earthquakes has not been instilled into their hearts, but when a big one does occur they will know fear and will regret not having constructed their buildings better. Geologists are aware of the increasing probability of a large quake coming though, and legislation has been passed to improve the situation. Kids are now taught the dangers of earthquakes at an early age, and earthquake evacuation drills are performed at most of the schools in the New Madrid area. New buildings are now having to be built according to tougher standards, and scientists are redoubling their efforts at studying the New Madrid fault to attempt to come up with a more accurate time schedule for when the big one will strike again. However, none of these changes will probably make much difference since so many other buildings that are not prepared to deal with the earth shaking already exist, and few other preparations are being made. Once the next powerful earthquake strikes the New Madrid area, even those lucky enough to survive will be left with severe emotional problems due to the sheer horror the quake will cause.
Related Sites on the Web
The Virtual Times: The New Madrid Earthquake
Saint Louis University Earthquake Center
Hidden Hazards in the Mississippi Valley
Geology of the Mississippi Embayment
Note: sometimes this link works, sometimes it doesn't
The Mississippi Valley-"Whole Lotta Shakin' Goin' On"