In the middle of the United States, where the Mississippi River flows and the St. Louis Arch meets the west, there is a rather surreptitious geological event occurring below the surface. The New Madrid Seismic Zone (NMSZ) is one of the least understood geologically active regions in the United States. It has the potential to cause serious destruction and effect millions– in a region, that for the most part would appear to be relatively quiet and safe.
Magnitude of areas effected by the New Madrid Earthquakes [Image Source]
Unlike the San Andreas Fault in California, which can be easily viewed from the surface, the NMSZ faults remain hidden under 100-200-foot thick layers of a soft river soil in the Mississippi River known as alluvium. Even if a significant seismic event were to occur, geologists would have to be there at a moments notice to see any significant fault scarps or topographical changes in the alluvium. This is largely due to it being highly susceptible to erosion from the flow of the river.
Aerial view of the San Andreas Fault in California. [Image Source]
The last time a significant earthquake occurred in this region was roughly 200 years ago (1811-1812) and while it didn’t effect massive populations (the region was sparsely populated), it changed the actual lay of the land and left behind a number of anecdotes from eyewitnesses. Over the course of 2 months (Dec. 16th 1811–Feb 7, 1812) the region would be shaken by 3-5 major earthquakes (all believe to be at least 7.0 on the Richter scale), followed by aftershocks ranging from 5.0-8.0. They could be felt as far as Washington D.C. and according to some reports, Quebec City- nearly 1,400 miles away.
Due to the New Madrid region being the frontier for new European settlements, the ability to record these events using scientific instruments or recognize changes to the scenery was limited. However, there was one notable piece of evidence that this geological event left behind that can still be seen today and it’s in the form of a lake.
Prior to these violent quakes, Reelfoot Lake in Tennessee was simply a forested area like any other part of the region. However, when the earthquakes happened, land began to rise and subside all over the New Madrid region; specifically, along the Reelfoot rift. This caused fluvial tsunamis (also known as a seiche) to move upstream. This gave the illusion that the Mississippi River was flowing in the opposite direction. There were reports of waterfalls appearing out of no where and that the banks of the Mississippi River practically vanished, causing mass flooding. As a result, Reelfoot Lake became a product of the earthquakes of 1811-1812. It should also be noted, it is the only natural large lake in Tennessee.
Illustration depicting the Madrid Earthquakes of 1811-1812. [Image Source]
One anecdote backing up this claim, came from a letter written by William Shaler who was retelling the story he heard from a Kentucky flatboat captain:
"...he soon found the current changed, and the boat hurried up, for about the space of a minute, with the velocity of the swiftest horse; he was obliged to hold his hand to his head to keep his hat on. On the current’s running its natural course, which it did gradually, he continued to proceed down the river, and at about daylight he came to a most terrific fall, which, he thinks, was at least six feet perpendicular, extending across the river, & about 1/2 mile wide."
A picture of Reelfoot Lake today. [Image Source]
Today the lake is a popular destination for locals to go fishing, boating and enjoy other activities. It also became an estuary for a variety of birds, the most notable being the American Bald Eagle. While geologists have been able to make the educated guess that earthquakes in this region appear to occur every 600 years, it is still difficult for them to ascertain whether this number is entirely accurate. This is in large part due to the fact that the NMSZ lacks significant topographical changes. This leads geologists to believe seismic activity in this region is in its infancy and has been occurring for roughly 64,000 years at most. However, there is some debate as to whether the earthquakes of 1811-1812 were a precursor to more earthquakes or if the region is only quieting down.
Given what information we have in hand today, through archaeological and geological studies of the area, it would appear the inhabitants of this region are in no immediate danger. However, it goes without saying that the NMSZ is capable of producing earthquakes of great magnitude. Fortunately, the modern era of the U.S. has yet to experience this. There are other examples of puzzling seismic activity in other areas of the United States as well e.g. Oklahoma and South Carolina.
Areas such as these, invite a legitimate challenge for geologists of the world. Not just because of the age of the NMSZ, but also due to the number of people that could be effected by it. Furthermore, the responsibility to create more awareness of this threat so the people of this region are better prepared, lay on their shoulders as well.
In places such as California, where the “big one” could occur at any moment, many of the people residing in the state have earthquake preparedness plans that involve everything from taking cover, having bottles of water next to their bed and turning off the gas at home. Being someone that has family from the one of the states (Illinois) that could be effected by a seismic event produced by the NMSZ, I can say without a doubt this isn’t something they think about often. Point being, I believe the NMSZ is worth being made aware of and that it should be discussed more often amongst the populations residing in this region.