Tennessee does not lie on a conventional “fault line” that marks the boundary between two tectonic plates, such as the San Andreas Fault in California. Far from any plate edges, western Tennessee is directly affected by the New Madrid Seismic Zone (NMSZ), which is a major source of earthquake activity within the continent’s interior. This zone is a complex system of buried faults that poses the greatest seismic risk in the United States east of the Rocky Mountains. The area’s seismic activity centers on an ancient weakness in the Earth’s crust that is currently being reactivated by continental stresses.
The New Madrid Seismic Zone
The New Madrid Seismic Zone (NMSZ) is the most active earthquake area in the Central and Eastern United States, covering a span of about 150 miles. This zone extends from northeastern Arkansas, through the Missouri Bootheel and western Kentucky, and directly into northwestern Tennessee. West Tennessee, particularly the region near the Mississippi River Valley and the city of Memphis, sits at the southern end of the most seismically active segments.
The fault system is not visible at the surface because it is buried beneath 100 to 200 feet of soft river sediments deposited by the Mississippi River. Sensitive seismographs continuously monitor the area, recording hundreds of small, mostly unfelt earthquakes each year. These micro-earthquakes outline a zig-zag fault system composed of several distinct segments deep underground.
The primary segments include a northeast-trending strike-slip fault and a northwest-trending reverse fault, such as the Reelfoot Fault, which extends near Dyersburg, Tennessee. These faults are not a single, continuous break but a collection of ancient fractures in the crust. The ongoing seismic activity confirms that these buried faults are capable of movement. A consequence of this geology is that ground shaking from an NMSZ earthquake can affect an area up to 15 times larger than a similar-sized quake in California.
Mechanisms of Intraplate Activity
Earthquakes that occur far from tectonic plate boundaries are known as intraplate earthquakes, which is the type seen in the NMSZ. The cause of this activity is rooted in an ancient geological feature called the Reelfoot Rift, a structure formed about 600 million years ago. This rift represents a failed attempt by the North American continent to split apart, leaving a deep, fractured zone of weakness in the crust.
The Reelfoot Rift is a zone where the continental crust is structurally weaker and more susceptible to stress than the surrounding rock. Although the rifting process stopped, the embedded faults have remained as a scar deep underground. Today, the entire North American Plate is subjected to enormous compressive forces transmitted from the plate boundaries, particularly from the spreading of the Mid-Atlantic Ridge.
These distant forces create an east-west compression across the continent. When this stress reaches the Reelfoot Rift, the ancient, buried faults are reactivated because they represent the path of least resistance. The seismic activity is concentrated along these zones of weakness between the depths of about 3 to 15 miles.
Seismic History and Current Hazard
The most significant seismic events in the region’s history were the sequence of the 1811–1812 New Madrid earthquakes, which included three main shocks estimated to be between magnitude 7 and 8. The first major event occurred on December 16, 1811, followed by others in January and February of the following year. These powerful quakes caused catastrophic changes to the landscape, including the temporary reversal of the Mississippi River’s flow and the creation of Reelfoot Lake in Tennessee through land subsidence.
The geological record, derived from studying features like sand blows, shows that events of magnitude 7 to 8 have occurred in the NMSZ approximately every 500 years over the last 1,200 years. The NMSZ still shows continuous small-to-moderate earthquake activity, confirming that the underlying geological processes remain active. Scientific assessments indicate a continuing concern for a destructive earthquake in the future.
Scientists estimate a 25 to 40 percent probability of a magnitude 6.0 or greater earthquake occurring within the NMSZ in the next 50 years. The probability of a repeat of the massive 1811–1812 sequence is estimated to be lower, between 7 and 10 percent within the same 50-year period. Vulnerable communities in Tennessee, such as Memphis, are at risk due to severe ground shaking and the potential for soil liquefaction.