The Mississippi region has been no stranger to seismic activity, especially within the New Madrid Seismic Zone.

On December 3, 1990, Americans were filled with uncertainty. Climate expert Iben Browning had forecasted a "50 percent chance" of a major 7.0 magnitude earthquake hitting the nation, likely near New Madrid, Missouri, in the southeastern part of the state.

As the date approached, fear spread across the country. The St. Louis Red Cross distributed 230,000 earthquake preparedness kits, the Memphis "Commercial Appeal" published daily columns on the upcoming event, and schools across four states allowed students to skip school on December 3rd.

The predicted disaster never occurred. Browning's earthquake failed to materialize, much to the relief of the public. Earth scientists weren't surprised, as Browning, lacking expertise in seismology, had been dismissed by the experts. So, why did his forecast capture the public's attention? After all, most earthquakes occur near the edges of tectonic plates, and New Madrid is located thousands of miles away from any such boundary.

Even though the region's location is unusual, it does have a history of powerful earthquakes. Situated on an ancient seismic zone that spans Missouri, Arkansas, and Tennessee, this area was the site of massive earthquakes in the 19th century. The New Madrid Seismic Zone, as it is known, has a fascinating geological past.

A Failed Rift

You might have heard of Pangea, the vast supercontinent that existed when all of Earth's landmasses were joined together roughly 300 million years ago.

Before Pangea, there was the supercontinent Rodinia, which formed between 950 million and 1.3 billion years ago. Around 750 million years ago, Rodinia began to break apart, leading to the creation of rifts and faults between the various continental plates. One of these divisions, known as the Reelfoot Rift, took shape around 600 million years ago in what is now the southeastern United States.

Occasionally, rifts can split continents apart, but that wasn’t the case with the Reelfoot Rift. It was halted in its progress. From a geological standpoint, it failed — but not before it weakened a portion of the continental plate and created faults that persist to this day.

"[It] left a lasting mark on the otherwise ancient and relatively uniform crust," says Susan Hough, a seismologist at the U.S. Geological Survey, in an email. The New Madrid Seismic Zone sits within the Reelfoot Rift. For earthquake researchers like Hough, this region remains a puzzle. As noted in her email, numerous other failed rifts exist across North America, including one beneath Lake Superior. Yet these have not triggered significant earthquakes in recent or historical times.

"So why has the Reelfoot Rift been active with earthquakes, while others have not?" asks Hough. "There are some theories, but no conclusive answers."

"Whole Lotta Shakin' Goin' On"

When an earthquake strikes a layer of wet, loose sand, the combined pressure of the grains and water can force them to burst through the surface like a geyser. This creates "sand blows", large cones of debris on the ground. By dating these ancient sand blows, geologists have determined that the central Mississippi Valley experienced some massive quakes around 1450 A.D., 900 A.D., 300 B.C.E., and 2530 B.C.E.

The next seismic event to match the magnitude of those pre-Columbian quakes wouldn’t occur until 1811. On December 16 of that year, at around 2:15 a.m. Central Time, a powerful quake shook northeastern Arkansas, causing chimneys to collapse, trees to fall, and landslides along the Mississippi. Two more similar tremors followed near New Madrid on January 23 and February 7, 1812.

The precise magnitude of the 1811-1812 quakes remains uncertain. Seismic events with a magnitude of 8.0 or higher on the Richter Scale are classified as "great". These can level small towns near the epicenter and are 10 times more powerful than 7.0 quakes, which still cause significant damage.

The 1811 and 1812 earthquakes in the New Madrid Seismic Zone are historically regarded as some of the most devastating quakes the U.S. has ever experienced. Some researchers even suggest they were stronger than the 7.9-magnitude San Francisco earthquake of 1906. However, Hough disagrees with that claim. "My research indicates that the largest of these quakes were closer to magnitude 7 than 8," she says.

Could History Repeat Itself?

Many buildings in New Madrid were swept into the Mississippi River by the 1811 and 1812 earthquakes. Despite this, the city proudly embraces its seismic history: Visit today and you can buy T-shirts that say, "It's Our Fault."

Should modern Americans be worried about the dormant rift? In 2009, the Federal Emergency Management Agency (FEMA) assessed the possible danger of a 7.7 magnitude earthquake originating from the New Madrid Seismic Zone. According to the report, such an event would likely demolish nearly 715,000 buildings across an eight-state area. The report also predicts that 2.5 million households would lose power, with 86,000 casualties and serious injuries.

But don't panic just yet. The likelihood of this apocalyptic scenario is up for debate. Geologists remain divided on the current activity levels of the zone. Nonetheless, as Hough emphasizes, it hasn’t caused significant damage in a long time.

"In recent geological times, the zone has experienced earthquake sequences like those in 1811-1812 roughly once every 400 to 500 years," Hough shares with us. "If you get a magnitude 7.2 (for example) every 400 years, you can expect a magnitude 6.2 earthquake every 40 years."

However, things haven't gone as predicted. The last seismic event in the region to come close to magnitude 6 occurred in 1895. Does that mean we’re overdue for another major quake? "Short answer: nope," Hough replies. There is no reliable way to predict earthquakes — something Browning failed to understand back in 1990. As Hough puts it, "[They] aren't babies. We don’t know when they’re coming, so we can’t say when they're overdue."