Every 500 Years, This Area Is Rocked By Massive Earthquakes - And Nobody Knows Why

Look at a map of faults and fault lines in the US, and much of it will look pretty much like you’d expect, assuming you’ve ever heard of the San Andreas Fault . Basically, there’s nothing between the East Coast and, oh, Denver or so, and then suddenly the country turns into a mess of seismic activity and potential .

There’s one anomaly to that rule: a big old hotspot right in between the Ozarks and Appalachia. Known as the New Madrid Seismic Zone, it’s been home to some of the largest and most destructive quakes in US history – and nobody really understands why.

What is the New Madrid Seismic Zone?

It was two a.m., on December 16, 1811, when the first earthquake struck the frontier town of New Madrid, Louisiana Territory. It was “a violent shock of an earthquake,” wrote New Madridian Eliza Bryan in 1816, “accompanied by a very awful noise resembling loud but distant thunder, but more hoarse and vibrating, which was followed in a few minutes by the complete saturation of the atmosphere, with sulphurious vapor, causing total darkness.”

They’re generally referred to as three earthquakes, but the truth is that it was more like one three-month-long biblical apocalypse. The initial quake – a magnitude 7.0 mother – was felt across an area of nearly a million square miles, or 3.5 Texes; the ground was felt shaking as far away as Canada, church bells rang from the motion in Boston, and chimneys were brought down in Cincinnati, Ohio. It was followed by aftershocks and weaker earthquakes until, on January 23, the area was hit by a second major quake of magnitude 7.3; after that, the “earth was in continual agitation, visibly waving as a gentle sea,” Bryan wrote, until on February 7, the strongest earthquake yet – magnitude 7.5 – hit the town again.

People and animals alike panicked and fled as the ground was “horribly torn to pieces,” Bryan recounted, with great fissures “vomit[ing] forth” black, coal-like substances mixed with sand and water. The Mississippi River rose up “fifteen or twenty feet perpendicularly,” she wrote, and appeared temporarily to reverse course; it was literally rerouted so much that it created Reelfoot Lake in Tennessee, and the banks of the river were covered in dead fish and wrecked boats.

In the end, the whole town had sunk about 15 feet lower than before – there are even photos of trees that have grown a second set of roots to deal with the new ground level – and the whole area was dealing with aftershocks for years afterward. It was, Bryan admitted, “a scene, the description of which would require the most sublimely fanciful imagination.”

And that’s only the most recent spate of quakes. Paleoseismology – the study of ancient earthquakes via evidence in the geological record – has revealed a pattern of seismic activity in the region: we now know that the New Madrid region was devastated not only in 1811, but also in about 1450 CE, 900 CE, 300 CE, and 2350 BCE.

Not only are earthquakes fairly regular in the area, then, but they’re also startlingly consistent. By studying liquefaction features – the size and composition of sand blows; their locations and distributions, and so on – researchers have been able to conclude that these historical earthquakes were strikingly similar to the events of 1811-12: they were “similar in location and magnitude to the 1811-1812 earthquakes,” the United States Geological Survey (USGS) notes, and likely “also […] were earthquake sequences.”

Overall, the USGS explains, “the New Madrid seismic zone generated magnitude 7 to 8 earthquakes about every 500 years during the past 1,200 years.” And yes: it’ll probably happen again; it’ll probably be bad; and it probably won’t be alone.

And here’s the thing: none of that really makes any sense at all.

Why do earthquakes happen?

We like to think we know what causes an earthquake. “Earthquakes result from a ‘stick-slip’ motion, where rocks ‘stick’ along fault planes while stress accumulates until a ‘slip’ occurs – a bit like pulling on a stuck door until it suddenly opens,” wrote Åke Fagereng, a researcher in Cardiff University’s School of Earth and Ocean Sciences, in a 2020 article for The Conversation.

“This slip also releases energy as the seismic waves that, in large magnitude earthquakes, create substantial damage.”

This is why earthquakes are so commonplace in places like Alaska, Southern California, Japan, and everywhere else in the so-called “Ring of Fire” – the infamous tectonic belt around the Pacific Ocean inside of which some 90 percent of the world’s earthquakes occur. It’s why places like Iran and Afghanistan see so much seismic activity – the countries are sat on top of multiple fault lines, with the Indian and Arabian tectonic plates subducting under the Eurasian plates.

All of which raises the question… why would Missouri be in trouble?

What’s going on in Missouri?

If earthquakes are caused by the interaction of various tectonic plates – what the heck is going on at the New Madrid Seismic Zone?

“It’s a big mystery,” USGS geologist Eugene Schweig told Wired in 2008. “New Madrid is about as far from a plate boundary as you can get.”

While they’re rare, earthquakes within tectonic plate interiors do happen – but why is a question that geologists are struggling with even today. “It is generally agreed that these intraplate earthquakes are caused by reactivation of old rifts,” explained Attreyee Ghosh, a professor from the Centre for Earth Sciences at the Indian Institute of Science, in a 2020 article for the European Geosciences Union. “However, what causes this reactivation is not well-understood.”

For the New Madrid Seismic Zone in particular, there’s a whole slew of potential answers, Ghosh noted: suggestions include “glacio-isostatic adjustment, weakening of mantle due to rifting or arrival of a plume, ridge push, gravitational body forces, large scale mantle convection as well as dynamic topography,” to name but a few. Her own preferred theory features long wavelength tectonic stresses, with intraplate earthquakes effectively being activated by massive forces propagated in from the tectonic boundary.

Whatever is reactivating these rifts, it’s clear that the New Madrid Seismic Zone is the result of old, old geology. Researchers have located three fault lines in the region which were formed about 500 million years ago – “a time when the North American plate tried and failed to pull itself apart,” explains Undark . But precisely why these fault lines are still active – or newly reactivated – is a mystery. And that’s a big problem.

See, without a clear mechanism for what’s causing the quakes, researchers are at a loss for ways to predict them. Being located smack dab in the middle of a tectonic plate makes normal methods all but moot: it’s only within the last decade that they were even able to confirm geological movement in the area at all, let alone figure out how much tectonic stress is being held in the ground – or where the breaking point may be.

“We know from research and past events that we’re capable of having large earthquakes in this area again, and at any time,” Brian Blake, executive director of the Central United States Earthquake Consortium, told Undark . “Our job, regardless of the mechanism that causes earthquakes, is to prepare.”

How much danger is the New Madrid Seismic Zone in?

So, how worried should Missourians – and Tennesseans, Arkansans, Kentuckians, and Illinoisans – be? Well, it depends on what precisely you’re asking.

The actual risk of an earthquake, especially one similar in severity to the three that battered the state back in the 1800s? That’s probably not very high: “we would estimate a 25 – 40 percent chance of a magnitude 6.0 and greater earthquake in the next 50 years,” notes the USGS Earthquake Hazards Program , “and about a 7 – 10 percent probability of a repeat of the 1811-1812 earthquakes in the same time period.”

In fact, it may even be lower than that. The extent to which the New Madrid Seismic Zone has been touted as a potential disaster area – by the media, and even by other scientists – has been criticized by some experts. There are questions over the methodology used by the USGS, which is based on probabilistic techniques; deterministic or scenario analyses, which are favored by geologists at the University of Kentucky, result in a lower risk profile, and – at least, according to the methods’ proponents – a more accurate one.

That said, earthquakes are notoriously difficult to predict – and even a very low risk of seismic activity can only tell half the true story. Alaska gets more earthquakes than any other state in the union, including some of the highest-magnitude ever recorded – but in 2018 , when Anchorage was hit by a magnitude 7.0 earthquake, there was only minimal damage and zero deaths.

How did they manage that? It’s simple: in a state where seismic activity is commonplace, earthquake preparedness is built into everyday life. Schools regularly drill students on what to do in a quake; buildings must adhere to strict codes and standards; family homes get kitted out with emergency provisions and exit strategies in preparation for disaster.

Not so in New Madrid. The eponymous Seismic Zone covers several major cities – Memphis, Tennessee; St Louis, Missouri; Little Rock, Arkansas, each with populations in the hundreds of thousands – and “we know that a lot of people are living in buildings that are not ready for a big earthquake,” Jeff Briggs, the earthquake program manager for the Missouri State Emergency Management Agency, told Undark .

And while efforts are afoot to shore up the area’s earthquake awareness, it’s slow going. Funding is low, and legislation is lacking; thanks to their rareness, earthquakes in general are simply “not as front of mind” as other natural disasters, Briggs explained.

Nevertheless, he added, when the next major earthquake hits – and all evidence suggests that it will – “it’s going to be the biggest natural disaster this state has ever experienced.”