About Every 500 Years, a Major Quake Body-Slams Middle America

Experts can’t explain why, and big cities aren’t ready for another.

This article was originally published by Undark Magazine.

The first earthquake struck while the town was still asleep. Around 2 a.m. on December 16, 1811, residents of New Madrid—a small frontier settlement of 400 people in what is now southeastern Missouri—were jolted awake. Panicked townsfolk fled their homes as buildings collapsed and the smell of sulfur filled the air.

The episode didn’t last long. But the worst was yet to come. Nearly two months later, after dozens of aftershocks and another massive quake, the fault line running directly under the town ruptured. Eliza Bryan, a 31-year-old resident, watched in horror as the Mississippi River receded and swept away boats full of people. In nearby fields, geysers of sand erupted, and a rumble filled the air.

By the end, the town had dropped around 15 feet. Bryan and others spent a year and a half living in makeshift camps while they waited for the aftershocks to end. Four years later, the shocks had become less common. At last, the rattled townspeople began “to hope that ere long they will entirely cease,” Bryan wrote in a letter.

Whether Bryan’s hope will stand the test of time is an open question.

The U.S. Geological Survey released a report in December 2023 detailing the risk of dangerous earthquakes around the country. As expected on the hazard map, deep-red risk lines run through California and Alaska. But the map also sports a big bull’s-eye in the middle of the country—right over New Madrid.

The USGS estimates that the region has a 25 to 40 percent chance of a magnitude 6.0 or stronger earthquake in the next 50 years, and as much as a 10 percent chance of a repeat of the 1811–12 sequence. Although the risk is much lower compared with, say, parts of California, experts say that when it comes to earthquake resistance, the New Madrid seismic region suffers from inadequate building codes and infrastructure.

Caught in this seismic splash zone are millions of people living across five states—mostly in Tennessee and Missouri, as well as Kentucky, Illinois, and Arkansas—and two major cities, Memphis and St. Louis. Mississippi, Alabama, and Indiana have also been noted as places of concern.

In response to the potential for calamity, geologists have learned a lot about this odd earthquake hot spot over the past few decades. Yet one mystery has persisted: why earthquakes even happen here in the first place.

This is a problem, experts say. Without a clear mechanism for why the New Madrid area experiences earthquakes, scientists are still struggling to answer some of the most basic questions, such as when—or even if—another large earthquake will strike the region. In Missouri today, earthquakes are “not as front of mind” as other natural disasters, says Jeff Briggs, the earthquake program manager for the Missouri State Emergency Management Agency.

But when the next big shake comes, he says, “it’s going to be the biggest natural disaster this state has ever experienced.”

[Read: California is 150 years overdue for a massive earthquake]

Sizing up earthquake risk isn’t easy—especially when a seismic zone is smack dab in the middle of a tectonic plate.

Up until recently, the 1811 and 1812 quakes were considered “freak events,” says Martitia Tuttle, a paleoseismologist at M. Tuttle and Associates, an earthquake-risk consulting company. Earthquakes occur when a subterranean strain that was building for centuries is released in seconds. That usually happens near plate-tectonic boundaries, where massive plates rub against each other.

But exceptions happen. For instance, the New York area was jolted by a sudden seismic shock in April, despite being thousands of miles from the closest plate boundary.

But just because areas such as New York are far from a plate boundary today doesn’t mean that was always the case—and many regions have the scars to prove it. Researchers have mapped out three currently active ancient fault lines—fissures in the Earth’s crust—in the New Madrid area that formed about 500 million years ago, at a time when the North American plate tried and failed to pull itself apart. Now those fault lines form weak zones where stress in the Earth can build and eventually break into earthquakes.

Why these fault lines are seismically active, yet neighboring ones aren’t, is less clear. “It’s really one of the most enigmatic seismic zones on the planet,” says Eric Sandvol, a seismologist at the University of Missouri. “We’re not supposed to have earthquakes here.”

Scientists have suggested that the movement of the North American plate westward is driving stress, or that earth bounding back after the crush of massive glaciers during the Ice Age is to blame. Some have also pointed to a pillow-shaped rock underneath the seismic zone as a factor.

But researchers simply “don’t have a smoking-gun stress source” and are unlikely to anytime soon, explains Eunseo Choi, a geophysicist at the Center for Earthquake Research and Information at the University of Memphis.

That matters because measuring stress buildup is a great way to predict earthquake hazards. The telltale ripping of earth along the surface—a sign of stress buildup—is easy to see in places such as California where tectonic plates meet. Yet until recently, researchers working in the New Madrid area weren’t even able to prove that the ground in the region was moving at all, because the movement there is far harder to detect. The lack of obvious stress buildup led some scientists to suggest that New Madrid might not be gearing up for another earthquake. Yet in 2015, scientists published data that showed that the ground near fault lines was in fact creeping—albeit slowly.

But researchers still don’t know exactly how much stress is building below the surface, Choi says.

With modern technology, people can look at celestial bodies light-years away, he says. “But ironically, we don’t really see that well just a few kilometers down from our surface.”

[Read: Earthquakes are unlike any other environmental disaster]

On a cloudy day in May, the geologist Roy Van Arsdale is driving on top of the Reelfoot fault line in western Tennessee. The Mississippi River valley extends on terrain as flat as Kansas for miles. Only some towns have the slightest bit of elevation.

Settlers in the region built anywhere they could to escape the annual spring floods. In New Madrid, that meant building where earthquakes had uplifted the earth. Van Arsdale pulls off the main road to park behind a prison complex. At odds with the rest of the countryside, a long, linear mound of earth juts out from beneath a prison fence and into a neighboring cornfield. It’s the fault line. If another major earthquake were to happen, Van Arsdale says, “they’d be in trouble.”

Van Arsdale has spent his career trying to understand why the region experiences earthquakes. In the 1970s, the nuclear-power industry planned on building power plants along the Mississippi and Ohio Rivers. But the specter of the 1811 and 1812 quakes raised concerns, so geologists like Van Arsdale were brought in to try to reconstruct the area’s seismic history.

The geologist treks across a small drainage ditch and onto the fault line. Dragonflies dart everywhere as Van Arsdale points to where, to the north, the fault line hits a levee—the massive engineered earthwork that holds back the annual Mississippi floods. When Van Arsdale and other research groups dug into this fault line decades ago, they found traces of the 1811 and 1812 quakes, as well as older ruptures dating back to the 15th and tenth centuries.

This work helped reveal that the New Madrid Seismic Zone has popped out major earthquakes—of 7.0 magnitude or greater—every 500 years or so for the past five millennia. Traces of older earthquakes may also exist, but scientists have yet to find them. Work by other researchers on sand blows—those geysers of sand that Bryan saw back in 1812—revealed that the 1811–12 pattern of three earthquakes hitting one right after the other wasn’t an aberration. The seismic events of the 15th and tenth centuries also likely involved multiple major quakes, one right after the other.

This research suggested that another major set of quakes was possible—and that if one happened, the area “should expect two more” in quick succession, Van Arsdale says.

A rough span of 500 years between major-earthquake events potentially puts a lot of time before the next big one. But many experts say the region is still largely unprepared for even moderate shaking.

Many parts of Alaska—which experiences large quakes more frequently—put in strict building codes after an earthquake devastated the state in 1964. Those building codes are thought to be why Anchorage survived a 7.0 earthquake in 2018 with only minimal damage and no deaths.

In contrast, many states in the New Madrid region began including earthquake provisions in their building codes only in the early 2000s, meaning anything built prior to that is prone to collapse. In Tennessee, jurisdictions can decide whether to opt in to building codes with earthquake provisions, while other states—like Missouri—don’t have statewide building codes at all, though there are efforts to change that.

That has experts worried. “Human bodies don’t stand up well to falling building parts,” says Chris Cramer, a geophysicist who works on earthquake hazards at the Center for Earthquake Research and Information at the University of Memphis. He estimated that a major earthquake near New Madrid would cost the U.S. an average of $10 billion a year for 100 years, from damaged buildings and lost infrastructure. For an aging Memphis—which is only roughly 40 miles from the southern edge of one fault line in the New Madrid system—even a moderate earthquake could cause considerable damage.

St. Louis is another city near the bull’s-eye. As with other places in Missouri, “we know that a lot of people are living in buildings that are not ready for a big earthquake,” says Briggs, who works for Missouri’s Seismic Safety Commission, a committee of experts appointed by the governor to help the state prepare for earthquakes.

Memphis and St. Louis have started to retrofit bridges in preparation. Newer buildings, especially taller ones, are built with shaking in mind. But “while progress has been made, there’s still a considerable way to go to ensure the resilience of buildings and infrastructure,” Alicia Tate-Nadeau, the director of the Illinois Emergency Management Agency and Office of Homeland Security, wrote in an email to Undark. To deal with this, that state has adopted international building codes with seismic provisions. These won’t become mandatory across Illinois until 2025.

But federal funding for earthquake preparation is also not thick on the ground, says Patrick Sheehan, the director of the Tennessee Emergency Management Agency. He highlighted that in 2024, the Federal Emergency Management Agency will have a little more than $2 million to distribute to states and territories for earthquake education and risk reduction. “That’s a pittance,” Sheehan says. “I think our nation could do a better job of investing in this.”

When contacted for comment, the FEMA press office confirmed that the 2024 fiscal year budgeted more than $2 million for individual state earthquake assistance, to be distributed through grants. But it said that the total amount allocated for the National Earthquake Hazards Reduction Program, or NEHRP—a major government initiative to reduce risk, educate the public, and research earthquake impacts in the U.S.—totals $8.5 million. The press office also added that there are natural-hazard mitigation funds through other programs that states, tribes, and territories can apply for as well.

Much of the funding distributed through the NEHRP goes toward raising awareness of earthquake risk. Arkansas, for example, qualifies for a grant of roughly $70,000, which can go toward increasing “community preparedness and knowledge of the threat of an earthquake,” wrote Hilda Booth, the earthquake program manager at the Arkansas Department of Public Safety, in an email to Undark.

Kentucky, Indiana, and Alabama’s emergency-management agencies did not respond to requests for comment. The Emergency Management Agency of Mississippi did not answer written questions sent by Undark.

In the New Madrid region, at least, “there’s a long way to go,” Briggs says. “I don’t know that we’ll ever get there.”

To work on earthquakes in the central United States is to trade in uncertainty. The region experiences about 200 small earthquakes every year. And still, when it comes to major quakes, “they can’t say that it will happen, because we don’t know that,” says Seth Stein, an earthquake seismologist and emeritus professor at Northwestern University.

Even Van Arsdale can see an earthquake-free future for New Madrid. His theory is that the Reelfoot fault and other rifts in the area were strained and activated only when the Mississippi River Valley eroded the land above them—creating a way for that pressure to be released. In this scenario, New Madrid might be winding down seismically.

But to others, the trend is clear: “It’s not whether they’ll occur, but when and where,” Tuttle says.

Not knowing the physics behind earthquakes in the area has made predicting future events tough. So, while scientists try to untangle the mysteries of the New Madrid Seismic Zone, some states and nonprofits are doing their best to get locals ready for the next earthquake.

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

In Missouri, Briggs and his agency have developed an emergency plan for the next big one. The agency’s headquarters in Jefferson City has an underground facility that they can use to coordinate relief after a disaster. In the meantime, his team heads out to test the structural integrity of schools and other significant buildings in the southeastern corner of Missouri to see whether they can withstand shaking.

Despite this preparation, there appears to be no sense of urgency in the town of New Madrid, which had 2,700 residents as of 2020 and has a largely empty main street.

On the levee, a sign incorrectly announces the location of a fault line. The actual rift runs west of town, says Jeff Grunwald, an administrator at the New Madrid Historical Museum. The 1811 and 1812 quakes are a major draw for the 5,000 or so annual visitors to the museum.

But locals think about earthquakes—and the risk they pose—“very, very, little,” he says. “People have lives to lead.”