The first earthquake struck while the town was still asleep. Around 2:00 am on Dec. 16, 1811, New Madrid — a small frontier settlement of 400 people on land now located in Missouri — was 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. Thirty-one-year-old resident Eliza Bryan 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.

In the end, the town had dropped at least 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 higher earthquake in the next 50 years, and as much as a 10 percent chance of a repeat of the 1811-1812 sequence. While the risk is much lower compared to, say, California, experts say that when it comes to earthquake resistance, the New Madrid 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 — including two major cities, Memphis and St. Louis. Mississippi, Alabama, and Indiana have also been noted as places of concern.

New Madrid residents 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.

In response to the potential for calamity, geologists have learned a lot about this odd earthquake hotspot over the last 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 New Madrid experiences earthquakes, scientists are still struggling to answer some of the most basic questions, like 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, said Jeff Briggs, earthquake program manager for the Missouri State Emergency Management Agency.

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

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,” said Martitia Tuttle, a paleoseismologist at M. Tuttle and Associates, an earthquake risk consulting company. Earthquakes occur when subterranean strain is building over 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 like New York are far from a plate boundary today doesn’t mean that was always the case — and they 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 around 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, while neighboring ones aren’t, is less clear. “It’s really one of the most enigmatic seismic zones on the planet,” said 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, explained Eunseo Choi, a computational geodynamicist at the Center for Earthquake Research and Information at the University of Memphis.

“It’s really one of the most enigmatic seismic zones on the planet.”

That matters because measuring stress buildup is a great way to predict earthquake hazards. The tell-tale ripping of earth along the surface — a sign of stress buildup — is easy to see in places like California, where scientists can track the movement of the earth thanks to key geological features in that area. 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. 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, said Choi.

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

On a cloudy day in May, 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 escape the annual spring floods. In New Madrid, that meant building where earthquakes have 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, “they’d be in trouble,” Van Arsdale said.

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 and reconstruct the seismic history of the area.

The geologist treks across a small drainage ditch and onto the fault line. Dragonflies dart everywhere as Van Arsdale points to where the fault line hits a levee to the north — the massive, engineered earthworks that hold back the annual Mississippi floods. When Van Arsdale and other research groups dug down 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 10th centuries.

This work helped reveal that the New Madrid Seismic Zone has popped out major 7.0 magnitude of greater earthquakes every 500 years or so for the last five millennia. Traces of older earthquakes could also exist, but scientists have yet to find them. Work by other researchers on sand blows — those geysers of sand Bryan saw back in 1812 — revealed that the 1811-1812 pattern of three earthquakes hitting one right after the other wasn’t an aberration. The 15th and 10th century earthquakes 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, said Van Arsdale.

Five hundred years between major earthquakes 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.

The New Madrid Seismic Zone has popped out major 7.0 magnitude of greater earthquakes every 500 years or so for the last five millennia.

In contrast, many states in the New Madrid region only included earthquake provisions in their building codes 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 state-wide 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,” said Chris Cramer, a geophysicist who works on earthquake hazard 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 around 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 bullseye. Like other places in Missouri, “we know that a lot of people are living in buildings that are not ready for a big earthquake,” said 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, director of the Illinois Emergency Management Agency and Office of Homeland Security, wrote in an email to Undark. To deal with this, the state has adopted international building codes with seismic provisions. These won’t become mandatory across the state until 2025.

But federal funding for earthquake preparation is also not thick on the ground, said Patrick Sheehan, director of the Tennessee Emergency Management Agency. He highlighted that in 2024, the Federal Emergency Management Agency will have a little over $ 2 million to distribute to states and territories for earthquake education and risk reduction. “That’s a pittance,” said Sheehan. “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.

“Human bodies don’t stand up well to falling building parts.”

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 towards increasing “community preparedness and knowledge of the threat of an earthquake,” wrote Hilda Booth, 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,” said Briggs. “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, ” said 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,” said Tuttle.

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,” said Brian Blake, 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 the 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.

“It’s not whether they’ll occur, but when and where.”

Despite this preparation, there appears to be no sense of urgency in the town of New Madrid, which now boasts some 2,700 residents and 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, said Jeff Grunwald, 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 said. “People have lives to lead.”

This story was supported in part by the Council for the Advancement of Science Writing and The Brinson Foundation.

Freda Kreier is a freelance journalist based out of Washington, D.C. Her work focuses on science and the natural world.

This article was originally published on Undark. Read the original article.