Matt Davis, and Caleb Johnson hunt for ducks on Big Lake, west of Blytheville, Ark. The New Madrid earthquakes created the shallow lake and its adjoining swamp. (Alan Spearman/THE COMMERCIAL APPEAL)

As mighty forests cracked and shattered and the Mississippi River flowed backward, the settlers of New Madrid, Mo., thought it was doomsday. Their accounts are frightening to read: the landscape undulating like a shaken carpet, rising and falling in waves 15 feet high. Widespread flooding as the earth broke. Fissures shooting “vast volumes of water and sand . . . as high as the tops of the trees.”

With the 200th anniversary of the New Madrid earthquakes this winter, scientists and emergency planners are asking: Could all this happen again?

The New Madrid temblors knocked chimneys off log cabins in the wilderness and drowned passengers on riverboats. Today they might crumble buildings from St. Louis to Memphis, rupture natural gas pipelines, wrench bridges into the Big Muddy, traumatize an estimated 12 million people.

The Federal Emergency Management Agency warns of a possible $300 billion in economic losses across eight states, should a magnitude 7.7 earthquake strike near New Madrid. (After the 1812 temblor, Congress doled out $50,000 to settlers, the first disaster relief in American history.)

How strong was it?

The New Madrid quakes started nine days before Christmas in 1811 and culminated in a massive shock on Feb. 7, 1812, which some experts believe was one of the largest quakes ever to strike the center of a continent.

Mid-continent events, far from plate boundaries such as the so-called Ring of Fire along which California trembles, are poorly understood. They can be just as destructive: The 1556 disaster in Shaanxi, China, had the highest death toll of any quake in history, an estimated 830,000. They happen along a crazy quilt of faults and hence are unpredictable. And they can be widely separated in time: Last August’s magnitude 5.8 quake at Mineral, Va., which damaged the Washington Monument and other buildings in the District, was the biggest in that seismic zone since 1875.

No one knows exactly what caused the New Madrid region — the name is pronounced “New MAH-drid” — to strain and rupture. And because there were no instruments recording the event, scientists disagree about how powerful the convulsions were. Recent estimates range from below 7 up to nearly 8.

We know that New Madrid jolted sleepers awake on the East Coast. President James Madison wrote Thomas Jefferson that the Feb. 7 tremor hit a few minutes after 4 a.m. at the White House, 700 miles from the epicenter, and “lasted several minutes.” To some scientists, this suggests a magnitude closer to 8 than to 7.

To get a better sense of what happened at New Madrid and whether a similar shock is likely to recur, paleoseismologist Martitia Tuttle looks for the fingerprints an earthquake leaves on the landscape. The Mississippi’s muddy floodplain, subject to erosion and soil movement, obscures faults at the surface. But there is another kind of evidence: patches of sand that in many places mottle the wide fields like a rash. Scientists call them sand blows.

During the quakes of 1811-12, pressurized water and sand spurted up from underground and left deposits at the surface. Tuttle and her scientific partners are adept at finding these old sand blows on riverbanks — she has paddled or motored along 895 miles of snake-infested backwaters — or from the air.

She slices into sand blows with shovel and backhoe, exposing the clogged fissures through which the sand once gushed upward. These vents can be three to 16 feet wide, among the largest ever found on the planet.

On Pemiscot Bayou in Arkansas, 60 miles south of New Madrid, Tuttle has found sand blows that buried huge trees. Broken trunks still stand, tilted and well preserved, amid the fine sediment that entombed them as the surrounding land slumped. Such slumping formed entire lakes in several places, including today’s Reelfoot Lake in Tennessee and Big Lake, Ark.

The size of the sand blows indicates especially powerful earthquakes in 1811-12, Tuttle says. “I have seen the geological record of these events, and I can attest that the ground was basically ripped apart.”

Further evidence for the earthquakes’ ferocity, Tuttle notes, is that sand blows can be found up and down the Mississippi River Valley from Illinois to Arkansas. Sand blows across a similarly vast area appeared during the 2001 earthquake in Bhuj, India, which killed 20,000 and was recorded at magnitude 7.7. Like New Madrid, this temblor happened mid-continent in a locale not severely shaken since 1819.

More to come?

As she works with archaeologists in probing sand blows, Tuttle finds arrowheads and even whole Indian mounds on top of some of them. Radiocarbon dating of buried leaves and twigs confirms that these blows are much older than 200 years, suggesting that New Madrid-size earthquakes have happened in this region repeatedly. In fact, Tuttle and her associates have documented that the valley rocked to monster quakes around the years 300, 900 and 1450 as well as 2350 B.C. The 1811-12 event was one in a series, which could point ominously to the future.

One of Tuttle’s colleagues, Haydar Al-Shukri at the University of Arkansas at Little Rock, had a hunch that similar, prehistoric earthquakes might have struck 50 miles south of the New Madrid seismic zone, in an area long thought to be geologically quiet. In the cotton fields of the Arkansas Delta he has recently found huge patches of gleaming white sand, cursed by farmers but invaluable as scientific evidence.

The largest of these covers 29 acres, so sprawling that Al-Shukri nicknamed it Daytona Beach.

Using a new dating technique called optically stimulated luminescence, Al-Shukri can tell how long it has been since samples of soil have been touched by sunlight. Daytona Beach formed about 3500 B.C., presumably the result of a magnitude 7.0 quake (about the strength of Haiti’s 2010 disaster) or stronger. His findings prove that such events can happen in places well outside the known seismic zones and that periods of violent activity may be followed by thousands of years of torpor.

This poses a conundrum for emergency management experts: How to plan for earthquakes if seismic activity wanders around the map?

To add to the confusion, Northwestern University geophysicist Seth Stein argues that there won’t be any future earthquakes at New Madrid. He has gathered GPS data that show no pressure developing in the Earth’s surface there; hence, he argues, no energy is building for a convulsion. He believes mid-continent seismicity moves around, so that as the New Madrid zone “turns off,” future quakes may happen elsewhere.

Others, though, say Stein puts too much weight on GPS data. Strain may be building deep inside the Earth, where GPS cannot peer. And GPS data have been gathered for only a few years. The U.S. Geological Survey continues to warn of the potential for “a major destructive earthquake” from Memphis north to southern Illinois, relying in large measure on the sand-blow evidence.

“Based on this history of past earthquakes, the USGS estimates the chance of having an earthquake similar to one of the 1811–12 sequence in the next 50 years is about 7 to 10 percent, and the chance of having a magnitude 6 or larger earthquake in 50 years is 25 to 40 percent,” a fact sheet says.

Most scientists seem to agree. “My goal is not to scare people,” Al-Shukri says. “I’m not saying it will come tomorrow. But it might come.”

Maynard has taught environmental history at Johns Hopkins and Princeton universities and written five books on American history and culture.