July 28, 2006 — The fortunes of a string of cities from Memphis to Quebec City may be written in a maze of Mississippi River mud and sand near New Madrid, Missouri.
Buried in the old river channels are clues to what may someday be the largest natural disaster in U.S. history, say geologists who study the deceptively serene seismic realm that is eastern North America.
Worst U.S. Disaster Ever?
It sounds like hyperbole, but after all it was New Madrid, not San Francisco, where the most powerful quakes in the history of the lower 48 states were centered.
The quakes that put New Madrid on the map — and almost wiped it off — were a cluster of landscape-scarring monsters that hammered the once sparsely populated region over a seven-week period from Dec. 16, 1811 to Feb. 7, 1812.
In the two centuries since, scientists studying the "New Madrid Seismic Zone" have usually reconstructed those quakes — which were up to 8 in magnitude — by relying on historical eyewitness accounts of collapsing chimneys, a backward-flowing Mississippi, and quicksands that swallowed whole islands.
"The New Madrid zone has the highest hazard rating east of the Rockies," says geologist Russ Wheeler of the U.S. Geological Survey in Memphis. "It's head and shoulders above anyplace else."
That’s not just because of what happened in 1811-12. There is now ample evidence of previous big quakes at New Madrid. That evidence includes contorted sediments, once-liquefied sands and, most recently, abruptly straightened river channels.
A Knotty Problem
From high above New Madrid, the Mississippi River is stamped on the land like a giant imprint of shabbily braided twine. It’s a knotty mess. But there is a method to the meandering maze, says geologist John Holbrook of the University of Texas at Arlington.
When the region is undisturbed by earthquakes, the Mississippi River wends its meandering way south. But when the Reelfoot Fault near New Madrid gives way, it can abruptly raise the land to the southwest, changing its slope.
"We think that’s what happened in 1811-12," says Holbrook. "That created a tsunami upstream."
That may explain some eyewitness accounts of the river flowing in the wrong direction. But such an abrupt change also leaves an enduring mark on the river, Holbrook says.
The new, more gradual slope causes the river to abandon its meanders and straighten just above the fault zone. After about a thousand years, the river erodes away from the bulge created by the quake and returns to a sinuous course.
That cycle, says Holbrook, ought to be detectable in river sediments. To find it, he and colleagues have spent years hand-drilling cores of sediment in the New Madrid area to reconstruct past river channels.
In the July issue of Tectonophysics they reported two cycles of straightening apparently caused by ancient, massive quakes.
"At about 2200 B.C. the river straightens out," says Holbrook. The river gradually returned to a meandering course over the next thousand years. "Then at about 900 A.D., the river straightens again and starts all over," he says.
The 900 A.D. event had already been noticed by paleoseismologists looking in the Mississippi Valley for evidence of the sudden shifts of stable sand into quicksand that can occur when it is shaken violently.
Paleoseismologist Martitia Tuttle had seen hints of the earlier event — which she had dated to 2350 A.D. — but not enough to be sure.
"That’s probably the same event," Tuttle says.
The difference is that Holbrook’s work can connect the paleoseismological clues directly to the Reelfoot Fault, the same fault that caused the 1812 disaster.
"There are liquefaction features in the area but we didn’t feel we had enough" to firmly say there was another distinct event or to connect them to the Reelfoot Fault, Tuttle says.
