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.
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