for National Geographic News
New "CAT scans" of the deep Earth may help figure out why one of the biggest earthquakes in American history occurred in the Mississippi Valley heartland, far removed from seismic centers, researchers report.
The scans show an ancient piece of the Earth's crust is descending into the deep mantle beneath central North America, pulling downward on the overlying crust to create earthquake-producing cracks.
The massive earthquake, estimated at a magnitude between 7.4 to 8.0, struck New Madrid, Missouri, on February 7, 1812. The area was lightly populated at the time, but today the affected cities would include St. Louis, Missouri; Little Rock, Arkansas; and Memphis and Nashville, Tennessee.
Since 1812, there have been thousands of small earthquakes in the New Madrid fault zone. But geologists have long been baffled about why they have been occurring in the center of a continental plate.
Normally earthquakes happen at the margins of continents, where plates of the Earth's crust are colliding, such as in California and regions close to the Pacific Ocean. (See a map of the region.)
CAT Scan of the Deep Earth
A team of U.S. and Canadian scientists peered into the Earth's interior with a technique similar to a medical CAT scan. The technique, called tomography, studies the speed with which seismic waves pass through the deep Earth.
This allows seismologists to construct three-dimensional maps of zones through which the waves travel, helping to decipher what the interior rock looks like.
Researchers also studied the rate of continental drift, minute variations in surface gravity, and variations in the thickness of the Earth's crust to determine as much as possible about the slow motions of the underlying rock.
The study team, led by Alessandro Forte of the University of Quebec in Montreal, concluded a plume of cold, dense rock is slowly sinking beneath the New Madrid area.
The downward movement is caused by the collision of two extremely slow currents of material in the Earth's mantle, several hundred miles beneath the surface of the Mississippi Valley. (How does an earthquake happen?)
One current originated tens of millions of years ago, beneath what is now California, and flowed eastward. The other came from the eastern U.S.
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