Midwest Faces Quake Danger From Shifting Fault, Experts Say
for National Geographic News
|May 10, 2007|
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.
"That pulls the surface down a bit," said study co-author Steve Grand of the University of Texas. "These earthquakes are due to the bending stress."
Forte and Grand's research appeared in a recent issue of the journal Geophysical Research Letters.
Not everyone is so sure about the findings.
"It's an interesting idea and should be one of the things we include in the possible explanations for New Madrid, but I don't think it's the answer," said Robert Smalley, a seismologist at the Center for Earthquake Research and Information at the University of Memphis in Tennessee.
One problem, he said, is the same forces have been active for millions of years, but big earthquakes have probably only been going on for a few thousand years at most. Otherwise, continued activity would have built a giant mountain range, and no such range exists.
Seth Stein, a geophysicist at Northwestern University, was not part of the study.
He noted the mid-continent is actually a patchwork of old seismic zones. Earthquake activity appears to migrate around among them, with one zone becoming active for a while.
The new theory is a "very plausible" explanation of how stress might originate, Stein said. But it doesn't explain why earthquakes have been occurring at New Madrid and not somewhere else.
"This is not arguing against the large-scale forces being due to mantle dynamics," he added.
The Next Big One
The billion-dollar question, of course, is when the next big earthquake might occur.
(Related: Big Earthquake Could Devastate U.S. Midwest, Experts Warn" [April 12, 2006].)
All scientists know for certain is the fault zone has produced five major earthquakes in the past 1,500 years.
Three occurred in rapid succession in the winter of 1811-1812. The others were scattered at intervals of about 500 years.
To put that in context, there's about a ten percent chance a building built today might have to survive a 7.0-magnitude earthquake in the next 50 years, said Gary Patterson of the University of Memphis center, who was not involved in the study.
Grand pointed out his and Forte's new theory does nothing to improve the ability to forecast the next earthquake: It suggests tectonic stresses might not be limited to the New Madrid area. There are also high stress areas about 100 miles (160 kilometers) to the south and 100 miles to the north.
"Our work may give a reason for earthquakes over a broader region than just the New Madrid fault," Grand said.
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