Although the New Madrid zone is in the middle of the North American plate, it is affected by the tectonic activity occurring at its boundary.
"We know now that the forces associated with plate tectonics are actually transmitted through the plate and into the interior," Zoback said.
"So ultimately the force that caused the earthquakes is related to the force that caused plate tectonics."
Strong quakes in the New Madrid zone are rare, but their frequency seems to have increased in the recent geologic past, Zoback says.
The long-term record shows that large earthquakes have occurred infrequently in the area over the last 65 million yearsperhaps once every million years.
But the short-term record shows that there have been at least two, maybe four, magnitude 7 events in the last 2,000 years.
The U.S. Geological Survey uses devices called seismographs to determine how much energy an earthquake releases, or its magnitude. A magnitude 7 or greater quake is considered a major event.
"The short-term record is saying big earthquakes are occurring quite frequently," Zoback said.
"Were searching for something that could trigger these earthquakes at a rate that is a thousand times faster than the long-term rate."
That search led Zoback to consider the disappearance of the ice sheet that once covered Canada and ran as far south as the middle of Illinois.
The ice sheet began retreating 18,000 years ago, and was completely gone about 10,000 years later.
While the ice never covered the New Madrid zone, the sheet was large enough to affect the Earth hundreds of miles to the south.
Removal of the ice freed the ground below from heavy pressure. Zoback thinks the continuous release of this pressure may be causing the more frequent earthquakes.
"Basically strain energy has been building up in the region, possibly for millions of years," Zoback said. "The removal of the ice sheet was the trigger [for it to be released]."
"The [pressure release] causes the plates to flex, like ripples on a trampoline," and that's when quakes occur.
On December 16, 1811, a powerful earthquake jolted the 400 residents of the town of New Madrid, Missouri. The intense tremor set church bells ringing in Boston, Massachusetts1,000 miles (1,600 kilometers) away.
It was the first of three massive quakes that rocked the central Mississippi Valley that winter.
Zoback thinks the best estimate for the first quake, in 1811, is about a magnitude 7.5. Other estimates have put it closer to a magnitude 8.
Earthquakes in central or eastern U.S. affect much larger areas than quakes in the western part of the country, Zoback says.
The 1811-1812 quakes were felt over more than 2 million square miles (5.2 million square kilometers), with chimneys falling in Cincinnati, Ohio, and sidewalks buckling in Baltimore, Maryland.
Miles of banks along the Mississippi River are reported to have caved in as a result of the events, and at certain points the land tilted so dramatically that the river ran backwards.
Five small towns, including Big Prairie, Arkansas, were destroyed.
Much of the area was undeveloped, however, so damage was relatively light considering the force of the earthquakes.
The damage caused by a similar event today would be far greater, experts warn. A shock as powerful as the one that struck in 1811 could devastate cities like Memphis, Tennessee, and St. Louis, Missouri, were it to hit today.
"There is still an abundance of unreinforced masonry buildings," said David Gillespie, a professor of social work at Washington University in St. Louis.
"Many of the bridges and overpasses are aging and need repair. Few of the bridges have been retrofitted for earthquake safety.
"There is no overarching communication system, so it's likely that key responders will not be able to communicate easily," he added.
Awareness of the earthquake hazard among Midwesterners has been promoted since the mid-1980s, but it still remains relatively low.
"California has been working on [earthquake] preparedness for over 70 years and still has a long way to go," Gillespie said. "The Midwestern states are just getting started."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES