Unearthing explanations for New Madrid earthquakes

On Dec. 16, 1811, residents of New Madrid, Mo., were wrested from sleep by violent shaking and a deafening roar. A short time later, church bells hundreds of miles away in Boston began to ring. It was the first of three massive earthquakes that rocked the central United States between December 1811 and February 1812, even changing the course of the Mississippi River in their aftermath.

"A big earthquake in the same region as the 1811-1812 earthquakes would have devastating consequences should they recur today because of the population centers in St. Louis and Memphis," Stanford University geophysicist Mark Zoback told an audience Feb. 20 at the annual meeting of the American Association for the Advancement of Science in St. Louis, Mo.

vWe simply need to know more about how these systems work in order to serve the public," added Zoback, the Benjamin M. Page Professor in Earth Sciences.

In a talk titled vTremors in the Heartland: The Puzzle of Mid-Continent Earthquakes," Zoback discussed what is presently known about the New Madrid seismic zone and his work creating geodynamic models of the region. Zoback began his career studying New Madrid. In 1976, shortly after receiving his doctoral degree, he participated in the first seismic work to identify the causative faults. In an article published in the February 2001 issue of Geology, Zoback and former graduate student Balz Grollimund presented a theory explaining why earthquakes occur in this area.

The New Madrid seismic zone, which is roughly at the juncture of Missouri, Kentucky, Arkansas and Tennessee near the Mississippi River, is unusual because most earthquakes occur at the edges of rigid tectonic plates that essentially float on the fluid-like interior of the Earth. The plates produce earthquakes when they move over, under or beside each other. In California, earthquakes occur along the San Andreas Fault because the Pacific plate moves horizontally past the North American plate, like two bumper cars brushing up against each other.

Understanding why earthquakes occur in the New Madrid zone, on the other hand, has proven more elusive. The zone is in the middle of the North American plate, thousands of miles from the edges where all the action usually occurs.

"What makes New Madrid unique are elements of the structure and properties of the Earth's crust and mantle that it inherited over long periods of geologic time," Zoback said. "It's sort of a legacy effect."

He explained that tens of thousands of years ago, the Laurentide ice sheet covered most of Canada and ran as far south as the middle of Illinois. According to Zoback, this massive glacier did not cover the New Madrid zone but was large enough to affect the Earth hundreds of miles to the south-in effect, the ice sheet was so heavy it pressed into the Earth's surface. Think of squeezing a rubber ball with your finger.

As the climate warmed, melting the ice, the ground was freed of the heavy pressure of the ice sheet. It is the constant release of this pressure that causes earthquakes in New Madrid, he explained. Zoback's model predicts that earthquakes could continue to occur in the region for the next few thousand years.

Because a major earthquake could strike the area, Zoback said the science community must help regional officials prepare for such an event.

"What the scientific community must do is continue the fundamental research trying to understand why these earthquakes occur," he said. "At the applied level, scientists need to work with state and local officials to make sure the importance of earthquake hazards are considered in the development of building codes and critical structures such as bridges, schools and hospitals."

Zoback also cautioned that local communities must understand that seismic events, such as those in 1811 and 1812, aren't simply history but are warnings of the potential for future earthquakes.

"It's one thing to know it was a part of your past," he said. "It's another to be prepared for it to be part of your future."

John B. Stafford is a science-writing intern at Stanford News Service.

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COMMENT:
Mark D. Zoback, Department of Geophysics: (650) 725-9295, zoback@pangea.stanford.edu

EDITORS NOTE:
Professor Mark Zoback will participate in a AAAS press conference, ''The 'Big One' Could Be in Memphis,'' at 9 a.m. CST Monday, Feb. 20, in Room 274 of the America's Center, 701 Convention Plaza, St. Louis, Mo. He also will participate in the AAAS symposium, ''Tremors in the Heartland: The Puzzle of Mid-Continent Earthquakes,'' at 2 p.m. CST, Feb. 20, in the America's Center. For more information, visit the web at http://www.aaas.org/meetings.

RELEVANT WEB URLS:

MARK ZOBACK WEBSITE http://pangea.stanford.edu/~zoback

USGS NEW MADRID WEBSITE http://neic.usgs.gov/neis/eq_depot/usa/1811-1812.html

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