Scientists Question Preparedness for New Madrid Quake

February 20, 2006
By Lee Bowman
Scripps Howard News Service

ST. LOUIS -- The Mississippi River basin runs right down the middle of the North American tectonic plate, thousands of miles from the nearest fault zone normally associated with earthquakes.

Yet south of here at New Madrid, Mo., during the winter of 1811-12, three of the most massive earthquakes ever to rock the United States threw people from their beds, toppled chimneys, even changed the course of the river. Shock waves from the events made church bells ring as far away as Boston.

Scientists are still trying to understand how the New Madrid seismic zone _ a rough ring around where Missouri, Kentucky, Arkansas and Tennessee meet _ behaves to keep a fairly regular string of small rumbles going deep beneath the surface, and threatens to repeat the magnitude 7 or 8 quakes of the past.

"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 around St. Louis and Memphis," Stanford University geophysicist Mark Zoback said Monday at the annual meeting of the American Association for the Advancement of Science.

He and other speakers discussed the level of risk for a major earthquake and the difficulties of preparing in a region where floods and tornados are more commonly perceived as disaster threats.

"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 participated in some of the first seismic studies that identified the faults beneath the region 30 years ago. Scientists now know that the North American plate twice tried to break apart in the middle as continents drifted and pushed together over the past 700 million years or so. Although the split didn't happen, the stress left scars in Earth's crust 10 to 15 miles deep that still have sections that try to slide alongside each other and move upward.

Zoback and colleagues first proposed five years ago that what's aggravated the faults in more recent geologic time has been a rebound effect on crust that was pushed down by ice-age glaciers that crept as far south as central Illinois.

Although the ice sheet didn't actually cover the area over the faults, it deformed the crust several hundred miles beyond. The ice has been gone for thousands of years, but the pressure it put on the land is still being released and causing small quakes along the fault line. But seismologists say the small quakes aren't eliminating the chance for a sudden big shift that could cause more quakes of magnitude 6 or greater in the region.

The U.S. Geological Survey estimates there is up to a 10 percent chance of repeating the 1811-12 quakes within the next 50 years. And the Federal Emergency Management Agency has proposed that building codes in the heart of the zone be as stringent as those used in California.

But scientists studying the fault zones in recent years have at times offered conflicting estimates of how great the risk really is, creating some confusion about preparedness and liability. For instance, officials estimate that only about 50 percent of Missouri residents living closest to New Madrid have earthquake insurance, while many insurance companies have cited lack of certainty about the risk in deciding to stop offering quake coverage in the region at all.

David Gillespie, a professor of social work at Washington University in St. Louis who specializes in disaster preparedness, said even with the history of New Madrid, it's hard to get most people in the region to take quake dangers seriously.

"Unfortunately, earthquake safety in the Midwest is event driven _ most people will not begin to care about the risk until an earthquake happens," Gillespie said, and cited modeling he's done that shows small towns tend to push tougher building codes aside in favor of economic development.

"Town leaders need to think long-term, 25 or 50 years out, about incremental improvements in safety measures that can be sustained. This is a different kind of planning, but necessary to be ready for the eventual catastrophic quake that will strike," Gillespie said.

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(Contact Lee Bowman at BowmanL(at)shns.com)

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