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SCOTT SIMON, host: Hurricane Irene has come ashore in North Carolina, as the storm makes its way up the Atlantic Coast, and hundreds of thousands of people who live in the hurricane's path have been evacuated, from North Carolina to New York State. Now we will be following developments throughout the morning. Unlike this hurricane, the 5.8 earthquake that rattled the East Coast this week took everyone, even, especially geologists, by surprise. And even when there are reasons to think that an earthquake could be just around the corner, scientists still have trouble making good predictions.

Two hundred years have passed since the last major earthquakes rocked the New Madrid Seismic Zone - that's a fault system that runs down to the central U.S. through parts of Illinois, Missouri, Kentucky, Arkansas and Tennessee. That quake was a magnitude 7.8. The region has had plenty of smaller quakes since then, but as St. Louis Public Radio's Veronique LaCapra reports, there is no clear answer to the question: when's the next big one coming?

VERONIQUE LACAPRA: The New Madrid earthquakes of the winter of 1811 to 1812 have become the stuff of legend. They were so powerful, the story goes, they made the Mississippi River run backwards.

SUSAN HOUGH: It was what we call a thrust fault. And it came up to the surface beneath the river and actually created a stair step in the river bottom, to where it set up waves that went coursing back upstream.

LACAPRA: That's Susan Hough of the U.S. Geological Survey in Pasadena. She says much of what we know about those early quakes comes from first-hand accounts, like this one from future president Zachary Taylor. He felt the shaking two-hundred-thirty miles away in Louisville.

UNIDENTIFIED ACTOR: (Reading) The sight was truly awful - houses cracking, chimney falling, men women and children running in every direction in their shirts for safety and a friend of mine was so much alarmed as to jump of a window and was very much hurt.

HOUGH: Hough's calculations put the largest New Madrid earthquake at about 7.0, the same magnitude as last year's earthquake in Haiti. The official USGS estimate is 7.7. The Federal Emergency Management Agency takes those numbers seriously, which is why this past May the agency helped lead an earthquake drill involving eight central U.S. states.

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LACAPRA: At the Springfield Airport in Missouri, C-130 military airplanes revved up their engines, preparing to evacuate the injured. FEMA regional administrator Beth Freeman says the exercise was designed to test how the region would respond to a natural disaster on the scale of the original New Madrid quakes.

BETH FREEMAN: You know, back then there weren't very many people that lived in the New Madrid area, and today we have quite thriving populations of Memphis, St. Louis, and all those other communities in between. So if those earthquakes would reoccur, it could be quite devastating.

LACAPRA: The problem is, scientists can't predict when, where or even if another earthquake will happen. Seth Stein, for one, doesn't expect another big quake at New Madrid anytime soon. Stein is a geophysicist at Northwestern University. He and others have been using GPS technology to measure how the ground moves or deforms along active faults.

SETH STEIN: Now, normally the way earthquakes work is that you store up energy, the ground deforms before a big earthquake, kind of like stretching a spring, and then it snaps, and you have an earthquake.

LACAPRA: Stein says that warping of the ground has been measured in California, Alaska, Oregon, and Washington - along every fault where we think a big earthquake is on the way. So that's what he and his colleagues expected to see when they started taking those same kinds of GPS measurements at New Madrid.

STEIN: And to our complete surprise, we see absolutely no motion of the ground.

LACAPRA: Stein says part of the explanation could be that seismic zones in the middle of a continent seem to behave differently from those in places like California, where the huge plates that make up the earth's surface thrust up against each other.

STEIN: Faults in the middle of the continents will be active for short periods of time geologically, maybe a few thousand years, and then they'll turn off and be inactive for times, and then start up again. So it looks like we maybe seeing the end of one of those cycles.

LACAPRA: But many other geologists don't agree. Robert Williams of the USGS in Colorado says you can't ignore the past. The earthquakes 200 years ago liquefied the soil underground, blasting jets of wet sand out onto the surface.

ROBERT WILLIAMS: There's been some great science done, with geologists digging into these 1811, 1812 sand blows, and then lo and behold discovering evidence for older sand blows caused by earthquakes of about the same magnitude as the 1811, 1812 sequence.

LACAPRA: They realized that before 1811, there had been quakes in about 1450, and again before that, in 900. Williams says that pattern of very large earthquakes means another big one could be on the way.

WILLIAMS: We can't predict earthquakes. So the geologic record is really the strongest piece of evidence we have to remain concerned about earthquakes there in the New Madrid region.

LACAPRA: So is there a risk, or isn't there?

JOHN VIDALE: It's just difficult to be confident in places like the central U.S.

LACAPRA: This spring, University of Washington seismologist John Vidale led an independent panel of experts who took yet another stab at evaluating the hazard at New Madrid. Yes, Vidale says, there are differences in the way fault zones in the middle of a continent behave, but

VIDALE: You know, given all the uncertainty it's prudent to think that what has happened in the past will continue.

LACAPRA: And maybe next time, we'll be ready. For NPR news, I'm Veronique LaCapra in St. Louis.

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SIMON: You're listening to WEEKEND EDITION from NPR News.

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