The magnitude-5.8 earthquake that rattled the eastern U.S. on Tuesday took everyone — even geologists — by surprise. But even when there are reasons to think an earthquake could be around the corner, scientists still can't make good predictions.
It has now been 200 years since the last major earthquakes rocked the New Madrid Seismic Zone — a fault system that runs down the central U.S. through parts of Illinois, Missouri, Kentucky, Arkansas and Tennessee. The region has had plenty of smaller quakes since then, but there's no clear answer to the question of when the next big one is coming.
The New Madrid earthquakes of the winter of 1811 to 1812 were a series of three major quakes — the strongest of which was a magnitude 7.7 — and have become the stuff of legend. They were so powerful, the story goes, they made the Mississippi River run backward.
"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," says Susan Hough of the U.S. Geological Survey in Pasadena, Calif. She says much of what we know about those early quakes comes from firsthand accounts, like one from future president Zachary Taylor. He felt the shaking 230 miles away in Louisville.
"The sight was truly awful: houses cracking, chimney[s] falling, men, women and children running in all directions in their shirts for safety, and a friend of mine was so much alarmed as to jump off a window and was very much hurt," he wrote.
Hough says scientists have used eyewitness accounts like Taylor's to calculate how big those earthquakes really were.
"Basically you compare what happened in a historical earthquake with the effects of a more recent earthquake that we have a magnitude for," she says. "And so just by that process of comparison, which gets fairly mathematical, you determine a magnitude."
Volcanoes Of Sand
That magnitude is just an estimate because there were no seismometers around 200 years ago to measure the shaking. Hough's calculations put the largest New Madrid earthquake at about magnitude 7.0 — the same magnitude as last year's earthquake in Haiti.
Robert Williams, a geophysicist with the U.S. Geological Survey in Colorado, says the official USGS estimate is 7.7. "Those quakes were big," he says.
They were so big, Williams says, you can see the changes they made to the landscape. The seismic waves caused the earth to heave up and cave in, creating new swamps and lakes. In many places, Williams says, the strong shaking liquefied the soil underground, and people saw jets of wet sand blast out onto the surface, like volcanoes.
"The sand blows were apparently quite spectacular," he says. Some people were "describing the ejection of sand being perhaps 10 to 20 feet high."
The earthquakes created thousands of sand blows. From an airplane, they look like pale, oddly shaped splotches, some of them 50 yards wide, in the fields around New Madrid.
They've turned out to be time capsules of the region's seismic past.
A Pattern Of Major Quakes
"Over the past 20 years or so, 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," Williams says.
Buried in the sand were fragments of burnt wood and Native American pottery. By dating them, scientists could date the sand blows — and the earthquakes that created them. 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. And it might not take 500 years.
"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," he says.
But other scientists insist there's nothing to worry about. Seismologist Seth Stein, for one, doesn't expect another big earthquake at New Madrid anytime soon. In fact, just the opposite: "It looks like we're seeing the fault system shutting itself down for a while."
Stein, a geophysicist at Northwestern University, says just looking back at the past can be misleading. To get a better idea of whether another quake is looming, he says, scientists need to look at what's happening in the present. He and others have been using GPS technology to measure how the ground moves — or deforms — along active faults.
"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," Stein says.
He says that warping of the ground has been measured in California, Alaska, Oregon and Washington — along every fault where experts 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.
"And to our complete surprise, we see absolutely no motion of the ground," he says.
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 Earth's surface thrust up against each other.
"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," he says. "So it looks like we may be seeing the end of one of those cycles. The bottom line is we don't expect anything for hundreds of years at a minimum, and quite possibly thousands of years, in terms of a very large earthquake like happened 200 years ago."
Risk Or No Risk?
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. Their conclusion? The New Madrid Seismic Zone does still pose a significant risk. Vidale says there are differences in the way fault zones in the middle of a continent behave, but scientists don't understand enough about them yet to be sure there's no danger.
"It's just difficult to be confident in places like the central U.S.," Vidale says. "Given all the uncertainty, it's prudent to think that what happened in the past will continue."
The Federal Emergency Management Agency agrees. That's why this past May, the agency helped lead an earthquake drill involving eight central U.S. states.
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.
"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," Freeman says. "So if those earthquakes would reoccur, it could be quite devastating."
Freeman says wherever and whenever that next big one happens, it's better to be prepared.
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