Big Idea: A Shock to the Heartland

This week's earthquake in Virginia calls to mind another part of the country that faces a surprising level of earthquake risk: the New Madrid Seismic Zone, near the middle of the Mississippi River.

By Amy Barth|Thursday, August 25, 2011
nmseismic
nmseismic
A map of the New Madrid Seismic Zone shows quakes greater than magnitude 2.5. Red circles represent quakes since 1973; blue shows those recorded earlier. Yellow denotes towns with populations greater than 10,000.
Courtesy U.S. Geological Survey

The magnitude 5.8 quake that struck central Virginia yesterday was felt from Florida, to Maine to Missouri. “This is probably the most widely felt quake in American history, even though it was less than a 6.0,” says Michael Blanpied, a USGS seismologist DISCOVER contacted today.

The reason for this intensity is that the East Coast, like the controversial New Madrid Seismic Zone in the central U.S., is located in amidst old faults and cold rocks in the middle of the North American tectonic plate. This is very different than far more common quakes plaguing coastal zones like California, caused by the constantly shifting outskirts of the continent’s plate. “Earthquake hazard is particularly high in the eastern and central U.S. because seismic waves travel so efficiently through the old, cold rocks in the middle of the plate, and that shaking carries very far,” says Blanpied. “Earthquakes in these zones are infrequent, but when they do occur, they shake such a large area, and so many people, it raises their importance.”

We would do well to take a hint from Tuesday's expansive shake-up. It's lucky that it struck in rural America. But a similar tremblor in the crowded cities of the central U.S. is a matter of when, not if. And the region is woefully unprepared to mitigate the damage.

The disastrous winter of 1811–12 is the stuff of legend in the Midwest. In the span of a few months, three major earthquakes rocked Missouri, Tennessee, and Arkansas, violently shaking 230,000 square miles stretching from St. Louis to Memphis. Witnesses claimed that the ground rolled in waves several feet high and the Mississippi River flowed backward. Some reports described buckling sidewalks in Charleston, South Carolina, and tremors that reached as far as Quebec. Had seismographs been available at the time, scientists believe those tremors would have registered magnitudes at least as great as the 7.0 quake that devastated Haiti in 2010 and possibly as high as 8.0. These would place them among the worst in U.S. history.

Two centuries later, the set of faults responsible for the tumult in the Midwest, known collectively as the New Madrid Seismic Zone, continue to rumble—only now they do so beneath millions of homes and some of the biggest ports along the Mississippi. In April an independent panel of geologists, seismologists, and engineers commissioned by an advisory group to the United States Geological Survey (USGS) published a report [pdf] disputing earlier claims that the seismic strain in the area had dissipated, concluding instead that the New Madrid Seismic Zone is “at significant risk for damaging earthquakes.” According to the USGS, the chances of a quake of magnitude 6 or higher within the next half-century are between 25 and 40 percent. “That must be accounted for in urban planning and development,” the panelists wrote.

That directive has gone ignored. In an area that does not fit the proto­type of a seismic hotbed, efforts to implement meaningful policy changes have stalled, leaving the area vulnerable to tremendous damage. “There are no dedicated programs to strengthen facilities or infrastructure in the Midwest in order to resist New Madrid–type earthquakes,” says Amr Elnashai, a structural engineer at the University of Illinois. “Politicians are worried about floods, hurricanes, and tornadoes—things that happen frequently.” Earthquakes are different, he notes. “They are low probability and high consequence, and politicians only hope they don’t happen on their watch.”

Assessing the risk of any seismic zone is difficult, but in New Madrid it is particularly challenging. Most earthquakes occur along the edges of continental plates, often near a coastline, where plates scrape and collide. In those regions, such as California, the faults that can result from that rock movement usually lie close to the surface, making them relatively easy to study. The New Madrid Seismic Zone, in contrast, sits in the center of the North American Plate, and most of the fault system lies four to nine miles beneath the surface. When ancient geologic forces failed to rip the continent apart, the pressure left behind deep rifts that can shift and shove against each other and trigger earthquakes. In 2009 a Purdue University geophysicist grabbed headlines when he published a paper suggesting that the seismic pressure in those rocks had dissipated to the point that the fault system could no longer produce intense tremors. The recent USGS report overwhelmingly rejected that notion, and most scientists agree that powerful quakes are still possible—though how large and when, exactly, are subject to debate. “The less we know, the more we have to guard against,” says USGS seismologist Michael Blanpied, who advised on the April panel report.

Embracing a similar philosophy, the Federal Emergency Management Agency (FEMA) commissioned a study in 2006 to estimate the cost and casualties of a hypothetical magnitude 7.7 quake in the Mississippi Valley. That job went to Elnashai, who began by creating a detailed database pinpointing the location of 603,756 structures, including all the hospitals, bridges, schools, and fire stations in the region. He attached corresponding information about the type of rock and soil beneath each structure, details of the structure itself, the amount 
of shaking anticipated in the area, and the expected population nearby. Elnashai then plugged all the data into a computer model designed to simulate a 7.7 quake—a magnitude approved by the USGS—and let it rip. The results were sobering: Some 715,000 buildings would be seriously damaged, 425,000 pipelines would be broken, and some 15 major bridges would fail. By Elnashai’s calculation, such a quake would injure 86,000 people, displace 7.2 million, and force 2 million into temporary shelters. The direct economic losses would total nearly $300 billion. “Everybody was surprised by the numbers,” he says.

Elnashai never got to take his study further. In 2009 the project was cut short when a new FEMA administration took over and reshuffled the agency’s funding priorities. The decision scuttled his plans to highlight those structures most in need of shoring up to help officials prioritize their investments in seismic retrofits. “The bottom line is that there is increased awareness, and not much else,” Elnashai says. “Our task was to run the numbers and simulate the effects, and that’s what we’ve been doing. The results are haunting, but they are just pushed aside.”

As the 200th anniversary of the New Madrid quakes approaches, the local population is arguably no better prepared than it was in 1811, when Missouri and Arkansas were not yet even states. “An earthquake in New Madrid is a matter of when, not if,” Elnashai says. “I’m a resident of this area, and I’m not too happy with what is happening.”

Not Your Average 
Earthquake Zones
Earthquakes are not restricted to tectonic plate boundaries. New Madrid is just one of several regions that have been struck by major tremors despite a seemingly safe location at the center of a plate.

St. Lawrence 
Valley, Quebec  The North American plate builds up tremendous pressure as it runs into neighboring plates. It releases most of that strain along its edges, but every now and then it de-stresses through ancient fault lines at its center. Southeastern Canada contains one such fault system and has suffered quakes as strong as magnitude 7.0 as a result.

Charleston, South Carolina  Deep beneath the South Carolina coastline lie fault lines dating back 150 million years. The area experiences about 20 small quakes a year, plus the occasional monster, such as the magnitude 7.3 shake in 1886 that killed 60 people.

Bhuj, India  Most seismic activity in India occurs near the Himalayas, where two plates are slamming 
into each other. But in 
2001, a huge burst of pressure was released through Jurassic-era fault lines near the center of the Indian plate, triggering a magnitude 7.7 quake that took 20,000 lives.

—Gillian Conahan

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