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.