This article first appeared in the St. Louis Beacon, April 26, 2011 - In the predawn hours of April 18, 2008, Kathy Chamberlin received an unpleasant awakening.
"I thought the house was falling down all around me," recalled the 27-year-old Maplewood resident.
The shaking abated quickly enough but Chamberlin and her family were left to turn on the television and hear about the event that had gotten them out of bed at half past four in the morning. She had known since grade school that St. Louis was close to the New Madrid Fault, but this was her first earthquake.
She thinks of the experience regularly.
"It's been in the back of my mind," she said. "After what happened in Japan, it's been more toward the front."
'We Don't Really Understand'
The quake Chamberlin recalls from three years ago, a 5.4 event centered about 20 miles from the town of Olney, Ill., wasn't a New Madrid temblor at all. It originated in the nearby Wabash Valley Seismic Zone, which lies near the Illinois and Indiana border, a good distance from its more infamous cousin. There's also a third quake-causing region in the neighborhood, the Eastern Tennessee Seismic Zone.
Though many St. Louisans are familiar with the New Madrid, they often aren't as well acquainted with the realities of earthquake preparedness as others living in seismically active areas. One reason may be that Missouri sees far fewer significant quakes. But another may be the fact that it seems oddly counterintuitive that the Midwest should be experiencing such events in the first place. It is, after all, smack dab in the middle of North America, hundreds of miles from any tectonic plate boundaries. Why would southern Missouri even have a fault?
The answer is simple enough, scientists say.
No one really knows.
"There are theories about the driving forces," said Robert Williams, a geophysicist at the U.S. Geological Survey in Colorado. "One of them is that remnants of old plates are in the upper mantle or lower crust that are pieces of material left over from a much earlier time millions of years ago. One piece is thought to be sitting deep under New Madrid and exerting forces locally."
Another idea holds that the crust is suffering a "rebound" effect from the retreat of glaciers during the last ice age.
"We don't really understand why earthquakes are occurring here," said Robert Herrmann, the Otto Nuttli professor of geophysics at St. Louis University. "On the other hand, we do understand the nature of the process for the earthquakes that are occurring."
Like its West Coast counterpart, the New Madrid appears to be a "strike-slip fault," or one which the sides move laterally in relation to one another, grinding along their edges as opposed to the recent earthquake in Japan where one plate is being pushed beneath another.
Herrmann said it seems similar to other strike-slip patterns seen elsewhere on Earth even though this one is in the middle of a plate, not along a boundary.
Still, it's frustrating.
"We don't understand the why," Herrmann said. "If you don't understand why you have earthquakes, you've made no progress on trying to understand the process by which you could have a repeat of earthquakes in the future."
The Big One
One possible guide to that future is found in a frightening past. New Madrid spawned the largest quakes ever recorded east of the Rocky Mountains. The four shocks, striking between December 1811 and February 1812, have become the stuff of local legend. People were rumored to have been awakened as far away as New York City, church bells rang in Boston and the president felt the quakes in the nation's capital. Regional lore even has it that the Mississippi River briefly flowed backward. The official webpage of New Madrid says the quakes are estimated to have clocked in at between 7.8 and 8.8 on the Richter Scale.
But the actual intensity is still a matter of some debate, especially since tools weren't around at the time to create proper measurements. Generally, they are thought to have been smaller than folklore suggests. The U.S. Geological Survey approximates the New Madrid quakes in the 7.0-7.7 range. In recent findings released to the Seismological Society of America, one expert estimated the most powerful of the quakes right around the 7.0 mark.
Researchers often look to the 2001 Gujarat earthquake in western India for comparison. That 7.7 temblor was also along an intraplate fault far from a plate boundary and, like New Madrid, had had no major quakes since the early 19th century.
The Gujarat quake had incredible reach. Gary Patterson, a geologist and director of education and outreach at the Center for Earthquake Research and Information in Memphis, said the vibrations echoed as far as Calcutta, some 1,200 miles away. By contrast, southern California earthquakes are barely felt in Las Vegas.
"Compared to plate boundary rocks, the continental interior ... transfers energy very efficiently," he said. "The deep crust here is relatively hard, dense and cold and allows energy to travel over great distances."
But Midwestern rock isn't the only reason for the New Madrid's reach. Midwestern soil creates another problem -- one that's left evidence of the two-century-old earthquakes, which is still visible today.
View parts of southeastern Missouri from above and you'll see what worries scientists so much -- strange sandy patches spread across the landscape. These are the remnants of sandblows or sandboils, powerful geysers of sand that erupted during the great New Madrid quakes.
"It's a secondary effect caused by severe shaking," said Williams. "The material in the subsurface, the upper 30 feet or so liquefies and the pressure builds."
This liquefaction can have consequences far more significant than small sand volcanoes. Buildings, roads and other fixed structures can suddenly find themselves rooted in a marshy quicksand as moisture trapped in the ground turns previously solid terrain gooey. Herrmann likens it to a walk on the beach.
"If you are right next to the water and you wiggle your foot, you end up sinking more," he said. "All you are doing is moving the particular grains of sand and moving water out from within those grains. That's liquefaction."
And it can cause a great deal of damage. One particularly sobering 2009 report from the Mid-America Earthquake Center estimated that a 7.7 quake could result in more than 80,000 injuries and fatalities and nearly $300 billion in damages across eight states while leaving as many as 7.2 million people displaced.
The Future
If scientists understand the how and the what, they are still working on the why. But the question on the mind of the general public is more immediate: when?
That's something no one can predict. Some have even speculated that seismic activity may be shutting down at New Madrid, an idea that circulated in the media a couple of years ago based on GPS readings that showed little movement in the fault.
But the fault seems anything but dead. Patterson said New Madrid produces about 200 quakes a year with more than 95 percent being too small for humans to feel. Moreover, in February, the USGS reported dozens of small earthquakes rattled Arkansas, including a 4.7 that was felt across five states. Known as a "swarm" such spates of tectonic activity occur periodically. The USGS indicated the February shakes may have been related to a larger swarm of hundreds of quakes that began last summer in the state.
"Scientists do not know why swarms start, why they stop or how long to expect them to last," said a release by the agency.
Williams said there is evidence of strong quakes along the fault around the year 900 and again in 1450. Then came 1811-12.
"The probability of a repeat of an 1811-12 size earthquake, which we think was about magnitude 7.5 to 7.7, is 7 to 10 percent in the next 50 years," he said. "But we are also worried about magnitude 6s perhaps even more because we think there is a higher probability, roughly 25 to 40 percent."
CERI's Patterson calls it a "low-probability, high consequence" event. He said it's prudent to prepare - even if we don't fully comprehend the mechanisms or the odds. It's all part of the mysteries inherent to life over an apparent chink in the North American Plate's tectonic armor.
"On very basic levels, our understanding of large earthquakes is pretty good when it comes to the broken edges of the earth's crust," he said. "There is currently no consensus theory on why large earthquakes should occur in the central United States."
David Baugher is a freelance writer in St. Louis.