Less-Prepared Central U.S. Also Prone to Earthquakes

BY Jenny Marder  April 28, 2011 at 12:01 PM EST

In 1816, a woman named Eliza Bryan wrote a letter to a friend in which she recounted “an awful noise, resembling loud but distant thunder, but more hoarse and vibrating… followed in a few minutes by the complete saturation of the atmosphere, with sulphurious [sic] vapor, causing total darkness.”

The horrifying event was an earthquake, which struck in 1811 in an area not widely known for being earthquake-prone: the Mississippi River Valley. Scientists have tracked the epicenter to the Arkansas-Missouri border, near Jonesboro, the Southern tip of the Arkansas boot heel.

While there’s more awareness of earthquake risk in the West and Pacific Northwest, a Central U.S. quake could impact a larger area and cause more damage, experts say. Building codes in the area are weaker, with dangerous brick buildings lining main streets, built without earthquakes in mind. Plus, the tectonic makeup in the East is such that rocks carry seismic energy farther.

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“It’s denser, colder rock,” said Robert Williams, a USGS geophysicist. “In California, the rock is more broken up, and the energy dies off more quickly. But the same sized quake will be felt over a much larger area, unfortunately, in the East.”

On Thursday, 11 states will participate in a mass earthquake drill, called The Great Central U.S. Shake Out. Its purpose is to educate people who live above a spiderlike network of fault lines, capable of producing a natural phenomenon largely unfamiliar to them, but highly plausible: a really big earthquake.

“Earthquake activity has been felt in all 50 states so none of us have the luxury of thinking this won’t happen here,” said FEMA Administrator Craig Fugate in a statement provided to the NewsHour Wednesday.

The U.S. Geological Survey estimates a 7 to 10 percent chance that a 7.5 magnitude quake or larger will strike the New Madrid Seismic Zone in the next 50 years. That zone covers a swath of the central part of the country that includes northeastern Arkansas, western Tennessee, western Kentucky, southern Illinois and southeast Missouri. This region already has as many as 200 smaller quakes per year, usually lower than a magnitude 3.

“There have been about a dozen magnitude 4s since the mid-1970s, when the seismic network started being deployed in greater numbers in that region,” Williams said. “It’s an active seismic zone.”

These are generally small earthquakes, with little threat to public safety. Not so for three massive earthquakes that rattled the region nearly 200 years ago. With no instruments to measure the quakes, the precise magnitude is unclear, but scientists believe they ranged from a 7 to 8 on the Richter scale. This is based on newspaper articles, letters, and journals describing eyewitness accounts of the events, along with studies of resulting ground defamation.

Geysers of sand, known as sand blows, erupted during the quakes, along with large areas of subsidence, where the ground caved in, later forming swamps and lakes.

The three quakes struck the New Madrid region near Memphis and southern Missouri, stretching up to St. Louis, and were followed by hundreds of aftershocks. The earthquake Bryan referenced occurred on December 16, 1811. Another struck northeast of New Madrid, Missouri on January 23, 1812, and yet another — and possibly the biggest — occurred on February 7, 1812, also near New Madrid.

Eyewitness accounts tell of 18-foot-wide cracks in the ground, houses “racked to pieces,” and cottonwood trees snapped from their roots. Chunks of dirt and trees from riverbanks slumped into the water, clogging waterways. One boatman who ran barges of goods along the Mississippi wrote that the February 1812 quake caused the river current to briefly run backwards. Another wrote of seismic waves generating river tsunamis that buried an island where a boat was anchored.

“We think the island had liquefied and disappeared,” Williams said. “[The boatman] reported that the place he had tied up to was gone.”

In 1814, William Clark, of the Lewis & Clark Expedition, who was then the Missouri Territorial Governor, asked for federal relief for New Madrid County residents. The following year, Congress allocated $50,000 for recovery — the first under the United States Disaster Relief Act.

These were not isolated events. It’s believed that sequences of powerful earthquake activity also occurred in the region in 1415 A.D. and again in 900 A.D.

Scientists think that first two 1811-1812 earthquakes were caused by a strike-slip fault. Strike-slip surfaces are vertical, and the plates move laterally past each other. The latter quake resulted from a reverse fault, meaning one plate was shoved upward relative to the other.

But Williams adds that it’s difficult to isolate the location of the network of faults running under the region. “The movement of the Mississippi River has, we think, obliterated some of the evidence of faulting by the meandering it’s done over the last couple hundred years,” he said.