River Gets Blame for Giant 1811 Earthquakes

Seismicity of the "stable" art of the North American plate in the U.S. Yellow circle show 200 years of seismicity. The largest events occurred in the New Madrid seismic zone (1881-1812) and Charleston (1886).

(Image credit: E. Calais/Purdue University/USGS.)

Massive earthquakes that struck the town of New Madrid, Mo., in 1811 can be traced to the actions of the mighty Mississippi River thousands of years earlier, a new study in the journal Nature suggests.

The work could affect scientists' understanding of the fault systems that caused the quakes.

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The new thinking works like this: At the end of the last ice age around 16,000 years ago, the Mississippi River washed away tons of soil, taking a giant weight off the central portions of the continent. Over 6,000 years, the river dredged 39 feet (12 meters) of sediment from the river basin — a "quite dramatic" erosion event that set in motion the events that would lead to the giant quakes, said geologist and study team member Roy Van Arsdale of the University of Memphis.

In the middle of the stick, Freed said, the upward curving top part is stretched and the bottom part is compressed. The land at New Madrid bent the same way as the sediment eroded. The area of land that was stretched contained faults, or cracks in the rocky plates of the Earth's crust. These faults were already close to rupturing, and when they eventually failed in 1811 they unleashed violent earthquakes.

"The theory is certainly interesting enough to merit further consideration," Susan Hough, a geologist with the U.S. Geological Survey who was not involved with the work, told OurAmazingPlanet, but she added that geological evidence "suggests strongly" that a set of faults in the area has produced multiple earthquakes over the last 2,000 years (with quakes coming in roughly 500-year intervals), implying some regularity in these faults as well. More work is necessary to explain that, she said.

The New Madrid earthquakes that struck between December 1811 and February 1812 were some of the strongest seismic events ever recorded in the continental United States. The largest of these quakes was estimated between magnitude 7.0 and 8.0 and made the Mississippi River flow backwards temporarily. Earthquakes in this part of the country rumble over a larger area than they do on the West Coast, due to the makeup of the underlying ground. When the quakes hit New Madrid, according to legend, church bells rang in Boston.

New Madrid still rumbles with roughly 200 tiny quakes annually — a 5.2-magnitude quake hit in 2008 — but a recent study suggested that what the town is feeling are actually 200-year-old aftershocks, since the fault may be shutting down. The New Madrid faults move more than 100 times more slowly than the San Andreas Fault; the slower a fault moves, the longer the aftershocks last. That's because the tectonic plates can't "reload" the fault, wiping out the effects of a previous earthquake and suppressing aftershocks.

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Brett Israel was a staff writer for Live Science with a focus on environmental issues. He holds a bachelor’s degree in biochemistry and molecular biology from The University of Georgia, a master’s degree in journalism from New York University, and has studied doctorate-level biochemistry at Emory University.