The Mystery of New Madrid:

New Theory Helps Explain 19th Century's Puzzling Earthquakes in Central U.S.

AUSTIN, Texas-Geologists have struggled to understand what caused a series of powerful earthquakes in the early 1800s in the central U.S. that reversed the flow of the Mississippi river, created new lakes and sinkholes, destroyed homes, and reportedly rang church bells 1,000 miles away in Boston.

The New Madrid earthquakes happened in a place they shouldn't have -- the middle of a continental plate. A study by researchers at the University of Quebec in Montreal, the University of Toronto, and the University of Texas at Austin's Jackson School of Geosciences might help shed light on why that area was susceptible to earthquakes.

Most earthquakes occur where Earth's tectonic plates collide or rub against each other -- such as along the Pacific Ring of Fire. That's why places such as Alaska and California tend to be the hot spots for earthquakes in the U.S. But some earthquakes occur far from the edges of tectonic plates. This was the case with the New Madrid earthquakes of 1811 and 1812 which had epicenters near New Madrid, Missouri. Their effects were noted across much of the U.S.

With millions of people living in the central Mississippi Valley today (including parts of Missouri, Arkansas, Kentucky, and Tennessee), there is concern that another magnitude 8 earthquake (the estimated magnitude of the New Madrid earthquakes) could be devastating.

A contemporary woodcut depicts a cabin collapsing and people outside panicking as one of North America's most powerful earthquakes shakes the ground.

The researchers aren't making any predictions about the potential for future earthquakes in this area.

Steve Grand and graduate student Nathan Simmons analyzed global seismic data to determine a 3-dimensional map of densities of the Earth's mantle -- a layer that behaves as a viscous fluid on geologic time scales sandwiched between the solid outer crust that we live on and the molten iron core at Earth's center.

Alessandro Forte, professor of earth and atmospheric sciences at the University of Quebec at Montreal, plugged these mantle density maps into a computer model to generate flow maps which show how material in the mantle moves or convects. This model showed that right under the area where the historic New Madrid earthquakes occurred, mantle material moving from below the Atlantic in the east and from the Pacific in the west converges and flows downward. Forte believes that this convergence of material causes downward bending of the crustal rocks near New Madrid. This bending of the outer crustal rock creates increased stress within the crust beneath New Madrid thus perhaps explaining why large earthquakes have occurred in this region.

Scientists already knew that a slab of dense rock is sinking below the New Madrid seismic zone. But until now, no complete flow model had been calculated to determine the stresses acting on the overriding crust.

Location of the New Madrid Seismic Zone. Map: USGS.

Grand also said it would be a good idea to look for similar flow patterns in other parts of the world to identify other areas with the right conditions for the kinds of earthquakes that occurred in the New Madrid area.

Grand noted that, as with many discoveries in science, he and his colleagues didn't set out to solve the mystery of the New Madrid earthquakes. Instead, it was the happy byproduct of an entirely different research project.

by Marc Airhart

For more information about the Jackson School, contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.