Midwest's New Madrid Earthquake Zone: Is It Dying Out?

Midwest's New Madrid Earthquake Zone: Is It Dying Out?: "A Midwest fault zone that unleashed a series of violent earthquakes in the early 19th century shows no signs of building up the stresses needed for the quakes many seismologists expect to someday rock the region again, two scientists say.

The researchers said that may mean the little-understood New Madrid Seismic Zone is shutting down or that seismic activity is shifting to adjacent faults in the nation's midsection.

Other scientists called those conclusions premature, in part because the study was based on a relatively narrow time period from the area that remains seismically active.

For their study, researchers from Purdue and Northwestern universities analyzed global positioning measurements of shifts in the Earth's surface taken from 10 sites within the New Madrid zone over eight years. That region in the central Mississippi Valley produced a series of earthquakes in 1811 and 1812 of an estimated magnitude 7.0 or greater.

Researchers expected to find surface features moving at least one to 2 millimeters each year. Such shifts would reflect growing subterranean stresses like the slow stretching of a rubber band that seismologists expect to someday spark more big New Madrid quakes.

Instead, they found annual shifts of 0.2 millimeter or less each year -- an amount so tiny it essentially represents no growing stresses in the seismic zone, said Eric Calais, a Purdue professor of earth and atmospheric sciences who led the study."

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