By
Eric Hand
ST. LOUIS POST-DISPATCH
12/14/2005
The earth under the New Madrid Seismic Zone either isn't shifting or is barely shifting at all, say three independent university analyses of global positioning system stations stuck in the ground and monitored for a decade.
So the cataclysmic shifts of past earthquakes remain unexplained, the mechanism for future earthquakes still a mystery.
The results contradict a University of Memphis study, which in June made headlines when it stated that two GPS stations on opposite sides of the Reelfoot fault had moved closer to each other at a rate that rivaled faults in California. The compression could coil up the faults for future earthquakes.
One of the detractors of that study, Purdue University geophysicist Eric Calais, said their results were certainly a statistical anomaly, probably an instrumental error, and, regardless, not anywhere close to the motions of the San Andreas fault, which slips more than 10 times as fast as the two Reelfoot fault GPS stations seem to be creeping toward each other.
"It's not fair in a scientific paper to scare people with things like that," he said.
Michael Ellis, one of the University of Memphis authors, says his group was only trying to show that the motions are consistent with the level of seismic hazard that geologists have already established for New Madrid.
A debate between the groups was published today in the journal Nature.
In California, earthquakes are caused by the more predictable process of plate tectonics. The Pacific Ocean plate grinds against the North American plate along the San Andreas fault, which ruptures regularly in earthquakes. But New Madrid sits in the middle of the quiet, rigid North American plate.
Ellis's paper was the first to suggest that the plate is stretching and straining near New Madrid. Yet all four groups agree that the tiny motions they're arguing about - as much an eighth of an inch per year - would need to be almost twice as big to build up the strain needed to unleash the huge magnitude 7 or 8 earthquakes that occurred in 1811 and at regular 500-year intervals for the past few thousand years.