INDIANAPOLIS -- 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 short time from the area that remains seismically active.
For their study, researchers from Purdue and Northwestern universities analyzed GPS measurements of shifts in the Earth's surface taken from 10 sites within the New Madrid zone over eight years. The region 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 millimeters 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.
"If the area is not moving today, if the rubber band is not being put under tension, that could mean that the fault zone is shutting down. It may be that it's produced several earthquakes in the past but now it's turning off," he said.
Calais said the findings could also mean seismic activity is shifting away from the New Madrid zone and into other nearby seismic areas. He said those could include a fault zone in Southern Illinois that produced a magnitude-5.2 temblor in April, or similar seismic regions in Arkansas, Oklahoma or elsewhere in the region.
The paper, which appears in the current issue of the journal Science, was co-authored by Seth Stein, a professor of Earth and Planetary Sciences at Northwestern University.
Other scientists called the study compelling and said it could lead to a better understanding of continental fault zones, but one cautioned that the eight years' worth of data is "just a blink of the eye" compared with the past 2,400 years.
That period has seen four or five episodes of multiple large earthquakes rock the area, said Chuck Langston, director of the Center for Earthquake Research and Information at the University of Memphis.
"We have just one little glimpse, one instant in time," he said. "And we're still having earthquakes in this area that look like they're lining up on faults, so there's no doubt that there are big fault structures there that remain active."
Langston also said that unlike California's San Andreas fault and others that occur along the boundaries of moving tectonic plates on the Earth's surface, the New Madrid zone is a continental fault that's found within one of those plates.
Little is known about what forces trigger earthquakes within continental faults, so it's possible that interior stresses can accumulate over a short period of time instead of continuously over longer periods, he said.
Bob Smalley Jr., a research professor with the University of Memphis earthquake center, said the study points to new ideas geologists should explore. But he said the conclusions would be more persuasive if the data covered 20 to 30 years.
"We should consider all of the things they're saying in their hypothesis, but they haven't reached the final answer yet," he said.