Now, they have a new tool to help them understand the fault.
With five years of global positioning system (GPS) data, researchers at the Center for Earthquake Research and Information (CERI) in Memphis are documenting the same kind of ground movement that can be seen along the San Andreas Fault in southern California.
The GPS study documented that lands on either side of the Reelfoot thrust fault are moving toward each other at a rate of about 2.7 millimeters a year, give or take 1.6 millimeters, said Robert Smalley. Smalley, a CERI associate research professor, conducted the study with Mike Ellis, John Paul and Roy Van Arsdale, all from CERI at the University of Memphis.
The Reelfoot thrust fault stretches from the river bluffs southeast of Reelfoot Lake, across the southern corner of the lake to the Mississippi River levee, north into the Kentucky Bend and over the river toward New Madrid, Mo.
The CERI scientists installed 11 GPS monitors in the New Madrid Seismic Zone. One was placed near a water tower at the Northwest Correction Center and another was placed at the airstrip beside Reelfoot Lake State Park's Airpark Inn. These monitors are on opposite sides of the Reelfoot scarp, or land that was uplifted during the 1811-12 earthquakes. The scarp runs along the fault and is highest from Reelfoot's Champey Pocket to the Mississippi River levee in Lake County where it is 15 to 30 feet high.
GPS units in Missouri also documented slight movement (about 1 millimeter a year) between Steele, Mo., and McCarty, Mo., both of which are located due west of Dyersburg. Those GPS units are on the strike slip arm extending southward from Reelfoot Lake to Marked Tree, Ark.
Smalley compared the ground movement at both locations to bending a stick over your knee. The ends of the stick come closer together and the center curves and changes in elevation. The ground is deforming, just like the stick. The study shows that the land is moving like the ends of the stick. What the study doesn't show is how much the land elevation is changing.
GPS units farther from the fault lines -- such as those in Troy and Covington, Tenn.; Piggott, Ark., and Charleston, Mo. -- showed no movement. "Only when you're close to the fault do you see them moving," Smalley said. Sites at similar distances from the San Andreas Fault would have shown considerable movement, reflecting the fact that two tectonic plates are pushing against one another, he said.
Smalley and other earthquake researchers believe that the lack of movement on outlying GPS monitors indicates that the strain along the New Madrid fault lines is not produced by outside pressures.
The GPS study reveals for the first time that the surface of the earth is deforming along the fault line, but it doesn't answer all of the questions scientists -- and local residents -- have. Smalley said the study is just a start. He hopes to acquire funding to increase the GPS monitor network and to continue collecting data.
The biggest outcome of the study is that it verifies what history and modern-day tremors seem to indicate. The New Madrid Seismic Zone is still active.
In 1999, a group of scientists from Northwestern University published findings in which they concluded that the New Madrid Seismic Zone wasn't as hazardous as had been reported. They urged that the threat level be downgraded.
The Northwestern study also used GPS monitors to document ground movement over a period of years in the 1990s. Smalley said the GPS technology was not as accurate as today. Those GPS studies couldn't measure changes smaller than several millimeters. he said. Furthermore, the GPS readings were taken periodically, rather than continuously.
Smalley said the less sophisticated GPS technology and the periodic readings didn't show any movement at the level of precision available, but he believes the researchers went too far when they strongly argued that the New Madrid earthquake hazard had been overestimated.
"That was the best they had at the time, but the interpretation was an extreme view," he said. "We believe GPS is now supporting all of the previous work that there is a seismic hazard."
The CERI study -- funded by the U.S. Geologic Survey and the National Science Foundation's Mid-America Earthquake Center -- was detailed Thursday in "Nature, The Science Journal."