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Study documents ground movement along Reelfoot fault

Tuesday, June 28, 2005

A map of the New Madrid Seismic Zone shows the Reelfoot thrust fault stretching from the southern end of Reelfoot Lake through Kentucky Bend and toward New Madrid, Mo. Scientists from the Center for Earthquake Research and Information in Memphis recently released the results of a five-year global positioning system (GPS) based study. It shows that lands on either side of the fault line are moving toward each other.
For so many years, earthquake researchers relied on historical evidence and modern-day tremors to estimate earthquake threats in the New Madrid Seismic Zone.

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.

A five-year global positioning system (GPS) based study has documented that lands on either side of the Reelfoot thrust fault (shown here with the red line marked with triangles) are moving toward each other at a rate of about 2.7 millimeters a year.
Unlike the San Andreas Fault, the fault lines in the New Madrid Seismic Zone can't be easily seen on the Earth's surface. The faults were covered long ago with wind-blown soils and river-washed sediments.

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."

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Having worked off & on with Roy Van Arsdale and Randy Cox for a number of years (since ~ 1995) to identify and map the major faults in East Arkansas and North Mississippi, I certainly agree that this group of investigators "have it right" - for the most part. The prospect of a recurring major earthquake is real al beit the timing is not understood. Some say the recurrence of major quakes is approximately every 500+- years; however, this estimate is poorly constrained.

In general, my point of contention is with the suggestion that external tectonic forces appear not to be involved with the Blytheville Seismic Zone. However, I would accept that statement if it were strictly related to external forces to the west and east rift boundaries of the Reelfoot Aulacogen, this leaving the possibility that a regional stress is being generated from the south and up the axis of the rift system. In my opinion, the key to understanding the cause of the seismicity requires mapping regional faults to identify which faults are involved in generating the current localized strain that defines the Blytheville seismic zone. Likewise, the proposed mapping would lead to a better understanding and measurement of the seismic risk.

The current news report confirms that deeper crustal stress is causing the measurable surface compression. Earlier reports by Van Arsdale and Cox have laid some of the groundwork for mapping the major faults but more work is needed to identify where the basement (the underlying continental crust composed of crystalline granite) creates a restraining juncture and/or bend(s) that essentially lock faults/basement thereby restricting slippage along the faults. The location(s) of fault/basement restriction causes enormous pressures to build until the stress is greater than the rocks can constrain. When the pressures reach the point of catastrophic failure the restricting fault ruptures thereby releasing the stored pressure (energy). The released energy is then propagated/sent through the underlying rock/basement and reaches the surface in the form of ground/surface waves with various directional motions -- an earthquake.

In closing, the 1811 and 1812 New Madrid earthquakes released catastrophic energy forces estimated at 8-10 moment seismicity -- devastation. As you know, historical reports provide insight in to the local and regional impact associated with these earthquakes; hence the serious concern regarding the potential catastrophic impact that another earthquake of similar magnitude would have on the region. Note also that the Memphis area is a major transport corridor for commerce (air, water, and land) as well as the location of several major oil & gas pipelines. These facts heighten our concern that a major earthquake in the New Madrid-Memphis area would potentially wreak havoc to the local communities as well as the National economy. More work is needed to understand the risk. I would hope that the CERI Team would follow-up on earlier regional mapping activities to further identify and confirm the tectonic stresses that are in play here.

-- Posted by slingsr on Tue, Jun 2, 2009, at 11:53 AM

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