The Associated Press News Service, April 22, 1999
New Madrid Fault Isn't Threatening
By RANDOLPH E. SCHMID
WASHINGTON - The threat of major new earthquakes on the New Madrid fault along the Mississippi River has been overestimated, according to researchers studying the source of devastating tremors nearly two centuries ago.
A series of great quakes in 1811-12, centered near New Madrid, Mo., shook the region from present day St. Louis to Memphis, Tenn., and beyond, reportedly causing the Mississippi to run backward for a time.
Minor tremors continue to occur and scientists have been concerned that a repeat of the powerful earlier quakes could cause devastation in the modern cities that have grown in the region.
But that threat may not be as bad as thought, according to a report in Friday's issue of the journal Science.
Newly gathered data ``imply that 1811-1812-size earthquakes are either much smaller or far less frequent than previously assumed,'' reports the team of scientists led by Seth Stein of Northwestern University.
``In either case, it seems that the hazard from great earthquakes in the New Madrid zone has been significantly overestimated,'' the study says.
Previously, scientists have estimated the magnitude of the 1811-12 quakes at 8, with 5 to 10 yards of horizontal ground movement at the time.
Stein's team used both satellite and ground-based measurements to check current movement along the New Madrid fault, and calculated it to be between zero and 2 millimeters per year. Two millimeters is bit less than a tenth of an inch.
Geological motions are very constant over time, Stein explained in a telephone interview. The very low motion along the fault, he said, indicates as much as a 14,000-year period between magnitude 8 quakes.
``The most plausible interpretation of our data is that the (1811-12) quakes were probably magnitude 7 instead of 8. That's the simplest explanation and what we think is most likely,'' said Stein, whose research was supported financially by the university consortium UNAVCO.
A magnitude 8 quake, capable of tremendous damage, is 10 times more powerful than a 7.
Stein noted that quakes in the eastern part of the United States are felt over larger areas than western ones because of the more rigid rock in the East. That may have led to overestimation of the early quakes, he said.
But the conclusions were challenged by Arch Johnston, a seismologist at the University of Memphis and expert on the New Madrid fault, who contended that the Northwestern team understated the uncertainty in their measurements.
Actual movement along the plate could be as much as 10 millimeters a year, Johnston said, which would indicate a major quake could occur every 400 to 600 years.
``We're very certain about our uncertainty,'' responded Andrew Newman, a graduate student who was part of Stein's team. He said the 2 millimeter movement estimate is ``at the 95 percent confidence level.''
``We disagree with Arch,'' added Stein.
The researchers also asserted that because the danger has been overestimated, changes should be considered in the National Seismic Hazard Maps for the New Madrid area.
That suggestion is ``premature and actually irresponsible,'' Johnston contended, noting that the seismic hazard maps are used when building codes are being developed to set safe construction standards.
But Stein stood by his calculations, contending the danger ratings of current seismic hazard maps seem very unlikely in light of current understanding.
Scientists mixed over New Madrid earthquake hazard
By WOODY BAIRD // Associated Press Writer //
MEMPHIS, Tenn. (AP) - Scientists have struggled for years to figure out when the next big earthquake might strike the central United States.
Now a geologist from Northwestern University believes he has evidence that the New Madrid Fault, at the center of the country's most active seismic zone east of the Rockies, is "cold and dying."
Other scientists disagree, and those differences show just how hard it is to understand what causes quakes in the central U.S. - and when another big one may erupt.
"It's easy to say, 'Oh, things must be cooling down, and because we don't understand it, it must be dying.' But we really don't know anything," said Chuck Langston, a seismology professor with the University of Memphis.
The New Madrid Fault, a network of deep cracks in the earth's surface from southern Illinois to northeastern Arkansas, produces hundreds of small quakes a year, most too weak to be noticed without scientific equipment.
But in 1811 and 1812, when the area was sparsely populated, it produced a series of big earthquakes estimated at magnitude 7.0 or greater.
Such a quake today would cause widespread damage, and scientists give a 10 percent probability one will strike over the next 50 years. Geological evidence indicates big quakes hit the region around the years 900 and 1450 A.D.
Chances for a magnitude 6.0 quake, which could cause serious damage depending on exactly where it struck, are put at 25 percent to 40 percent over the next 50 years.
The most recent large quake, estimated at magnitude 6.5, struck in 1895 near Charleston, Mo.
But Seth Stein, the Northwestern professor, believes the New Madrid could be "running out of steam" and the small recent quakes are lingering aftershocks of the great 1811-1812 temblors.
"If the fault started storing up energy tomorrow, it would take a thousand years to be ready for a good size earthquake," he said in an interview from his office in Illinois.
Eugene Schweig of the U.S. Geological Survey said Stein's assumptions rely on incomplete data and are just another piece of an ongoing debate over the cause of New Madrid quakes.
"The earthquakes are happening regardless of what the explanation is," Schweig said. "We know they occur with some regularity and that we're going to have them again. That's what we should be focusing on."
Unlike much better understood faults like the San Andreas in California, the New Madrid is not at the juncture of shifting tectonic plates, so why it produces earthquakes is still a mystery.
Stein's latest study, on which he presented a paper in December at the annual meeting of the American Geophysical Union, focuses on heat flows deep within the New Madrid Fault.
Hotter rocks are weaker than colder ones, and some scientists have theorized that that could help explain why the New Madrid is so active. But Stein and his associates found nothing unusual about heat flows within the fault.
Add that to his conclusions from an earlier study finding an absence of significant ground motion along the New Madrid, which could build up energy within the fault, and a picture begins to emerge, he said.
"Faults have these kinds of pulses of activity and they're active for a while. They turn on and they turn off," Stein said. "If we put all this stuff together, it looks like we're seeing the end of the recent pulse of activity."
Robert Smalley, a Memphis geophysicist, isn't impressed with that theory.
Arguing about "a temperature effect" on the New Madrid "is just a waste of time," Smalley said, because the temperature of the earth changes so slowly.
"It's a complete red herring," Smalley said. "The temperature now is the same as it was in 1811 and 1812 and 500 years before that and 500 years before that."
And slight ground motion may or may not be enough to build up stresses for a big quake, he said, since "we don't understand why they happen here in the first place."
Smalley is working with Glen Mattioli, a geology professor at the University of Arkansas, on a project to upgrade field equipment to study ground movement in the New Madrid zone.
If there is no ground motion to build up energy within the fault, Stein may be on to something, Mattioli said, but the data is incomplete.
Ground motion sensors, linked to global positioning satellites, are stationed throughout the New Madrid zone, but more up-to-date equipment is needed, Mattioli said, and he is looking for financing to supply it.
"I'm not convinced we have the full story yet, but I would have to say at this stage the evidence is pointing toward less (ground) deformation rather than more, so that would tend to favor Stein's argument that the seismic hazard is lower rather than higher," Mattioli said.
But even small amounts of ground motion might lead to a quake "that still could cause significant damage in a region that is not properly prepared."
Earthquake specialists have long urged residents of the New Madrid region to get ready for the big one, but Stein contends some of those warnings have been overblown.
"It's not a non-problem, but it's not a fantastically serious problem either," he said."You might call it a moderate-level problem."
For Smalley, the scientific evidence is insufficient to conclude that the New Madrid Fault has had its day.
"It could be dying out, but we don't know that," Smalley said. "We can't prove that it isn't and he can't prove that it is."
On the Net:
Seth Stein on the New Madrid Seismic Zone: http://www.earth.northwestern.edu/people/seth/research/nmsz.html
University of Memphis Center for Earthquake Research and Information: http://www.ceri.memphis.edu/
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