RESEARCHERS SAY NEW DATA SHOW THREAT OF QUAKE IS BUILDING ALONG NEW MADRID FAULT
STUDY SAYS THAT STRAIN IS MOUNTING MORE THAN PREVIOUS EVIDENCE INDICATED
By William Allen
St. Louis Post-Dispatch Science Writer
Section: NEWS
Edition: FIVE STAR LIFT
Page: A1
Scientists find that the New Madrid Fault is moving about 5 or 6 millimeters a year -- about a quarter of an inch.
Scientists are firing another salvo today in the war over whether the "big one" is coming from the New Madrid Fault.
In today's issue of the journal Science, a team led by researchers at the University of Colorado at Boulder reports new evidence of a major threat from the fault.
"You've got enough stored energy to shake the living daylights out of the lower Mississippi River Valley," said Karl Mueller, a structural geologist at the university who led the team.
The study by Mueller and his colleagues shows that more strain is building in the fault than was found by a study published April 23 in Science. That study, led by researchers at Northwestern University, found that the risk of a major earthquake in the fault has been overestimated. But one of the Northwestern researchers, Seth Stein, said today's report was "completely consistent" with his group's study.
At issue is the scientific basis for warnings to prepare for a major quake in the New Madrid Fault. The fault got its name from New Madrid, Mo., a town where one of many powerful quakes in 1811-12 was centered. Also at issue is how much to prepare people and buildings.
The New Madrid Fault runs for more than 100 miles from northeastern Arkansas, through the Missouri Bootheel and into the southern tip of Illinois. Like a multipronged lightning bolt, the fault zigzags and branches off in many directions and at various lengths deep beneath the ground.
A major quake along the fault could cause damage hundreds of miles away, many scientists say.
New technology and government funding have allowed scientists to examine the fault more closely.
In the April Science paper, researchers using data gathered by satellite reported finding little or no ground motion in the fault over six years and concluded that the hazard from great earthquakes "has been significantly overestimated."
But the evidence published today "shows the New Madrid seismic zone is indeed a threat, which contradicts" the April study, Mueller said. He was joined in the research by Jocasta Champion of Colorado; Margaret Guccione of the University of Arkansas, Fayetteville; and Keith Kelson of William Lettis and Associates, Walnut Creek, Calif.
Mueller and his colleagues looked at the fault using what scientists call "fault-related fold theory."
Put simply, that means they look at the shape of the wrinkles and folds near the top of the earth's crust and use their shape to estimate the shape of a fault miles below. They also can estimate how the fault is moving.
The concept is like setting a layer cake down across the edge of a table. The part of the cake hanging off the table will move down. By cutting the cake and looking at the wrinkles in the layers, the amount of relative movement between the two parts can be deduced.
The scientists studied the shape of the land surface in the Lake County area of western Tennessee.
Then they cut the cake, digging trenches hundreds of feet long and 12 feet deep through part of the region near Reelfoot Lake in the flood plain of the Mississippi River. They carefully mapped the wrinkles in the layers of sediments deposited by the river over the past 2,400 years.
Combined with other information, they found that the fault at that location is moving at about 5 or 6 millimeters a year - about a quarter of an inch. That might not seem like much until you consider that it equals nearly 3 feet of movement since the great New Madrid quakes of 1811-12.
"The spring is being stretched," Mueller said. "You're adding energy to the earthquake engine."
Stein said of the two studies, his and Mueller's: "It's very gratifying that you get essentially the same answer," even though the techniques the two groups used were different.
The difference, he explained, is that his group found a 7.0 magnitude quake could occur every 500 years on average, whereas Mueller's group found a quake of magnitude 7.2 to 7.5 could occur during that period.
That still means federal earthquake hazard maps of the region show a risk that's too high, Stein said. These maps assume a quake of magnitude 8.0 or greater is possible.
In May, an expert panel from the Central U.S. Earthquake Consortium State Geologists recommended against relaxing building codes or easing preparations for the Big One in the New Madrid Fault.
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