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Less Quake Danger Along River?

Living in the New Madrid earthquake zone along the Mississippi River may not be as dangerous as once thought.

Devastating quakes rocked the region from St. Louis to Memphis, Tenn., and beyond in 1811-12. The Mississippi reportedly flowed backward for a time.

A new analysis of the region in Friday's issue of the journal Science says the likelihood of a repeat of that event has been overstated.

Newly gathered data "imply that 1811-1812-size earthquakes are either much smaller or far less frequent than previously assumed."

"In either case, it seems that the hazard from great earthquakes in the New Madrid zone has been significantly overestimated," reports the team of scientists led by Seth Stein of Northwestern University.

Minor tremors continue to occur in the area, raising concern that a repeat of the powerful earlier quakes could cause devastation in the modern cities that have grown there.

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 a 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 had been previously estimated). That's the simplest explanation and what we think is most likely," said Stein.

A magnitude 8 quake, capable of tremendous damage, is 10 times more powerful than a 7.

Stein noted that quakes in the Eastern United States are felt over larger areas than Western ones because of the more rigid rock in the East. That may have resulted in the overestimation of the early quakes, he said.

The conclusions were challenged by Arch Johnston, a seismologist at the University of Memphis, who said 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 400-600 years.

"We're very certain about our uncertainty," responded Andrew Newman, a graduate student who was part of Stein's team.

Stein's team also suggested 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 responded. He noted that the seismic hazard maps are used when building codes are being developed to set safe construction standards.

Stein stood by his calculations, saying the danger ratings of current seismic hazard maps seem very unlikely in light of current understanding.

By Randolph E. Schmid

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