New Madrid earthquake probability lowered

Thursday, January 9, 2003

LITTLE ROCK -- The likelihood of a major earthquake occurring along the New Madrid fault has been vastly overstated for several years, according to a specialist with the U.S. Geological Survey.

Instead of the 90 percent probability of such a quake within 50 years that experts have been citing -- indicating a large New Madrid earthquake was almost imminent -- the chances are actually between 7 and 10 percent, said Joan Gomberg, a geophysicist based at the University of Memphis.

"The probabilities that were most widely used before were devised in 1985," Gomberg said. "We didn't have accurate data. The principle reason the probabilities have changed is that we've learned a lot since then."

She said probabilities were figured within about a 50-year window and don't specify a particular spot.

Changes in technology

Gomberg said advanced technology has allowed researchers to examine historic and geologic records to unravel the fault's seismic past. The result is a complete account of earthquake activity throughout the region.

The data also indicates the probability of a magnitude six earthquake over the next 50 years is between 25 and 40 percent. But Gomberg said this shouldn't cause alarm.

"When we talk about hazards, the hazard is how hard the ground will shake -- the probability of occurrence and the potential for damage is not one to one," Gomberg said.

Haydar Al-Shukri, director of the Arkansas Earthquake Center at the University of Arkansas at Little Rock, said a big New Madrid quake would likely cause widespread damage.

By comparison, a large quake in California would wreak more focused havoc.

"A quake of the same magnitude in New Madrid has the potential of causing much more widespread damage than a quake in California," Al-Shukri said. "The ground motion will be much more severe because (the ground in the New Madrid region) has the tendency to not absorb seismic energy so the wave energy will travel longer distances."

Gary Patterson, a geologist with the Center for Earthquake Research and Information at the University of Memphis, said the new data confirms what researchers have always known, that the New Madrid fault doesn't act like California.

Patterson said geologists have been studying the New Madrid fault for about 25 years. But he said California has been studied with instruments since the 1906 San Francisco earthquake.

"California has about 75 years of instrumental data we don't have in New Madrid," Patterson said.

Patterson said another problem in studying the central region is the lack of activity, making probabilities harder to figure.

"It's an entirely different region," Patterson said. "Science changes all the time, it's no surprise probabilities change and they'll probably change again the more we learn."

The New Madrid zone runs through Alabama, Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri and Tennessee.

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