Scientists say Midwest quakes poorly understood - WMC Action News 5 - Memphis, Tennessee

Scientists say Midwest quakes poorly understood

CHAMPAIGN, Ill. (AP) - Scientists say they know far too little about Midwestern seismic zones like the one that rumbled to life under southern Illinois Friday morning, but much of what they do know is striking.

The fault zones beneath the Mississippi River Valley have produced the largest modern U.S. quakes east of the Rockies, a region covered with old buildings not built to withstand seismic activity.

And, when quakes happen, they're felt far and wide, their vibrations propagated over hundreds of miles of bedrock.

Friday's quake was a magnitude 5.2 temblor centered just outside West Salem in southeastern Illinois, a largely rural region of small towns that sit over the Wabash fault zone. The area has produced moderately strong quakes as recently as 2002.

But it hasn't been studied to nearly the degree of quake-prone areas west of the Rockies, particularly along the heavily scrutinized Pacific coast.

"We don't have as many opportunities as in California," said Genda Chen, associate professor of engineering at the University of Missouri-Rolla, which sits near the well-known and very active New Madrid fault zone.

"We cannot even borrow on the knowledge they learn on the West Coast" because quakes that happen in California - where tectonic plates beneath the Earth's surface collide - are so different from Midwestern quakes that happen far away from the edges of the nearest plates.

It isn't entirely clear, for instance, whether the Wabash faults are related to the New Madrid faults or not.

Some scientists say yes, noting that the Wabash faults, which roughly parallel the river of the same name in southern Illinois and Indiana, are a northern extension of the New Madrid zone. Others say they're not.

And some scientists, like Illinois State Geological Survey geophysicist Tim Larson, say maybe.

"They're distinctly separated, at least in the upper rock," he said. "But at depth there could be some relationship between them."

The New Madrid fault zone produced a series of mammoth quakes in 1811 and 1812 that reached an estimated magnitude 7.0, the strongest known quakes to have occurred east of the Rockies. The quakes changed the course of the Mississippi River and were felt in New England.

That distance of well over a thousand miles sounds impressive, but experts say quakes that happen in the Midwest commonly radiate out for hundreds of miles because of the bedrock beneath much of the eastern United States.

"Our bedrock here is old, really rigid and sends those waves a long way," said Bob Bauer, a geologist with the Illinois State Geological Survey who works in Champaign.

He compared the underground rock, which in much of the Midwest lies anywhere from a few thousand feet to just a few feet below the earth's surface, to a bell that very efficiently transmits seismic waves like sound.

"California is young bedrock," he explained. "It's broken up ... like a cracked bell. You ring that, the waves don't go as far."

The question of whether Friday's quake was centered along a branch of the New Madrid zone or not is of more than academic interest. The area even now produces smaller, very regular quakes, and experts say it still has the potential to produce the kind of quake that could devastate the region.

The Wabash faults have the potential to do the same, at least based on distant history, said Columbia University seismologist Won-Young Kim.

The strongest quake produced in recent history by the Wabash was a magnitude 5.3 in southern Illinois in 1968, but researchers have found evidence that 4,000 to 6,000 years ago, much stronger quakes shook the region, Kim said, as strong as magnitude 7.0 or more.

And it's possible that the region, if given time to build up enough energy, could do it again, though knowledge about the region is too thin to say whether it's likely, he said.

That 1968 quake was one of the few in recent history in the region to have killed someone, according to U.S. Geological Survey spokesman Gary Patterson. A teenager was killed by a parapet that fell from an older brick building.

The older buildings that cover the Mississippi Valley are among the most vulnerable to quakes in the region. They were built decades before building codes even considered the need to stand up to earthquakes, said Amr S. Elnashai, director of the Mid-America Earthquake Center at the University of Illinois.

"There was no seismic design requirement until the mid '80s," he said.

The earthquake center models how earthquakes might play out across an eight-state region around the New Madrid fault zone for the Federal Emergency Management Agency, he said. Researchers use detailed information about thousands of buildings in the area.

Those models assume few buildings in the region meet tough seismic standards, he said.       And trying to bring even public and commercial buildings up to those standards would be a monumental task, Elnashai said.

He said any push to change that "would have no impact on our buildings for the coming 25 years, until some percentage are built to this magic recipe.

"So we are really stuck with what we have."

 

(Copyright 2008 by The Associated Press.  All Rights Reserved.)

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