Midwest quake shakes up scientists

4/20/2008

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

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

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

Friday's quake shook things from Nebraska to Ohio, rattling nerves but causing little damage and seriously hurting no one.

It was a magnitude 5.2 temblor centered just outside West Salem in southeastern Illinois, a largely rural region over the Wabash fault zone. The area has produced moderately strong quakes as recently as 2002.

But it hasn't been studied to the degree of quake-prone areas west of the Rockies.

"We don't have as many opportunities as in California," said Genda Chen, associate professor of engineering at the University of Missouri-Rolla, 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 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 they are related, but others say they're not.

The New Madrid fault zone produced a series of quakes in 1811 and 1812 that reached an estimated magnitude 7.0, putting them among 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 more than 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.