April 21, 2008

Scientists Seek Understanding of Midwest Earthquakes

Although the fault zones beneath the Mississippi River Valley have produced some of the country's largest modern earthquakes east of the Rocky Mountains, scientists acknowledge they don't yet fully understand the Midwestern seismic zone that caused Friday's earthquake in southern Illinois.

However, what they do know is rather unsettling.  

The Midwest region is covered with old structures not built to withstand seismic activity.  And, when earthquakes do happen, they are felt at greater distances, with heavy shaking propagated over hundreds of miles of bedrock.

Friday's quake was felt from Atlanta to Nebraska, but fortunately did little damage and caused no serious injuries.

The magnitude 5.2 quake was centered near the town of West Salem in southeastern Illinois, a rural region of small towns sitting over the Wabash fault zone. The region has experienced moderately strong earthquakes as recently as 2002.

But the fault zone hasn't been studied to nearly the degree of those areas west of the Rockies, particularly along the Pacific coast, where earthquakes are more prone to occur.

"We don't have as many opportunities as in California," said Genda Chen, associate professor of engineering at the University of Missouri-Rolla, during an Associated Press interview. The university is near the highly active New Madrid fault zone.

"We cannot even borrow on the knowledge they learn on the West Coast" she said, explaining that while quakes in California occur where tectonic plates beneath the Earth's surface collide, the Midwestern quakes happen far away from the edges of the nearest plates.

For instance, scientists are not even sure about the relationship between the Wabash and New Madrid faults. Some scientists believe the Wabash faults are a northern extension of the New Madrid zone, while others dispute the theory. The Wabash fault runs roughly parallel to the river of the same name in southern Illinois and Indiana.

In 1811 and 1812, a series of earthquakes produced by the New Madrid fault zone reached an estimated magnitude of 7.0, making them among the most powerful earthquakes to have ever occurred east of the Rocky Mountains. The quakes were so strong they were felt as far away as New England, and changed the direction of the Mississippi River.

Seismologists say Midwest earthquakes commonly radiate out for hundreds of miles due to the bedrock under much of the eastern United States.

"Our bedrock here is old, really rigid and sends those waves a long way," Bob Bauer, a geologist with the Illinois State Geological Survey, told the AP. Bauer equated the Midwest bedrock, which lies from a few feet to a few thousand feet below the earth's surface, to a smooth bell that 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 or not Friday's quake was centered along a branch of the New Madrid zone is not merely academic. The region consistently produces smaller quakes, and experts worry about the potential for a larger quake that could devastate the region.

According to Columbia University seismologist Won-Young Kim, the Wabash faults have the potential to do the same, at least based on historical records.

In recent history, the strongest quake produced in the Wabash faults was in 1968, when a magnitude 5.3 tremor hit southern Illinois. However, Kim said that researchers have discovered evidence of a much stronger magnitude 7.0 or greater quake that hit the region 4,000 to 6,000 years ago.

While a similar quake in the region is still a possibility, Kim told the AP that scientists do not yet have a full understanding of whether or not it is likely to occur.


On the Net:

"Earthquake Rocks U.S. Midwest"

Associated Press