Newswise — The 100th anniversary of the 1906 earthquake that devastated San Francisco is a time to reflect on the awesome power of the San Andreas fault. But there's a fault in the Midwest that packs an even greater punch, according to an earthquake expert at the University of Missouri-Rolla.
Dr. J. David Rogers, an associate professor of geological engineering at UMR, says unique geology in the Midwest increases the shaking intensity of earthquakes because energy from the New Madrid seismic zone, which produced a series of magnitude 8.0 quakes in 1811-1812, moves through the dense bedrock underlying the mid-continent region at very high speeds, then becomes trapped in the soft sediments filling river channels and valleys.
The geology of California is thoroughly fractured by a series of faults, which, fortunately, serve to dampen seismic energy, according to Rogers, who is originally from the Bay Area.
"In the 1906 San Francisco earthquake, zones of intense shaking were isolated to pockets of soil and fill that lay over bedrock," says Rogers. "Shaking was intensified in these small zones, causing the ground to liquefy and breaking fire mains, which led to the disastrous fire that raged for days after the quake, destroying most of San Francisco."
Most of St. Louis, located about 125 miles away from the epicenters of past New Madrid quakes, lies on 30 to 150 feet of loose, sandy sediment. The Missouri River flows into the Mississippi River just north of the city, which didn't have tall buildings or a large population at the time of the 1811-1812 earthquakes.
The New Madrid seismic zone, which is still capable of producing large earthquakes, zig-zags through Missouri, Arkansas, Tennessee, Kentucky and Illinois.
The infamous New Madrid earthquakes of 1811-1812 rang church bells in Boston, which is 1,200 miles from St. Louis. The New Madrid quakes were also responsible for knocking chimneys down in Cincinnati, Ohio, and lifting the Mississippi River 20 feet, causing it to temporarily run backward.
Rogers and several graduate students have been modeling synthetic seismic events in the New Madrid region. Most of their scenarios are modeled after an 1895 earthquake with a magnitude of 6.4 that was centered in Charleston, Mo.
The preliminary results are sobering, says Rogers, who was recently appointed to Missouri's Seismic Safety Commission by Gov. Matt Blunt. Data indicates ground shaking would be magnified about 600 percent within the flood plain of the Missouri River, a development that would predict soil liquefaction and cause most of Missouri's existing long-span bridges to collapse.
"You don't even need a really big earthquake to do significant damage in the region," Rogers says. "It could happen tomorrow."
A moderate to strong New Madrid earthquake in 2006 would be a national disaster much like Hurricane Katrina, according to Rogers. "But you don't get a warning from an earthquake," he says "At least with a hurricane, you can see the thing coming.
"It's more than just knocking down buildings and structures in St. Louis. The real economic threat to the entire country is the disruption of communication and transportation lines, and underground pipelines, that move through the Midwest."
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