April 18, 2008
By Douglas Wong
St. Louis Post-Dispatch
When St. Louis residents were awoken this morning by the tremors from the earthquake that struck near the southern Illinois town of Bellmont, many probably wondered: Is this the Big One?
But as the tremors passed, mostly leaving only rattled nerves, St. Louisans returned to their normal status of wondering when another New Madrid type quake will hit the region.Almost everyone has heard stories of the seismic event centered near the Missouri Bootheel town that produced the largest earthquakes ever measured in North America, between December 1811 and January 1812.
The stories range from fact to legend: that the Mississippi flowed backward, church bells ringing across the country. However, no one disputes the fact that the quakes occured and may occur again.
The U.S. Geological Survey has projected there is a 25 percent to 40 percent chance of a magnitude 6.0 earthquake along the New Madrid Fault in roughly the next 50 years, and up to a 10 percent chance during that time of an earthquake similar in strength to the ones in 1811-1812.
So when beds started shaking this morning, the natural reaction was to think we may be experiencing that once in lifetime event.
But seismologist say the 5.2 magnitude quake with an epicenter 128 miles east of St. Louis, was just a moderate quake. And the quake wasn't part of the New Madrid fault. A similar event occurred in the 1968 when a magnitude 5.5 earthquake struck Southern Illinois.
For most people in the region, the shaking lasted less than 10 seconds. But others felt it a bit longer.
This is explained by the various soil and bedrock types in the region.
In April 2006, Post-Dispatch reporter Eric Hand wrote that squishy river valley soils in St. Louis can amplify earthquake waves by as much as a factor of 10 -- perhaps more than any other place in the world. Other areas, with bedrock, don't feel the amplified effects.
"Where you build has everything to do with how much you get shaken," J. David Rogers, a University of Missouri at Rolla engineer, said in that story. Rogers had developed models comparing the effects of amplification on structures in the region.
