Scientists may have overestimated the risk of a future devastating earthquake striking near the town of New Madrid, Missouri, according to a report in today's issue of Science. The study shows that the crust in that region has buckled too little to portend a monster quake anytime soon.
A series of violent earthquakes rocked the New Madrid area in 1811 and 1812. Based on witness reports, scientists have estimated the temblors to have measured about 8 on the Richter scale. From studies of sand blows--columns of sand that shoot through the soil during an earthquake--they also deduced that similar-magnitude quakes had occurred some 400 and 800 years ago. These and other studies led scientists to deduce that magnitude-8 earthquakes happen at New Madrid every 500 to 1000 years. Such energetic tremors would cause even very stable structures to crumble and would be felt as far away as Chicago.
Seismologist Seth Stein and his student Andrew Newman at Northwestern University in Evanston, Illinois, and their colleagues decided to reevaluate that risk. In 1991, 1993, and 1997 they recorded satellite radio signals at 23 "monuments" along the fault zone--brass plaques fixed to rock, or stainless steel rods driven into the ground. They found that the monuments had barely budged, having moved at most 2 millimeters per year. Because the magnitude of an earthquake depends on the speed at which a tectonic plate slips across a fault line, the team calculated that a magnitude 8 earthquake would occur less than once every 2500 years. However, Newman notes, a quake of magnitude 7 could occur every 500 to 1000 years.
"This is an important paper," says Lanbo Liu, a geophysicist at the University of Connecticut, Storrs, who has also studied the New Madrid seismic zone. "As the quality of data available becomes better, the slip rate [across the fault] becomes lower than previously thought." However, Liu is cautious about making any definitive statements about the earthquake risk, because he says the slip rate can vary over decades.