Study changes picture of U.S. quake hazards

A view of damage caused by the historic Charleston earthquake of 1886. Picture taken on the south side of Broad Street.

WASHINGTON (Reuters) - The risk of earthquakes in the U.S. Midwest may be more widespread than geologists have believed, but a "big one" may be less likely at Missouri's New Madrid fault, researchers said on Wednesday.

They found that rivers that swept away sediments at the end of the last ice age could have triggered a series of large earthquakes that began in 1811 in the New Madrid seismic zone.

This suggests that these fault segments are unlikely to fail again soon, but the same process could trigger earthquakes on nearby fault segments, they reported in the journal Nature.

When glaciers melted at the end of the last ice age between 16,000 and 10,000 years ago, monstrous rivers formed and washed away 40 feet of sediment.

Eric Calais of Purdue University in Indiana and colleagues developed a computer model that shows this could have caused the crust underneath to slowly lift and cause the magnitude 7 and greater quakes that shook the Missouri-Arkansas border region in 1811 and 1812, causing the Mississippi River to run backwards and ringing church bells as far away as Boston.

"Models indicate that fault segments that have already ruptured are unlikely to fail again soon, but stress changes from sediment unloading and previous earthquakes may eventually be sufficient to bring to failure other nearby segments that have not yet ruptured," Calais and colleagues wrote.

Areas such as Charleston, South Carolina, hit by a highly damaging quake in 1886, may be susceptible to more activity cased by the processes described by Calais, geophysicist Mark Zoback of Stanford University in California wrote in a commentary.

Scientists have a good understanding of earthquakes at major faults where one of the Earth's tectonic plates touches another one -- such as in California, Indonesia and Haiti.

Less well understood are intraplate faults -- faults in the middle of a plate -- like the New Madrid fault.

"Much still needs to be done to reduce earthquake hazards for those living along active plate boundaries. To recognize that, one needs only to look at the devastating consequences of the 2004 earthquake and tsunami in Sumatra (230,000 dead in 14 countries), or the earthquake in Haiti earlier this year (approximately 200,000 dead and 2 million left homeless)," Zoback added.

But the uncertainty can be even worse in intraplate regions. "In the past decade alone, tens of thousands of people have died in each of the earthquakes that hit Bhuj, India (2001), and Bam, Iran (2003), as well as in the magnitude 7.9 Wenchuan event that occurred in China in 2008," Zoback wrote.

He said the New Madrid seismic zone is the best studied of such locations.

(Reporting by Maggie Fox, Editing by Sandra Maler)