July 29, 2010

Midwest At Greater Risk Of Earthquakes Than Thought

Researchers said on Wednesday that the risk of earthquakes in the U.S. Midwest may be more widespread than geologists had previously thought. 

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

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

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

Eric Calais of Purdue University in Indiana, along with colleagues, developed a computer model that shows this could have caused the crust beneath to slowly lift and cause the magnitude 7 earthquakes that shook the Missouri-Arkansas region in 1811 and 1812. 

"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.

Geophysicist Mark Zoback of Stanford University in California wrote that areas like Charleston, South Carolina may be susceptible to more activity caused by the processes.

Scientists have a good understanding of earthquakes at major faults where one of the Earth's tectonic plates touches another.

The intraplate faults are less understood, such as 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.

However, 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 these types of faults.


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