The study found that the frequency of earthquakes started rising in 2001 across a broad swath of the country between Alabama and Montana. In 2009, there were 50 earthquakes greater than magnitude-3.0, the abstract states, then 87 quakes in 2010. The 134 earthquakes in the zone last year is a sixfold increase over 20th century levels.

The surge in the last few years corresponds to a nationwide surge in shale drilling, which requires disposal of millions of gallons of wastewater for each well. According to the federal Energy Information Administration, shale gas production grew, on average, nearly 50 percent a year from 2006 to 2010.

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