A federal report says oil and gas production may explain a sharp increase in earthquakes in the nation's midsection.

The researchers found the rate of quakes has jumped six-fold from the late 20th century through last year, with a particularly sharp rise from 2009 to 2011. That includes mild quakes.

The researchers said the timing and geography suggest some kind of link to oil and gas production, but it's not clear why the two might be related.

Some previous work has linked earthquakes to injecting huge amounts of wastewater deep into the earth.

The study from the U.S. Geological Survey has not been published, but it is scheduled to be presented at a scientific meeting later this month.

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