Oil and gas production may explain a sharp increase in small earthquakes in the nation's midsection, a new study from the U.S. Geological Survey suggests.

 

The rate has jumped six-fold from the late 20th century through last year, the team reports, and the changes are "almost certainly man-made."

 

Outside experts were split in their opinions about the report, which is not yet published but is due to be presented at a meeting later this month.

 

The study said a relatively mild increase starting in 2001 comes from increased quake activity in a methane production area along the state line between Colorado and New Mexico. The increase began about the time that methane production began there, so there's a "clear possibility" of a link, says lead author William Ellsworth of the USGS.

 

The increase over the nation's midsection has gotten steeper since 2009, due to more quakes in a variety of oil and gas production areas, including some in Arkansas and Oklahoma, the researchers say.

 

It's not clear how the earthquake rates might be related to oil and gas production, the study authors said. They note that others have linked earthquakes to injecting huge amounts of leftover wastewater deep into the earth.

 

There has been concern about potential earthquakes from a smaller-scale injection of fluids during a process known as hydraulic fracturing, or fracking, which is used to recover gas. But Ellsworth said he is confident that fracking is not responsible for the earthquake trends his study found, based on prior studies.

 

The study covers a swath of the United States that lies roughly west of Ohio and east of Utah. It counted earthquakes of magnitude 3 and above.