Dateline: June 30, 2014

The average annual number of magnitude 3 and greater earthquakes in the central United States is now six times higher than at any time during the past century, according to the U.S. Geological Survey (USGS), where scientists are investigating the possibility that at least some of the small quakes might actually be manmade.

What concerns the USGS is that some, if not most, of the earthquakes are being triggered by the long-used, but recently controversial oil exploration and production practice of fracturing, or "fracking."

In fracking, large volumes of water mixed with various chemicals are injected into the earth under high pressure literally fracturing rock formations in order to release trapped oil and gas. Wastewater escaping during fracking is typically disposed of by injected it one or more deep disposal wells.

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It is these wastewater disposal wells rather than the fracking process itself SGS suspects may be inducing the small earthquakes.

In fact, says the USGS, the potential for injected wastewater to cause small earthquakes has been known since the early 1960s.

"In preliminary findings, our scientists cite a series of examples for which an uptick in seismic activity is observed in areas where the disposal of wastewater through deep-well injection increased significantly," stated the USGS in a press release. "These areas tend to be in the middle of the country - mostly in Colorado, Texas, Arkansas, Oklahoma and Ohio."

On the other hand, the USGS found that not all oilfield wastewater disposal wells induce earthquakes. Out of some 40,000 wastewater disposal wells drilled for fracking operations, only a "tiny fraction" has induced earthquakes large enough to be a danger to the public.

"These earthquakes are fairly small -- large enough to have been felt by many people, yet small enough to rarely have caused damage," noted the USGS.

But if oilfield wastewater disposal wells can trigger small earthquakes that rarely cause damage, couldn't they also trigger major earthquakes than cause devastation?

While the USGS says it cannot eliminate the possibility and more research is needed, there have been no conclusive examples of wastewater injection triggering major earthquakes, even when located near known faults.