After several minor-to-moderate earthquakes shook the U.S. interior in 2011, a number of reports suggested a link between hydraulic fracturing—a technique used to extract natural gas from shale-gas deposits—and increased seismic activity in areas typically not prone to such events.

Released on June 15, a study by the National Academy of Science's National Research Council (NRC) concludes, however, that hydraulic fracturing alone has a low risk for inducing earthquakes that can be felt by people. However, the underground injection of wastewater produced by hydraulic fracturing and other energy technologies do have a higher risk of causing such earthquakes, the study finds. Moreover, carbon capture and storage may have the potential for inducing more significant seismic events, because significant volumes of fluids are injected underground over long periods of time, the study authors conclude.

Hydraulic fracturing, commonly known as fracking, extracts natural gas by injecting a mixture of water, sand and chemicals in short bursts at high pressure into deep underground wells, which allows natural gas to be released from the shale and flow up the wells.

The researchers call for more investigation, particularly regarding the potential for induced seismicity in large-scale carbon capture and storage projects. They also recommend that federal and state agencies better coordinate their responses to seismic events related to energy development. There is a risk that the number of earthquakes could rise as drilling for oil and gas increases.

Sponsored by the Dept. of Energy, the study was performed at the request of Sen. Jeff Bingaman (D-N.M.), chairman of the Energy and Natural Resources Committee.

At a June 19 hearing before the committee, Murray Hitzman, a professor at the Colorado School of Mines and the lead author of the report, said about 35,000 hydraulically fractured shale-gas wells exist in the U.S. In this country, only one case of "felt induced seismicity" has been identified in which fracking "is suspected." Globally, there has been only one case—in Blackpool, England—in which a causal relationship was confirmed. "The very low number of felt events relative to the large number of hydraulically fractured wells for shale gas is likely due to the short duration of injection of fluids and the limited fluid volumes used in a small spatial area," Hitzman says.

However, a few seismic events in 2011 were linked to wastewater disposal wells, Hitzman adds. Examination of these cases suggests a causal link between the injection zones and previously unrecognized faults underneath the ground.

Mark Zoback, a professor at Stanford University's department of geophysics who did not participate in the NRC study but who testified at the hearing, adds, "I believe with proper planning, monitoring and response, the occurrence of small-to-moderate earthquakes associated with fluid injection can be reduced and the risks associated with such events effectively managed."

Zoback notes that in the Marcellus shale region, wastewater typically is recycled, and that could be a potential solution to avoiding the problems associated with injecting the fluid underground, as is common practice in Texas and other states.

But large-scale carbon capture and storage (CCS), which the Obama administration is counting on as part of its overall greenhouse-gas reduction strategy, could prove problematic, Zoback believes.

The issue is not whether carbon dioxide can safely be stored underground but whether there is enough geological capacity to store sufficient volumes of carbon dioxide to have a beneficial impact on climate change. "It must be recognized that large-scale CCS will be an extremely expensive and risky strategy for achieving significant reductions in greenhouse-gas emissions," Zoback says.