U.S. Geological Survey scientists said the frequency of earthquakes of magnitude-3 or greater increased by a factor of six in the decade beginning in 2001.
"While it appears likely that the observed seismicity rate changes in the middle part of the United States in recent years are man-made, it remains to be determined if they are related to either changes in production methodologies or to the rate of oil and gas production," the U.S. Interior Department stated.
Scientists said they found a link between the injection of wastewater from the development of shale natural gas into deep disposal wells. This practice could increase pressure on existing faults deep underground.
Interior Department Deputy Secretary David Hayes said the accuracy of recent media reports on the link between fracking and earthquakes "varied greatly." The Interior Department notes that, despite recent fervor, temblors associated with wastewater injection were first recorded in the 1960s.
Only a "tiny fraction" of the estimated 40,000 waste fluid disposal wells in the United States have induced temblors large enough to warrant concern, the Interior Department states.
"We also find that there is no evidence to suggest that hydraulic fracturing itself is the cause of the increased rate of earthquakes," said Hayes.