Doyle Rice

USA TODAY

Three million Americans, primarily in Oklahoma and Kansas, are at risk from human-induced earthquakes this year, the U.S. Geological Survey said Wednesday.

That's the conclusion of a new report that cites wastewater disposal from fracking as triggering the quakes. The number of Americans affected this year is less than last year, when the agency reported 7 million were at risk.

The drop is the result of fewer earthquakes occurring in 2016 than in 2015, the USGS said. Wastewater injection may have decreased in 2016 as a result of new regulations for its disposal, or slowed due to lower oil prices and less overall production.

“The good news is that the overall seismic hazard for this year is lower than in the 2016 forecast, but despite this decrease, there is still a significant likelihood for damaging ground shaking in the U.S. in the year ahead,” said Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project.

Oklahoma hit with 70 quakes in a week

Despite the decrease in the number of such earth-shaking events last year, Pawnee, Okla., was rocked by a 5.8-magnitude earthquake, the state's largest ever recorded. In addition, the state logged the highest number of large earthquakes, of magnitude 4 or greater, USGS reported.

During the fracking process, waste water is collected and later disposed of by injecting it into deep underground wells at high pressure. That water fills pores in dormant faults, causing them to slip and unleash the quakes, according to the USGS.

Oklahoma earthquake reignites concerns that fracking wells may be the cause

The drop-off in earthquakes is a positive sign that "collaborative efforts between industry, scientists, and regulators are working," said Katie Brown, a spokesperson for Energy In Depth, a program of the Independent Petroleum Association of America, a trade group.

However, the forecast for the number of human-induced and natural earthquakes this year is "hundreds of times higher than before induced seismicity rates rapidly increased around 2008,” Petersen said. “Millions still face a significant chance of experiencing damaging earthquakes, and this could increase or decrease with industry practices, which are difficult to anticipate.”

Environmental groups such as the Sierra Club and the Natural Resources Defense Council want all fracking curtailed. Fracking "is known to contaminate drinking water, pollute the air, and cause earthquakes," the Sierra Club says on its website.

The USGS report said an additional half million people face potential damage from natural earthquakes in 2017, which brings the total number of Americans at risk from both natural and human-induced earthquakes to 4 million for the year.

The report, published Wednesday in Seismological Research Letters, marks the second year the USGS has released an earthquake forecast.

Contributing: Rick Jervis, USA TODAY