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

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

The 2017 forecast decreased compared to last year because fewer earthquakes occurred in 2016 than in 2015, the USGS said. Wastewater injection may have decreased in 2016 due to 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.

Despite the decrease in the overall number of earthquakes in 2016, Oklahoma had its largest earthquake ever recorded in the state (a 5.0 magnitude quake that hit Cushing) as well as the greatest number of large earthquakes, the USGS said.

An industry group said the dropoff in earthquakes was a positive sign: “The lower risk of induced seismicity is a clear sign that the 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.

"In Oklahoma, the number of earthquakes last year dropped by 31%, and there were zero felt earthquakes in North Texas in 2016," she said. "This study is in line with what numerous experts have said: the risk of induced seismicity is small, rare, and manageable.”

The USGS report also said that 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.

This is the second year that the USGS has released an earthquake forecast.

This research was published Wednesday in Seismological Research Letters.