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 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.

Despite the decrease in the number of quakes in 2016, a 5.8-magnitude quake rocked Pawnee, Okla., the state's largest quake ever recorded. In addition, the state recorded the highest number of large earthquakes,of magnitude 4 or greater USGS reported.

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.

“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.

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.

This is the second year that the USGS has released an earthquake forecast. The research was published Wednesday in Seismological Research Letters.