The risk of man-made earthquakes will decline slightly in 2017, though parts of the central United States remain threatened, according to a new report.

The study from the United States Geological Survey (USGS) finds that 3.5 million people — primarily in Oklahoma and southern Kansas — live in an area vulnerable to significant damage from a man-made earthquake. An additional half million people live in areas vulnerable to damage from natural earthquakes.

Man-made earthquakes are primarily the result of wastewater injection, a process where the liquid byproduct of fracking is disposed of deep underground. The frequency of earthquakes in the central U.S. has increased hundreds fold since the dawn of widespread fracking nearly a decade ago.

Read more: See How Oil Drilling Created an Earthquake Crisis in Oklahoma

Regulations on wastewater injection and declining oil and gas prices both likely contributed to the decline in earthquakes, according to Mark Petersen, who heads the USGS National Seismic Hazard Mapping Project. “I don’t know when or if it will go back to the pre-2008 era,” he said on a conference call for journalists. “But I think it’s hopeful that we can considerably decrease the number of earthquakes in that region.”

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Write to Justin Worland at justin.worland@time.com.