Earthquakes are becoming a lot more frequent in America, particularly in Oklahoma. Two new studies suggest the wastewater from oil and gas production that is injected into the ground is to blame.

The first study, published in Science, found earthquakes were far more likely to be associated with ‘high rate’ injection, which pumped 300,000 barrels of wastewater a month into the ground, compared to lower-rate injection wells. The second study, published in Science Advances, found that Oklahoma’s increasing number of earthquakes was linked with the injection of wastewater into the 2,100-meter-deep (7,000 feet) sedimentary formation known as the Arbuckle formation. Nature’s Alexandra Witze says the studies “provide the strongest evidence yet that oil and gas companies have caused a rash of earthquakes.”

The Science paper assembled a database of roughly 180,000 injection wells in an area that ranged from Colorado to the East Coast and compared that to the data on 7,175 earthquakes, which occurred between 1973 and 2014. Researchers linked wells quakes that had happened within 15 kilometers (9 miles) and found more than 18,000 wells were associated with earthquakes.

“We saw an enormous increase in earthquakes associated with these high-rate injection wells, especially since 2009, and we think the evidence is convincing that the earthquakes we are seeing near injection sites are induced by oil and gas activity,” said Matthew Weingarten, who led the study, in a statement.

For the second study, researchers analyzed three areas that have experienced the greatest amount of seismic activity in Oklahoma. While the state had one or two magnitude 4 earthquakes per decade from 1882 through 2009, this rate increased to 24 in 2014. Researchers concluded that increases in wastewater disposal resulted in increased earthquakes, but there was no observed increase in the number of quakes in control areas, which did not have much wastewater disposal. While these quakes have damaged some homes and businesses, the vast majority were not felt.

"What we've learned in this study is that the fluid injection responsible for most of the recent quakes in Oklahoma is due to production and subsequent injection of massive amounts of wastewater, and is unrelated to hydraulic fracturing," Mark Zoback, the Benjamin M. Page Professor in the School of Earth, Energy & Environmental Sciences, said in a statement.

Researchers hope that the new work could help find solutions to the rapid increase in earthquakes. Zoback points out, however, that so much water has been injected that the pressure continues to spread throughout the Arbuckle formation, so “the earthquakes won't stop overnight, but they should subside over time.”

"People can't control the geology of a region or the scale of seismic stress. But managing rates of fluid injection may help decrease the likelihood of induced earthquakes in the future,” Weingarten added.