At a Glance The risk of earthquakes in Oklahoma and southern Kansas now equal that of California.

An increase in tremblors in Oklahoma is attributed to hydraulic fracturing, more commonly known as fracking.

Oklahoma and southern Kansas's earthquake risk is now equal to that of California, with 3 million people at risk from man-induced tremblors, according to a new U.S. Geological Survey forecast released Wednesday.

According to the report published in the journal seismological Research Letter , the agency noted that the increase in earthquakes in the region is thought to be the result of hydraulic fracturing, more commonly known as fracking, and the disposal of wastewater deep underground, a byproduct of the oil extraction process.

"Injected fluids cause pressure changes that can weaken a fault and therefore bring it closer to failure," the report notes.

“By understanding the relationship between earthquakes and wastewater injection, informed decisions can be made on processes such as controlling the volumes and rates of wastewater injected and determining which wells are most susceptible to inducing earthquakes,” Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project, said in a press release .

Oklahoma experienced one of its worst years in history for earthquakes, with the largest quake ever recorded near Pawnee, which measured 5.8 on the Richter Scale. In November, a 5.0-magnitude temblor shook the town of Cushing, known as the "Pipeline Crossroads of the World."

<img class="styles__noscript__2rw2y" src="https://dsx.weather.com//util/image/w/1280x720_18.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0" srcset="https://dsx.weather.com//util/image/w/1280x720_18.jpg?v=at&w=485&h=273&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 400w, https://dsx.weather.com//util/image/w/1280x720_18.jpg?v=ap&w=980&h=551&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0 800w" > (USGS) Before 2000, there were only about two earthquakes per year of magnitude-2.7 or greater, but that number has jumped to 2,500 in 2014 and 4,000 a year later.

In 2016, the number decreased to 2,500 earthquakes but scientists attribute that to a decrease in wastewater being injected underground.

Despite the decrease, the USGS report says the forecast earthquake risk in 2017 “is still significantly elevated” compared to the seismic risk before 2009.

“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 CEUS in the year ahead,” Petersen said.

(MORE: Fracking Triggered Hundreds of Earthquakes, Study Shows )

According to the report, the USGS considers a magnitude-2.7 earthquake the level at which ground shaking can be felt. An earthquake of magnitude-4.0 or greater is capable of causing minor or more significant damage.

“Most of the damage we forecast will be cracking of plaster or unreinforced masonry. However, stronger ground shaking could also occur in some areas, which could cause more significant damage," Petersen said.

Environmental groups like the Sierra Club want all fracking stopped. Fracking "is known to contaminate drinking water, pollute the air and cause earthquakes," the Sierra Club notes on its website.

"If drillers can’t extract natural gas without destroying landscapes and endangering the health of families, then we should not drill for natural gas," Allison Chin, Sierra Club president, said.

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