In less than 28 hours, Oklahoma has been pummeled by earthquakes.

The wave started on Tuesday night, when five quakes struck the central part of the state, and extended into the early hours of Thursday as two more hit, according to the United States Geological Survey.

All of the earthquakes were between magnitude two and five and no significant damage was reported. However, the shaking is part of a troubling recent phenomenon. The disposal of wastewater from hydraulic fracturing (also known as fracking), appears to have spiked the likelihood of earthquakes in Oklahoma, potentially raising the state to the same earthquake threat level as California, according to a recent USGS forecast.

Until recently, earthquakes in Oklahoma were few and far between. In 2010, the state experienced just 41 tremors. By comparison, each year the southern California area alone has about 10,000 earthquakes.

But "seismic activity has surged in [Oklahoma] in recent years," reporter Joe Wertz of StateImpact Oklahoma told NPR on Thursday. In the last few years, Oklahoma has weathered hundreds of significant quakes each year, along with parts of several other Midwestern states.

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7 PHOTOS Fracking See Gallery Fracking Workers at a Statoil USA site in Texas's Eagle Ford shale formation dig out ground in mid-June to prepare to tie in a pipeline extension for a fracking project into an existing pipeline, in Runge, Texas, U.S, June 7, 2016. Producers have been hoping for a sustained rally in crude oil to support more drilling. REUTERS/David Gaffen A fracking rig flares gas during an anti-fracking protest by Greenpeace activists outside the Houses of Parliament in London, Britain February 9, 2016. REUTERS/Stefan Wermuth CRESCENT, OK - MARCH 31: Oil field workers pull pipes from a fracking well March 31, 2016 near, Crescent, Oklahoma. The United States Geological Survey estimates that more than 7 million people live in areas at risk of human-induced earthquake damage. (Photo by J Pat Carter/Getty Images) Local residents and members of Resist AIM Pipeline standing on the sidewalk at the entrance of a pipeline construction site demanding the halting of the project as requested by Governor Andrew Cuomo. (Photo by Erik McGregor/Pacific Press/LightRocket via Getty Images) People protest against fracking and neighborhood oil drilling in Los Angeles, California, U.S., May 14, 2016. REUTERS/Lucy Nicholson Anti-fracking protesters write messages on a wall with chalk during a demonstration outside County Hall in Preston, Britain June 24, 2015. Lancashire County Council is debating an application by shale gas firm Cuadrilla Resources to frack on the Fylde coast, local media reported. REUTERS/Andrew Yates People protest against fracking and neighborhood oil drilling in Los Angeles, California, U.S., May 14, 2016. REUTERS/Lucy Nicholson Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

"Scientists link the quake boom to the widespread oil industry practice of pumping waste fluid into underground disposal wells," Wertz told NPR.

The rise in earthquakes, in other words, can be attributed to the injection of large quantities of wastewater into wells deep below the Earth's surface. According to USGS, the majority of the underground wastewater comes from oil and gas operations — it is created when clean water mixes with dirt, metals, and other toxins below the Earth's surface during extraction operations.

The contaminated water becomes too dangerous to dump anywhere, since it could seep into regular groundwater, so companies shoot the wastewater deep into the earth, between layers of hard rock. That buried water can fracture and move previously stable rock, causing earthquakes under certain circumstances.

Much (but not all) of that wastewater injection is associated with the fracking boom, which has led the practice to become more common in recent years, especially in Oklahoma. Although the state isn't the only one experiencing a spike in wastewater injections, Oklahoma is full of eons-old fault lines that went quiet long ago. Wastewater operations seem to be shaking some of those faults loose, making the land especially vulnerable to earthquakes.

The 2017 USGS predictions for Oklahoma are actually less intense than they were in 2016, because earthquakes were somewhat less frequent there than expected last year. USGS scientists suggest that might be due to stricter regulations that have been put in place to control wastewater injection.

Environmental Protection Agency administrator Scott Pruitt previously served as Oklahoma's attorney general, and has come under fire from some in the state for failing to take action against wastewater injection and fracking.

Rafi Letzter contributed to an earlier version of this post.

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