It was a few minutes past 7:00 am on September 3, 2016, when the earth started shaking in Pawnee, Oklahoma. Goods fell from grocery store shelves, and sandstone facades tumbled to the ground.

The quake was the largest recorded in Oklahoma history, measuring a magnitude 5.8. The magnitude of an earthquake is measured as the size of energy released at the quake’s source.

Oklahoma has had a serious increase in earthquakes over the past several years, an uptick attributed to oil and gas companies re-injecting water from drilling operations back into the ground. Geologists have linked the practice to hundreds of quakes across the state.

Soon after the quake struck, state regulators ordered well operators in the immediate area to shut down operations. But the exact cause of the quake remained uncertain.

Now, geologists have another piece of the puzzle.

In a paper published in Seismological Research Letters on Tuesday, geologists gathered data from injection wells in the area and used computer models to see how the water injected into the surface affected underlying faults.

Oil and gas drillers in the area typically inject wastewater into a porous layer of rock that sits on top of a much harder layer of granite. The harder granite contained the fault that slipped in the Pawnee earthquake in September. Researchers already knew that pumping water underground increases the amount of pressure underground, adding stress to a fault.

pawnee earthquake map
The epicenter of the 5.8 magnitude Pawnee earthquake (star), and shaking. USGS

“As pore fluid pressures increase, the fault might destabilize and eventually produce an earthquake well before it would have happened naturally,” says lead author Andrew Barbour, a research geophysicist with the USGS. “What we’re showing is in addition to that fluid pressure increase at that fault, the rate of injection can also enhance that effect.”

Barbour and colleagues found that three years before the 2016 earthquake, in 2013, several wells began injecting wastewater at a fast clip. Barbour isn’t sure why there was a need for speed at these wells, but his model shows that the speed might have pushed the earthquake to happen sooner.

“The rate at which you inject a fixed volume does effect the end result,” Barbour says. “The changes in stress along a fault become stronger and more rapid when you inject at a more rapid pace.”

Constant rates of fluid injection at other wells in the area prior to 2013 also played a role, Barbour says. The model shows that while the more steady injection sites helped increase the stress over time, the sudden increase and then abrupt decrease in pumping rates at the more variable wells helped jolt the seismic system into action.

Researchers are studying events like Pawnee intensely in order to better understand how human-induced earthquakes occur. Some groups are deploying additional sensors to monitor future quakes, and researchers like Barbour are looking to physics and computer models to try and understand not only how human activity is causing these disasters, but how to identify potential warning signs as well.