In the United States, most earthquakes happen along the West Coast, especially in California, because of major fault lines in Earth's crust. Oklahoma, in the middle of the country far from any major fault line, historically had almost none. From the 1970s through 2008, Oklahoma averaged only about 2 earthquakes of magnitude 3.0 or greater per year.
Then the numbers started climbing. In 2009, Oklahoma recorded 20 earthquakes. By 2010, there were 42. The count jumped to 109 in 2013, then exploded to 585 in 2014. In 2015, Oklahoma hit 903 earthquakes, making it the most seismically active state in the continental United States — surpassing California. The largest event, a magnitude 5.8 earthquake near Pawnee on September 3, 2016, was felt across seven states.
When scientists mapped the earthquakes, they noticed a striking pattern. The quakes were not spread randomly across the state. Instead, they clustered near high-volume wastewater disposal wells used by the oil and gas industry. These wells inject large amounts of wastewater deep underground. Researchers found that earthquake frequency tracked closely with the volume of wastewater being injected into these deep wells.
The injected wastewater increases pore pressure — the pressure of fluid inside tiny spaces in underground rock. This added pressure pushes against existing fault lines that had been stable for thousands of years, reducing friction until the faults slip and produce an earthquake.
Imagine pushing a heavy box across the floor. The box stays still because friction holds it in place. Now imagine someone squirts slippery soap under the box. The soap reduces the friction, and the box suddenly slides. That is similar to what happens underground. Fault lines are cracks in rock that stay locked in place because of friction. When wastewater is pumped deep underground, it seeps into tiny spaces in the rock and pushes outward. This added pressure — called pore pressure — acts like the soap. It reduces friction on the fault until the rocks suddenly slip, causing an earthquake.
In 2015 and 2016, the Oklahoma Corporation Commission ordered oil and gas companies to reduce the volume of wastewater injected at disposal wells in affected areas. The earthquake rate responded. By 2017, the count dropped to 304. By 2019, it was down to 81. The decline in earthquakes roughly matched the reduction in injection volume, reinforcing the connection scientists had identified in the data.