Not long ago earthquakes in Oklahoma were rare. Not anymore. Twenty earthquakes of 3.0 magnitude or greater shook Oklahoma in 2009. The state has seen a 40-fold increase in seismicity since 2008. The cause? Humans, according to new research in Science. The study confirmed what geologists have been speculating for years: that underground water disposal by oil and gas companies causes earthquakes.

Millions of gallons of wastewater are produced every month in Oklahoma as a result of extracting oil and natural gas from the ground. The companies inject this wastewater into wells to dispose of it, which raises the groundwater pressure and can stress geologic faults. “Normally, earthquakes occur naturally because of plate motions,” says Katie Keranen, a geophysics professor at Cornell University and the study's lead author. “But if you inject enough water into the earth, you can influence the cycle of earthquakes.”

After using hydrogeologic models with the seismic data, Keranen and her colleagues concluded that four disposal wells southeast of Oklahoma City likely caused the Jones swarm, a group of earthquakes that accounted for 20 percent of seismicity in the central and eastern U.S. between 2008 and 2013. The team also found that wastewater injection induced earthquakes as far as 30 kilometers away from the wells, much farther than previously thought.

Geologists have used the term “induced seismicity” to describe earthquakes triggered by mining, dams, underground nuclear tests and wastewater injection. As oil and gas extraction methods become more common and more studies connect their disposal methods to seismic activity, the term will be one to watch.