Injecting wastewater into the ground may be an effective way of disposing of salty fluids created through oil and gas production, but it also comes with a perilous side effect: earthquakes. Now, researchers have found that the rate at which we dispose of this liquid may greatly impact the chance that one of these quakes will occur. The faster we inject this water into the ground, the higher likelihood of induced Earth rumblings.

For years, researchers have known that humans can at least partially cause seismic activity. Whenever fluid is added to or withdrawn from the ground, the state of stress on the crust changes. And if that fluid pressure changes inside a fault, friction can decrease, causing the two sides of the fault to push away from each other. Wastewater and fluid injection, dam and reservoir construction, mining, and hydraulic fracturing all contribute to these pressure changes.

Fluid injection wells are important tools for the oil industry. They help dispose of saltwater byproducts from oil and gas activities, as well as pump fluids into existing oil reservoirs — increasing pressure and consequently boosting oil production. These wells are similar to fracking wells, which also inject fluids into the ground to fracture shale rocks and release natural gas. However, wells used for oil recovery inject fluids for years or decades at a time, while fracking wells only inject fluids for a few hours or days.

"The idea was to look at this on a broad scale," study author Matthew Weingarten, a doctoral student at University of Colorado-Boulder, tells The Verge. "It’s really only a smaller subset of injection wells that are linked to earthquakes, so we wanted to try to figure out if there is some kind of well operation that increases the likelihood."

Weingarten and his research team catalogued the production rates of 180,000 injection wells located between Colorado and the East Coast. They found the wells that pumped more than 300,000 barrels of wastewater into the ground per month were statistically associated with earthquakes. Wells that pump that much wastewater are considered "high rate"; about 18,000 wells — mostly in Oklahoma and Texas — were associated with the most seismic activity.

"We understand the physics of induced seismicity pretty well," says Weingarten. "If you increase absolute pressure, you’re going to have an induced earthquake; the physics show these high-rate injection wells preserve the ambient fluid pressure over a larger magnitude than lower-rate wells."

Weingarten notes that numerous factors were considered, including the wells’ total volume production and proximity to fault lines, but injection rates stood out. He suggests policy makers and regulators could maybe manipulate these rates to lessen induced earthquakes, though he isn’t making any formal recommendations.

Financial incentives may hinder attempts to limit the rate of wastewater injection. Typically, only 30 percent of oil can be extracted from a mature reservoir, but fluid injections increase oil recovery to between 50 and 70 percent. Still, Choy agrees that lowering injection rates may be something to consider, as the number of earthquakes associated with injection wells have increased over the past few decades. "Since this is a process that appears to be controllable, you could mitigate the effects. Obviously there’s some economic imperative to continue the process, but the least you can do is try to [control the rate]."