Disposing waste water from oil production into underground wells in Southern California likely contributed to a swarm of earthquakes, according to a new report.

While this appears to be the first time that researchers have found evidence linking oil production to California earthquakes, in recent years other states like Oklahoma have faced dramatic increases in earthquakes that have coincided with fracking and oil booms. Over the past six years the number of earthquakes in Oklahoma has grown from a small handful annually to over 900 last year.

The findings suggest that waste water disposal by oil and gas companies in California should be monitored more closely and potentially limited in certain areas. That’s something that the U.S. oil industry, which has been hit by some of the worst quarterly and annual earnings in decades, would likely push back on.

When companies drill and extract oil and gas from underground wells, salty water often bubbles up in the well. By some estimates, for every barrel of oil that’s made, there’s another 20 barrels of salty waste water that needs to be disposed.

Many companies get rid of that waste water by injecting it into deep underground wells. Scientists are discovering that large volumes of water injected deep in certain areas can increase the water pressure within the underground fractures. The pressure can build up over time and cause a fault deep within the earth to slip. That’s when an earthquake occurs.

The new research, published on Thursday in the journal Geophysical Research Letters, was conducted by a group of scientists from University of California at Santa Cruz, University of Southern California, California Institute of Technology, and others.

The scientists studied an unusual grouping of earthquakes in 2005 in Kern County, California that were larger than magnitude 4.0 and close to both a fault line and several waste water injection sites. Some of the water disposal sites had only started to be used five months before the earthquake swarm.

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Kern County is home to the largest amount of oil production and waste water fluid injection in the state. The volume of waste water injection in that region spiked dramatically between 2001 and 2010.

The scientists found that the amount, the strength, and the location of the earthquakes were unusual enough to have likely been caused by the nearby disposal sites. California has naturally-occurring earthquakes, so people could easily overlook any man-made earthquakes absent closer study.

The pressure of disposal water on underground fractures can be a bit tricky. Scientists focused on Oklahoma found that pressure can build up over large distances, and that earthquakes can occur far from disposal sites, despite the potential connection. The oil and gas industry has used such discrepancies to argue that water disposal isn’t contributing to earthquakes.

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In the report published this week, the researchers noted that there is a larger volume of oil and gas waste water injected into California than into Oklahoma. There are also more injection wells in California, and those injections are deeper underground than in Oklahoma.

These findings are concerning and lead to the conclusion that, like in Oklahoma, waste water disposal sites in California should be monitored, reviewed and potentially limited. Oil companies could either avoid certain injection wells that are causing the pressure build up, lower their rates of waste water injection in certain wells, or inject the water into more shallow wells.