By Emily Atkin, ThinkProgress

A team of scientists with the U.S. Geological Survey have found evidence “directly linking” the uptick in Colorado and New Mexico earthquakes since 2001 to wastewater injection, a process widely used in the controversial technique of hydraulic fracturing, or fracking, and conventional drilling.

In a study to be published in the Bulletin of the Seismological Society of America on Tuesday, the scientists presented “several lines of evidence [that] suggest the earthquakes in the area are directly related to the disposal of wastewater” deep underground, according to a BSSA press release. Fracking and conventional natural gas companies routinely dispose of large amounts of wastewater underground after drilling. During fracking, the water is mixed with chemicals and sand, to “fracture” underground shale rock formations and make gas easier to extract.

The USGS research is just the latest in a string of studies that have suggested the disposed water is migrating along dormant fault lines, changing their state of stress, and causing them to fail.

For their research, the four California-based USGS scientists monitored the 2,200 square mile Raton Basin, which goes from southern Colorado into New Mexico. They pointed out that the Basin had been “seismically quiet” until 1999, when companies began “major fluid injection” deep into the ground. Earthquakes began in 2001 when Colorado wastewater injection rates were under 600,000 barrels per month, and and since then there have been 16 earthquakes that could be considered large (above a magnitude of 3.8, including two over a 5.0 magnitude), compared with only one — a 4.0 magnitude quake — in the 30 years prior.

“These earthquakes are limited to the area of fluid injection, they occur shortly after major fluid injection activities began, and the earthquake rates track the fluid injection rates in the
Raton Basin,” the paper said, noting the scientists’ comparisons of the timing and location of earthquakes with the timing and location of injected wastewater. By the mid-2000s, Colorado’s wastewater injection rates were up to 1.9 million barrels per month.

Taking that and the unexpected frequency of the earthquakes into consideration, the paper noted that it was “highly unlikely” that the quakes could have been due to any random fluctuations underground.

“Detailed investigations of two seismic sequences places them in proximity to high-volume, high-injection-rate wells, and both sequences occurred after a nearby increase in the rate of injection,” the study’s accompanying press release said. “A comparison between seismicity and wastewater injection in Colorado and New Mexico reveals similar patterns, suggesting seismicity is initiated shortly after an increase in injection rates.”

The study does note that despite the strong and direct link, the findings are not definitive, echoing language often used by climate scientists to describe why it’s nearly impossible to say that individual weather events are caused by climate change. “Although there are many lines of evidence showing that the seismicity in the Raton Basin has been induced by wastewater injection activities in the area, it is very difficult to say whether an individual earthquake was caused by injection because natural seismicity has also been recorded there,” the study says. “For future research, a longer-term study with dense network coverage on both sides of the border would be especially useful in understanding the inducing relationship between the earthquakes and fluid injection in the Raton Basin.”

The U.S. government announced back in May that it would begin to track the risks that so-called “frackquakes” pose, and start including them on official maps that help influence building codes. Before then, the USGS had never taken man-made earthquakes into account during its regular quake mapping activity. It made the decision to do so after finding that two strong earthquakes in heavily-drilled areas of Colorado and Oklahoma in 2011 might have been the result of wastewater injection. “For future research, a longer-term study with dense network coverage on both sides of the border would be especially useful in understanding the inducing relationship between the earthquakes and fluid injection in the Raton Basin.”

Since then, drilling for natural gas and fracking has proliferated across the country, as have earthquakes in the places where those booms are occurring. Oklahoma, a hotbed for fracking, is currently experiencing anywhere from 5 to 20 small earthquakes every day, according to the state’s Geology Survey. What’s more, Cornell University scientists have linked more than 2,500 small earthquakes that have hit Oklahoma in the past five years to the wastewater disposal process.

These quakes are usually too small to be felt, but scientists have warned that they stand to get stronger as more wastewater injection happens — a likelihood considering the growing expansion of fracking.