In January 2015, there were a dozen little earthquakes in the Dallas–Fort Worth area in less than 48 hours. They weren’t the first, or the last, that would strike the region, either. Prior to 2008, there had been no reported earthquakes in the Fort Worth Basin. In the ten years since, there’ve been hundreds.

The biggest geological difference in North Texas in the years since 2008 has been the amount of hydraulic fracking and horizontal drilling going on in the area. Both processes, which allow access to oil further below the surface, are often followed by wastewater injection into underground formations through injection wells, which has been known for decades to cause earthquakes—a 1990 study from the Environmental Protection Agency makes that clear—but the fact that there’s a causal relationship between fluid injection and earthquakes doesn’t necessarily mean that every earthquake that happens in a drilling- and fracking-intensive area is automatically the result of fracking and drilling.

That’s what the Texas Railroad Commission—which regulates the oil and gas industry—has pointed to in the past. The Dallas Morning News reported on the agency’s approach to fracking last year, quoting commissioner Ryan Sitton:

As for the link between quakes and disposal wells in Texas, he said, “Unfortunately in the world of science and research, we very rarely have things that we consider to be conclusive.” So far, he added, “there’s been a large amount of assumptions, simplistic analysis and hypothesis in place of real data.”

When the U.S. Geological Survey—a federal agency whose responsibilities includes studying earthquakes—categorized the North Texas quakes since 2008 as man-made back in September, a Railroad Commission spokesperson told the Morning News that it hadn’t yet had a chance to review the studies that led to that conclusion.

In a recent statement, the Railroad Commission said, “Our rules are designed to prevent or minimize any risk, as well as give us the necessary authority to amend permit conditions up to and including shutting a well down as a result of seismicity.” The RRC regulates disposal wells and its rules for “seismicity-related” disposal wells went into effect in 2014. As part of this rules enforcement, the commission has required almost half of the newly permitted wells in these areas to reduce maximum daily injection volumes and pressure or to record volumes and pressures daily (instead of monthly).

As of this week, they’ll need to take one more study under advisement. According to a landmark study published in the peer-reviewed journal  Science Advances, the earthquakes in North Texas have been occurring in faults that, geologically, are considered “dead.”

The study used an ultrasound technique to trace the history of the fault lines in the Fort Worth basin dating back 450,000,000 years. These “seismic reflections” created data images they could use to look at the rock layers to determine how long it had been since the faults were active. According to SMU seismologist Beatrice Magnani, who was the lead author of the paper, there hadn’t been activity along those faults for 300 million years.

The study went on to verify that, not only had there not been major seismic activity along the faults over the years, but also to examine the data closely to be certain that the small-scale earthquakes that have been occurring in DFW hadn’t been happening over those hundreds of millions of years. As Scientific American explains:

[T]he recent north Texas earthquakes were so small they caused offsets of just a fraction of a centimeter. The seismologists wanted to be sure these faults had not been producing this kind of tiny earthquake all along. They were able to do this because the offsets representing rock moved by quakes are cumulative: each new quake adds more distance. The researchers took the 300-million-year time span and calculated the maximum number of small to medium sized quakes it would take to produce a cumulative offset just shy of 15 meters. That fell into the range of 3,800 to 6,000 earthquakes—or, roughly, an earthquake every 50,000 to 79,000 years. Even if temblors occurred that frequently, the probability of a natural earthquake sequence occurring in north Texas in the previous 10 years was only one in 6,000 and the probability of two sequences was one in 60 million.  Since north Texas has had five earthquake sequences during that 10-year span, the scientists write that it is “exceedingly unlikely” that the recent quakes were natural.

This isn’t the first study to conclude that the earthquakes in North Texas are man-made, it’s just the most conclusive one. A UT study from 2016 found that human energy-exploration activity had been responsible for earthquakes throughout the state, dating back to well before the 2008 timeline commonly cited, looking at statistical analysis dating back to the seventies. A study last year led by Stanford University used satellite imagery to demonstrate a clear link explaining exactly how earthquakes are triggered by wastewater disposal injections from drilling and fracking activity. An April 2016 study led by UT researchers ruled out other explanations for the quakes.

All of which is to say that, while this comes the closest we’ve seen to a smoking gun on proving the link between fluid injections and earthquakes—the sudden reactivation of fault lines that had been dormant for many millennia is hard to explain away—it can also be added to the stacks of existing research to help explain exactly what’s going on with those quakes.