With real-time monitors, scientists have linked a swarm of small earthquakes west of Fort Worth, Texas, to nearby natural gas wells and wastewater injection.
"It's what we figured all along, it's not really new news to us," said Azle Mayor Alan Brundrett, "It's just confirming our suspicious that we've had."
In 84 days from November 2013 to January 2014, the area around Azle, Texas, shook with 27 magnitude 2 or greater earthquakes, while scientists at Southern Methodist University and the U.S. Geological Survey monitored the shaking. It's an area that had no recorded quakes for 150 years on faults that "have been inactive for hundreds of millions of years," said SMU geophysicist Matthew Hornbach.
When the volume of injections decreased significantly, so did the shaking.
"Hopefully we don't have anything else to worry about because the gas companies in our area are smart enough to know that they better keep the volumes down or they're going to create some more earthquakes and then they're going to have to deal with the consequences," said Brundrett.
The scientists concluded that removing saltwater from the wells in the gas production process and then injecting that wastewater back underground "represent the most likely cause" for the swarm of quakes, according to a study published Tuesday in the journal Nature Communications.
The scientists determined this based on where and when the earthquakes happened; computer models that track pressure changes; and company data from nearby wells. Hornbach said the timing and location of the quakes correlates better to the drilling and injection than any other possible reason.
"There appears to be little doubt about the conclusion that the earthquakes were in fact induced," USGS seismologist Susan Hough, who wasn't part of the study team, said in an email. "There's almost an abundance of smoking guns in this case."
This adds to other studies that linked injecting wastewater from energy wells to a tremendous jump in earthquakes in Oklahoma and southern Kansas, where there have been more than 950 magnitude 2 or higher quakes so far this year, according to the USGS.
On Tuesday, the Oklahoma Geological Society said it is "very likely" that most of the earthquakes that have shaken the state in recent years have been triggered by the subsurface injection of wastewater from oil and natural gas drilling operations. The society released a statement following an investigation into dozens of earthquakes recorded in central and north-central Oklahoma.
Unlike other research that linked quakes to the injection of wastewater, the SMU study also sees a secondary link in another part of the drilling process, when massive amounts of brine is taking out of the ground with the gas, said study co-author William Ellsworth of the USGS. Removing the saltwater changes the underground pressure, Hornbach said.
But the deep injection of the wastes still is the principle culprit, Ellsworth said. The controversial method of hydraulic fracturing or fracking, even though that may be used in the drilling, is not physically causing the shakes, he said.
Meanwhile, SMU seismologists are still examining the cause of ongoing earthquakes in suburban Dallas, where nearly 400 seismic events have already been recorded in that area in recent months.
SMU now has more than two dozen recording devices near the Texas Stadium site in northeast Irving, and the scientists are part of a working group that meets twice a week and includes representatives from the cities of Dallas, Irving and Coppell.
“What we are doing right now is preparing, making sure that we have our resources in case we have a major earthquake, how to we respond,” said Rocky Vaz, director of Dallas Emergency Management.
Reporting the results from their Azle study, scientists at SMU shied away from any early conclusions about the Dallas County earthquakes.
“We’re too soon into the Irving study to be able to speculate on how strongly we’ll be able to comment on cause,” said SMU geophysicist Heather Deshon.
Any results are still more than a year away, but Dallas Mayor Mike Rawlings is among those anxious to hear them.
"You have an earthquake, you say why is this? People are anxious. We want to be able to say, 'Ah! We have an insight'" Rawlings said. "'We shouldn’t do this, we should do this.'"
In February, researchers released preliminary results that showed a narrow fault line extending from Irving to West Texas. Researchers previously identified disposal wells as the source of seismic activity at the Dallas-Fort Worth International Airport.
The findings come amid heightened debate over oil and gas regulations, including efforts in some communities to ban drilling. In Texas, lawmakers are considering bills that would limit cities' abilities to do so.
"It is worrisome, especially with the House Bill 40 that was passed – that takes away the city's authorities to regulate the oil and gas industry in our city," Brundrett said.
The Texas Railroad Commission, the state's oil and gas regulator, hired its first seismologist last year to investigate potential links between quakes and fracking after Azle residents asked the agency to halt oil and gas activities. The seismologist has not offered any conclusions.
NBC 5's Jocelyn Lockwood and Kevin Cokely contributed to this report.