The Texas Railroad Commission has officially cleared two oilfield companies of responsibility for earthquakes that rattled two North Texas towns — despite research suggesting otherwise.

In a unanimous vote Tuesday, the three-Republican panel agreed with examiners for the regulatory agency who found no concrete evidence linking wastewater disposal wells drilled by Houston-based EnerVest and ExxonMobil subsidiary XTO Energy to the burst of seismic activity that riled up residents in Reno and Azle from late 2013 through early 2014.

More than two-dozen mini earthquakes during that period shook towns atop North Texas’ gas-rich Barnett Shale, putting pressure on regulators to address fears that oil and gas activities — namely disposal wells, deep resting places for liquid oil and gas waste — had triggered those and other temblors across the state.

The agency ordered hearings after a team of researchers led by Southern Methodist University concluded that industry activity “most likely” unleashed the earthquakes. The peer-reviewed research, published in April, linked the earthquakes to wells operated by XTO Energy and EnerVest.

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The SMU study said the operators’ withdrawal of brine — naturally salty water removed during oil and gas drilling — and the high-pressure injection of huge volumes of wastewater from gas wells likely spurred the quakes. 

It added to the growing body of research linking disposal wells to mostly small earthquakes in Texas and other states. The number of disposal wells has surged amid Texas’ drilling bonanza. Drilling areas in South and West Texas have also seen more earthquakes.

Scientists have known for decades that injecting fluid deep underground could trigger earthquakes. Neighboring Oklahoma has seen a prodigious increase in earthquakes and has surpassed California as the country’s most quake-prone state. The USGS and Oklahoma Geological Survey say wastewater disposal probably contributes to the trend.

The Railroad Commission’s June “show cause” hearings required the companies to offer evidence rebutting the researchers’ findings, showing why the operators should keep their permits. No other parties presented evidence. The commission invited the SMU study’s authors to participate, but they declined, saying they did not want to wade into policy decisions and that the research speaks for itself. The researchers continue to stand by their study.

In two rulings, commission examiners sided with the energy companies, writing that “natural tectonic processes” surprised North Texans unaccustomed to shaky ground.

The SMU study is a “commendable first-order investigation” of the issue, the examiners wrote in both decisions, but “presents data indicating a weak temporal correlation between injection and seismic activities — too small, however, to imply a causal relationship without further corroborating evidence.”

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The commission’s decision Tuesday allows the companies to continue their usual business in the Barnett. Both companies applauded the vote.

“As we have said since day one, we believe there was no connection between our well and the seismic activity,” Ron Whitmire, an EnerVest spokesman, said in an email.

An XTO spokeswoman, Suann Guthrie, said the commission’s decision “accurately reflects the conclusions that should be drawn from the evidence XTO presented.”

Ryan Sitton, the lone commissioner to speak ahead of the vote, praised the hearing examiners for their work and touted the agency's broader effort on earthquakes.

“We’ve obviously done a lot of work here at the commission over the last, gosh, year-and-a half,” he said.

The Reno and Azle-area temblors sparked a public outcry and kick-started the agency’s careful response to the phenomenon. That included hiring an in-house seismologist and approving requirements that companies submit more information before drilling disposal wells. 

Environmentalists and other critics have called for more action, and some said Tuesday's decision showed the agency was sticking its head in the sand on the issue.

"The Railroad Commission is losing all credibility. Independent scientists, the U.S. Geological Survey and even the state of Oklahoma have all linked wastewater injection wells to earthquakes," said Luke Metzger, director of Environment Texas. "But, once again, the oil and gas industry is denying the science rather than acting responsibly."

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The commissioners continue to suggest that the jury is still out about the link between disposal wells and earthquakes.

“I don’t think we know the answer yet,” Commissioner Christi Craddick said last month during an on-stage interview at the Texas Tribune Festival.

Asked why Texas has not followed the lead of quake-shaken Oklahoma and Ohio in embracing the scientific consensus on those questions, Craddick said Texas needs more state-specific data because of its unique geology and geography.

“We’re a very science-driven agency, and to jump to a conclusion or not know a right answer before we make decisions is not the way we do businesses at the Railroad Commission," she said. 

Disclosure: ExxonMobil is a corporate sponsor of The Texas Tribune. Southern Methodist University was a corporate sponsor in 2013. A complete list of Tribune donors and sponsors can be viewed here.