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“There is a significant possibility” recent earthquakes in North Texas link to oil and gas activity, federal officials said.

That is what the U.S. Environmental Protection Agency (EPA) said in its annual evaluation of how the Texas Railroad Commission oversees thousands of injection and disposal wells.

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“In light of findings from several researchers, its own analysis of some cases and the fact that earthquakes diminished in some areas following shut-in or reduced injection volume of targeted wells, the EPA believes there is a significant possibility that North Texas earthquake activity is associated with disposal wells,” the report said.

Scientists have known for decades that injecting fluid deep underground could trigger earthquakes, and a growing body of research linked disposal wells to seismicity in Texas and other states.

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Texas is the third-most at-risk state for man-made earthquakes, according to the U.S. Geological Survey. The state stands behind Oklahoma and Kansas.

Several Texas drilling regions have felt more earthquakes, most of them small. But temblors in the Dallas-Fort Worth area have drawn the most attention, particularly those that struck in the past two years.

“EPA is concerned with the level of seismic activity during 2015 in the Dallas/Ft. Worth area because of the potential to impact public health and the environment, including underground sources of drinking water,” the agency said in the report.

The shaking created political challenges for the Texas Railroad Commission, which oversees the powerful oil and gas industry.

Since 2014, the agency has added a staff seismologist and approved regulations requiring disposal well operators to submit more geographical information. But the agency, which has conducted its own investigations, has not publicly tied any of the shaking to industry activity, even as regulators in other petroleum states acknowledge a connection. The Railroad Commission has pushed back against conclusions of some outside studies.

Responding to the EPA report Monday, the commission said it “takes the issue of induced seismicity very seriously and has in place some of the most stringent rules on disposal wells.”

Since approving those 2014 rules, the commission has received 56 disposal well applications in historically seismic areas, spokeswoman Gaye McElwain said in an email. Of those, the agency has issued 28 permits with “special conditions” — those related to injection volumes and pressures, for instance. Eleven applications ended up withdrawn or returned, while three went to hearings. “Ten permits were issued without special conditions, and four applications are pending,” McElwain said.

In its assessment, the EPA “commended” the Railroad Commission for establishing new regulations on disposal wells and clarifying its authority to shut down certain operations it ties to earthquakes. But the federal agency recognized that its own findings about North Texas seismicity run counter to what the Railroad Commission has publicly stated.

Earthquakes are generated by slipping faults, or fractures. Experts said injecting fluids at high pressures can relieve pressure in some faults, causing them to slip. And “naturally fractured injection formations may transmit pressure buildup from injection for miles,” the EPA assessment noted, highlighting one such formation in North Texas — the Ellenburger — which is a popular disposal zone.

In its report, the EPA recommended, “close monitoring of injection activity” going forward, coupled with “appropriate data analysis methods, in a coordinated effort to detect possible correspondence with seismic activity.”

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