A series of small earthquakes in Cleburne in 2009 and 2010 could have been caused by the oil and gas industry injecting wastewater into the ground, according to a study released by Southern Methodist University on Tuesday.
The analysis did not reach a definitive conclusion and also suggested the quakes could have been the result of longtime faults under the area simply shifting on their own.
"It is plausible it is due to the increased injection at all the wells across Texas but it's also possible this is just the onset of stresses shifting within Texas,” said Ashley Justinic, who authored the study as an SMU graduate student.
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Researchers did rule out drilling rigs themselves and a technique known as hydrofracturing – or fracking -- which involves shooting high-pressure liquid into underground rocks to extract oil and gas.
“Although wastewater disposal may be linked to the Cleburne earthquakes, there has been no evidence that hydrofracturing, drilling, or natural gas production played any role in the events,” the report concluded. “Microearthquakes caused by hydrofracture occur only around the well while it is being fracked and are very rarely large enough to be felt. Wastewater injection, on the other hand, has triggered felt earthquakes in other areas.”
The first quake to hit Cleburne was June 2, 2009, the report said. A series of quakes – including five in one day -- followed.
SMU researchers installed a series of sensors to record the tremors and track their precise locations, which were along an established fault line, Justinic said.
Two injection wells were located near the epicenter of the quakes. They began operating in 2005 and 2007 – two years before the tremors started.
One well was shut in July 2009 but the quakes continued for months, the study noted.
The SMU research did not look at the recent series of 20 earthquakes around Reno, just north of Azle.
Reno Mayor Linda Stokes said the Parker County area has seen a lot of oil and gas drilling recently, but she added that she does not know if drilling is contributing to the quakes.
"It's a little unnerving,” Stokes said. "The very first ones there was a loud bang like an explosion before they happen. And then here lately, they've just been kind of rolling through. All the sudden everything starts vibrating around you.”