Study suggests bedrock stress a factor in fracking caused earthquakes

New research is digging in to why fracking causes earthquakes insome areas but not in others.

A paper published Monday in Geophysical Research Letters suggeststhe likelihood of an artificial earthquake is heavily influenced byhow stable the ground was before the energy industry showed up.

"Some places appear to be particularly responsive to[artificially-]occurring earthquakes while other places aren't,"said Honn Kao, a seismologist with the Geological Survey of Canadaand lead author.

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Scientists have known for some time that injecting fluids todispose of wastewater or to free underground reserves of oil and gascan cause earthquakes.

Regulatory records show there have been hundreds of seismicevents since 2015 in a heavily fracked area ofnorthwestern Alberta.

Those earthquakes around the Fox Creek area have registered as highas 4.5 on the Richter scale strong enough to rattle dishes andpictures.

Alberta's energy regulator has tightened restrictions on frackingin the area.

Meanwhile, other regions see thousands of wells fracked while theearth remains still.

While the link between fracking and earthquakes iswell-established, precisely how that link works remains mysterious.

Other studies have asked if it's related to local geology orparticular fracking practices, but Kao said he's found a much moreimportant contributor.

"The background tectonic loading rate appear to be one of thepredominant factors that control the region's response to injection-induced earthquakes," he said.

Tension 'stored like a coiled spring'

In other words, the deep, underground shifting of Earth's rockytectonic plates create zones where tension is concentrated andstored like a coiled spring, called tectonic deformation. The suddenshattering of rock through fracking or the injection ofhigh-pressure wastewater releases that pent-up energy in the form ofan earthquake.

The finding could help explain why western Alberta and northeastB.C. have a high rate of fracking-induced earthquakes and placessuch as Saskatchewan, which has thousands of fracked wells, doesn't.

"The Canadian side of the Rocky Mountains has a much highertectonic deformation rate," Kao said. "As you go from the CanadianRocky Mountains eastward, the deformation rate drops quiterapidly."

Of all the fracking-induced earthquakes he and his colleaguesstudied, 98 per cent occurred in a 150-kilometre band down theRockies where the subsurface rocks are naturally stressed.

Those stresses aren't the only way earthquakes are caused.

Artificial temblors are common in Oklahoma, which has little of theunderground tension found in Alberta.

But there, Kao said, fluid injection may be big enough to causeproblems on its own. Injection rates are 100 times higher there thanin Canada, he said.

Underground stress

Underground stress is probably best understood as a majorcontributing factor, Kao added.

"It's more of a competition of all these different factors."

The work has implications on how fracking should be regulated,Kao said. Regulations may need to be different in different regions,based on the tectonic tension deep underground.

Kao said, in theory, those smaller artificial quakes might be agood thing. They may be relieving tension that would otherwise buildup to a larger, more dangerous event.

"We are reducing the occurrence of the future big earthquake,"he said. "Theoretically."