18 Nov 2016
Lethbridge Herald
THE CANADIAN PRESS — CALGARY
Research suggests hydraulic fracking can cause earthquakes in at least two ways — and one of them can cause tremors months after the activity stops.
“The seismicity is persistent after the operations are completed,” said David Eaton, a University of Calgary seismologist, whose paper has been published in the journal Science.
Eaton has been studying earthquakes that have shaken the Fox Creek region of northwestern Alberta for years.
The largest, measuring between 4.2 and 4.8 on the Richter scale, occurred in January. The area, which is in the centre of the Duvernay oil and gas field, has experienced hundreds of tremors since 2013.
Scientists have long known the shakers are associated with oilfield practices.
In the United States, underground waste-water disposal seems to be the cause. In Alberta, research points to hydraulic fracking, which involves pumping high-pressure fluids underground. That creates tiny cracks in rock and releases natural gas or oil held inside.
How the widely used technique creates earthquakes has largely remained a mystery — until now.
Eaton and his co-author Xuewei Bao used a mathematical algorithm to isolate and locate more than 900 earthquakes in the Fox Creek area.
“That gave us the ability to image the fault structure,” Eaton said. “We could see that there were steeply dipping faults that extended from the injection level down into the Precambrian basement.”
The pair also realized there were two hairline faults that hadn’t been spotted in previous work.
One fault, some distance from the fracking site, quaked as fluids were pumped down and stopped when the pumping did.
Eaton said those quakes were caused by stress changes on the rock from the pumping. When the pumping ended, the stress was reduced.
But the other fault, very close to the site, remained active for months.
The researchers combined their precise fault-mapping with equally precise data on how much fluid was pumped underground, when it was pumped and where.