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Aerial photos of Nexen's b-77-H, 18 well pad site at Dilly Creek in the Horn River Basin. The company is actively drilling for shale gas in the north east region of British Columbia, Canada.Dave Olecko

At least two earthquakes in British Columbia over the past year – including one last week – are among the largest ever caused by natural gas fracking in North America and were both strong enough to force temporary shutdowns of operations.

But while the province's oil and natural gas ministry as well as the shale gas industry have both played down the severity of fracking-induced quakes – insisting they are rare and present no threat to people or buildings – experts caution much is still not known about just how strong a fracking-induced earthquake could be.

Hydraulic fracturing, or fracking, is primarily used in shale gas deposits in northeastern B.C. It involves injecting high-pressure fluid into a well to crack rock, increasing the flow of natural gas – and, in some cases, triggering earthquakes.

"The key question is that if we have earthquakes of that size [from fracking], is it conceivable that we could have one that is actually a damaging earthquake?" said John Clague, an earthquake expert at Simon Fraser University, near Vancouver. "I'm afraid we don't have the answer to that."

Mr. Clague added that while it's "extremely unlikely" a fracking-related quake would ever reach higher than magnitude 6, he said seismic activity of 5 or 6 is still worrisome because it can be damaging.

"There's a huge difference between a 4.4. and a 5," he said. "To be perfectly honest, we don't know what the upper limit of a fracking-induced earthquake can be."

Progress Energy, which is owned by Malaysia's Petronas, paused its operations on Aug. 17 after an earthquake that occurred 114 kilometres from Fort St. John.

The B.C. Oil and Gas Commission says hydraulic fracturing by the same company triggered a 4.4-magnitude earthquake that was felt in Fort St. John and Fort Nelson in August, 2014.

The commission did not respond to repeated requests for comment.

The shale gas industry triggered 231 earthquakes in northeastern British Columbia between August, 2013, and October, 2014, none of which were linked to any injuries or property damage, according to a commission report released last year. There have been similar earthquakes in Alberta, where fracking has been linked to two 4.4-magnitude quakes this year alone.

While the majority of these quakes were too small to feel, Mr. Clague said last year's 4.4-magnitude quake and last week's 4.6 are among the first in the 4-magnitude range to happen in Canada.

The Canadian Association of Petroleum Producers (CAPP) maintains quakes of such magnitude are exceedingly rare and pose no threat to public safety.

"What's important to understand is that none of these felt seismic events have resulted in property damage or damage to buildings, they haven't injured anybody, and they also haven't caused any environmental damage," said Markus Ermisch, a media relations adviser for CAPP.

Regulations require hydraulic fracturing operations to halt as soon as seismic activity of 4 or greater is detected. Companies can start fracking again only after implementing measures to reduce seismic activity or eliminate the well operations causing the quake. A response plan is needed for quakes between 2 and 4. No response is required for any quake near a fracking operation that is less than magnitude 2.

"The regulations are in place to mitigate the risk," Mr. Ermisch said.

Experts are researching whether relatively weak quakes can prompt larger ones.

"We don't know for sure," said David Eaton, a geophysics professor at the University of Calgary who is researching maximum quake levels from fracking. "More research is urgently needed and that's something we're working on right now."

Although the science is years away from being able to map fault lines near fracking wells in a way that could predict how big a fracking quake could be, Prof. Eaton said the technology is coming.

"There's quite a substantial team of experienced people working on this."

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