Earthquakes west of Fox Creek, Alta., in the winter of 2015 were triggered by hydraulic fracturing near a fault system that industry and researchers didn't know existed at the time, says a University of Calgary study published Thursday in the journal Science.

In addition to mapping a previously undetected fault system, the two researchers involved also say that small earthquakes can continue to occur months after hydraulic fracturing, or "fracking," has stopped.

"What we are now able to do is to pinpoint the faults themselves and look at the dynamics of activation," said David Eaton a professor of Geophysics at the University of Calgary, who along with former university scholar Xuewei Bao, co-authored the study.

The research was prompted after an earthquake rattled Fox Creek on Jan. 23, 2015. At the time, its magnitude was measured as being 4.4 on the Richter scale, and residents reported that they felt shaking and saw their furniture and lamps sway back and forth.

After the quake was recorded, Eaton and two other researchers grabbed seismic equipment and headed up to the Fox Creek area, hoping to record aftershocks. They were able to use some of that data for their study, but also relied on information from 12 seismic stations in the area, including four operated by Repsol Oil and Gas Canada.

They used the data along with specific algorithms to map the fault system underground and track the earthquake activity. They found there was a fault system running parallel to two wells drilled by Repsol and that fracking was triggering tremors.

Fracking involves injecting a mix of water, sand and other additives into the ground at a high pressure. Eaton says stress eventually caused the fault to slip, triggering the earthquake felt by residents in the community. It occurred two weeks after the fracking had stopped, but other smaller seismic events persisted for months.

Eaton says this appears to be because the fracking fluids created an increase of pore pressure along the fault, which triggered smaller tremors.

"This is a kind of level of detail of the process that is going to be extremely important," said Eaton.

He said it will help scientists and industry understand exactly what is causing the earthquakes, and it will help regulators design strategies to mitigate the risk. He believes the regulations in Alberta don't explicitly address seismic events occurring months after an operation has stopped.

Under current rules, if there is an earthquake in the Fox Creek area above a 4.0, nearby operations are halted. On Jan. 12, 2016, a hydraulic fracturing operation at at Repsol site was shut down after 4.8 magnitude earthquake.

At the time, the company released a statement saying it had immediately shut down operations and was investigating.

Asked for comment on this latest research, Repsol issued a statement saying the company is strongly supportive of the research and that "the safety and wellbeing of the people living and working in proximity" to their operations is their top priority.

In recent years, there have been hundreds of small and moderately sized earthquakes in the Fox Creek area, and Duncan Kenyon, a director for unconventional oil and gas with the Pembina Institute, said this new research highlights the need for even more work.

"We are really at the early stages of understanding it," Kenyon said.

"We are kind of chasing after issues after the fact and hoping that things don't get really bad in the meantime."