Big earthquakes are rare in Alberta, but small ones happen often even if you don't feel them

Marion Osborne was reading a book on a quiet night in October when a loud bang made her jump. The windows of her house rattled for about 10 to 15 seconds.

Osborne knew it couldn't have been her furnace, but she checked anyway before scanning for anything outside, as her brain tried to register what happened.

"It scared the crap out of me," said Osborne, who lives with her husband in Rocky Mountain House, Alta., a town 215 kilometres southwest of Edmonton.

At 9:20 p.m. on Oct. 20, seismographs registered a natural earthquake about 40 kmnorthwest of Rocky Mountain House. Initially, the earthquake measured a magnitude of 4.2, but Natural Resources Canada later upgraded it to a magnitude 5.0.

The upgrade made it the second-strongest earthquake ever in Alberta.

Though earthquakes of such magnitude are rare in this province, data shows smaller quakes occurfrequentlyeven ifpeoplemight not feel them.

CBC News analyzed publicly available data from Earthquakes Canada, which has tracked natural and induced earthquakes since Jan. 1, 1985.

Since that date, nearly 1,000 natural and induced quakes have occurred in Albertaor on its borders, data shows.

That figure is likely lower than the actual total. Depending on an earthquake's magnitude, depth and the number of seismographs installed in the province, some earthquakes may go undetected, said Camille Brillon, a seismologist with Natural Resources Canada.

The province's strongest natural earthquake a magnitude 5.4 occurred in April 2001 near the Alberta-B.C. border, about 40 km northeast of Dawson Creek, B.C.

How earthquakes occur

The Earth's crust is made up of tectonic plates. These platesare constantly moving relative to each otheralong what are calledfaults fractures between two blocks of rock. Interactions between the tectonic platescausethemto deform and energy to build up.

If the energy overcomes the strength of the rocks, the rocks can break suddenly, releasing the energy as an earthquake.

Most earthquakes happen along faults, but some earthquakes happen far from plate boundaries, Brillon said. The latter are usually due to stresses in the Earth'scrust reactivating zones of weakness in the rock created millions of years ago, when there was more movement between the tectonic plates.

"We don't see any cyclical nature of earthquakes, they just happen all the time," shesaid.

Relative to other parts of Canada, Alberta and the other Prairie provinces record fewer and weaker quakes, data shows.

British Columbia records the most by far. More than 55,000 natural earthquakes have occurred in the province, along its borders and off the Pacific coast, since 1985.

The amount of seismic activity or seismicity in B.C. is caused primarily by converging tectonic plates beneath the Pacific Ocean. The Juan de Fuca Plate is being pulled under the North American Plate.

Alberta is situated on top of the North American Plate and has less seismicity as a result, said David Eaton, a University of Calgary geophysics professor.

Most quakes in Alberta occur near Rockies

In Alberta, most earthquakes occur along the Rocky Mountains and along the B.C. border in the southwest.

Earthquakes there are related to faults created when the Rockies were formed about 60 million years ago, said Rebecca Salvage, a University of Calgary postdoctoral researcher of geophysics.

"There's likely to be an increased stress in that area, which are remnants of those mountain-building processes," Salvage said. Northern Alberta, in comparison, experiences fewer earthquakes because that region doesn't have the same geological constraints.

Some of the earthquakes around the Rockies also relate to the last ice age about 15,000 years ago and the melting of continental ice sheets, Eaton said.

"Some of the seismicity that we're seeing now actually dates back to then, because there is a sudden change in the level of stress because of the removal of the glaciers," Eaton said.

This phenomenon is called the isostatic or post-glacial rebound. Kienan Marion, a U of C geophysics PhD student, likened it to an ice cube floating in a glass of water: if it's pushed down, it pops back up again.

The geological bounce-back just happens much more slowly, she said.

There are other clusters of earthquakes, however, that occur in specific areas, such as west of Fox Creek, a town 260 km northwest of Edmonton.

The region has seen a significant increase inearthquakes since 2013, most of which are believed to be induced by nearby fracking operations.

Alberta researchers linked the earthquakes around Fox Creek to the volume of hydraulic fracturing fluid and location of well pads in 2018.

The largest induced earthquake had a magnitude 4.8, according to the Alberta Energy Regulator (AER). It occurred near Fox Creek in January 2016.Fracking in the area was shut down for nearly three months before the AER approved a plan for operations to resume.

Brillon says it's important for people to be aware that humans can induce earthquakes, but the public shouldn't have to worry about any massive quakes happening as a result.

Rocky Mountain House area needs further study: experts

Marion Osborne was surprised to learn via social media that what she felt back on Oct. 20had been an earthquake. But at the time, she shrugged her shoulders and went back to reading her book.

"I've heard that there have been earthquakes in this area on occasion," she said. "I never really expected that I would ever feel one."

Some researchers suggest it's worth further study. Data shows there is a cluster of earthquakes that have occurredsouthwest of Rocky Mountain House, in what's called the Rocky Mountain House Seismogenic Zone.

The first case of induced earthquakes occurred in that zone, and they are associated with gas production that took place decades ago, according to the AER website.

"There's a big gas pool that's located here, which we've known about for a long time. As the gas was removed from that reservoir, that actually caused the earthquakes in this region," said Kienan Marion, whose research focuses specifically on the Rocky Mountain House area.

Since 2014, there have been more earthquakes occurring in that zone, which interests researchers, she said.

It's unusual because the increase comes well after peak gasproduction, said Salvage, whose work focuses on induced seismicity.

There are few examples of suchquakes after peak production, especially in Alberta, because companies are still trying to produce as much oil and gas as possible, she said.

The earthquake that occurred on Oct. 20 was natural and not within the cluster.