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

Marion Osborne was reading a book on a quiet October night when she jumped up with a loud bang. The windows of her house shook for 10 to 15 seconds.

Osborne knew it couldn’t be her oven, but she checked anyway before scanning for anything outside, as her brain tried to record what had happened.

“It scared me of the bullshit,” said Osborne, who lives with her husband in Rocky Mountain House, Alta, a town 215 kilometers southwest of Edmonton.

At 9:20 p.m. on October 20, seismographs recorded a natural earthquake about 40 km northwest of Rocky Mountain House. Initially, the earthquake had a magnitude of 4.2, but Natural Resources Canada later upgraded it to 5.0.

The upgrade made it the second most powerful earthquake ever in Alberta.

Although earthquakes of this magnitude are rare in this county, the data shows that smaller earthquakes occur more frequently — even if people don’t feel them.

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

The data showed that, since that date, nearly 1,000 natural and induced earthquakes have occurred in or on Alberta’s borders.

This number is likely to be less than the actual total. Depending on the size and depth of the earthquake 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 most powerful natural earthquake—magnitude 5.4—occurred in April 2001 near the Alberta-British Columbia border, about 40 km northeast of Dawson Creek, British Columbia.

How do earthquakes happen?

The Earth’s crust is made up of tectonic plates. These plates are constantly moving relative to each other along the so-called faults – fractures between two masses of rock. Interactions between tectonic plates lead to their deformation and accumulation of energy.

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

Most earthquakes occur along faults, Brilon said, but some earthquakes occur far from plate boundaries. The latter is usually due to pressures in the Earth’s crust reactivating areas of weakness in rocks that arose millions of years ago, when there was more movement between tectonic plates.

“We don’t see any cyclical nature of earthquakes, they happen all the time,” she said.

The data shows that compared to other parts of Canada, Alberta and other prairie provinces recorded fewer and weaker earthquakes.

British Columbia records the highest number so far. More than 55,000 natural earthquakes have occurred in the county, along its borders and off the Pacific coast, since 1985.

The amount of seismic activity – or seismic activity – in British Columbia is mainly caused by the convergence of tectonic plates under the Pacific Ocean. The Juan de Fuca Plate is being pulled down the North American Plate.

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

Most earthquakes in Alberta occur near the Rockies

In Alberta, most earthquakes occur along the Rocky Mountains and along the British Columbia border in the southwest.

The earthquakes there are related to faults that formed when the Rocky Mountains formed about 60 million years ago, said Rebecca Salvage, a postdoctoral researcher in geophysics at the University of Calgary.

“There is likely to be increased pressure in that area, which is a remnant of these mountain-building processes,” Salvage said. In contrast, Northern Alberta experiences fewer earthquakes because that region does not have the same geological constraints.

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

“Some of the earthquakes we see now are actually from that time, because there is a sudden change in the stress level due to the removal of the glaciers,” Eaton said.

This phenomenon is called equilibrium or postglacial recovery. Kianan Marion, a doctoral student in geophysics at the University of California, has likened it to an ice cube floating in a glass of water: if pushed down, it pops up again.

Aerial view of Clearwater River, west of Rocky Mountain House, Alta. The researchers say the Rocky Mountain House seismic zone should undergo further study. (Amber Bracken/The Canadian Press)

She said the geological rebound is happening more slowly.

However, there are other groups of earthquakes that occur in specific areas, such as West Fox Creek, a town 260 km northwest of Edmonton.

The area has seen a significant increase in earthquakes since 2013, most of which are believed to be caused by nearby fracturing processes.

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

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

Brillon says it’s important for people to realize that humans can cause earthquakes, but the public shouldn’t worry about any massive earthquakes that occur as a result.

Rocky Mountain House area needs further study: experts

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

“I heard there are earthquakes in this area sometimes,” she said. “I didn’t really expect that I would feel one.”

Some researchers suggest that it deserves further study. The data shows that there is a cluster of earthquakes that have occurred southwest of the Rocky Mountain House, in what is called the Rocky Mountain House seismic zone.

The first induced earthquake occurred in that region, and it is linked to gas production that occurred decades ago, according to the AER website.

said Keenan Marion, whose research focuses on the Rocky Mountain House area.

Since 2014, more earthquakes have occurred in that region, she said, which is intriguing researchers.

Salvage, whose work focuses on induced earthquakes, said it’s unusual because the increase comes so long after peak gas production.

She said there are few examples of such earthquakes after peak production, especially in Alberta, because companies are still trying to produce as much oil and gas as possible.

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

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