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 km northwest 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 occur frequently — even if people might 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 Alberta or 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.

The Earth's crust is made up of tectonic plates. These plates are constantly moving relative to each other along what are called faults — fractures between two blocks of rock. Interactions between the tectonic plates cause them to 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's crust 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," she said.