The large Western Quebec seismic zone has produced earthquakes for 300 years, according to the U.S. Geological Service.
The worst quakes occurred in 1935, a magnitude 6.2, at the northwestern end of the seismic zone, and in 1732, a magnitude 6.2. The latter quake caused significant damage in Montreal.
Earthquakes cause damage in the zone about once a decade. Smaller earthquakes are felt three or four times a year.
In the Ottawa-Gatineau, Que., region, the risk for seismic activity is the third highest in Canada, based on earthquake hazard and population, according to the Seismological Society of America.
Earthquakes east of the Rockies, although less frequent than in the West, are typically felt over a much broader region, said the USGS.
East of the Rockies, an earthquake can be felt over an area as much as 10 times larger than a similar magnitude earthquake on the West Coast.
A magnitude 4.0 eastern earthquake typically can be felt as far as 100 km from where it occurred, and it infrequently causes damage near its source.
A magnitude 5.5 eastern earthquake usually can be felt as far as 500 km from where it occurred, and can sometimes cause damage as far away as 40 km.
Earthquakes occur on faults within bedrock, usually many kilometres deep. Most of the bedrock in the Western Quebec seismic zone was formed as several generations of mountains rose and were eroded down again over the last billion or so years, according to the USGS.
The Western Quebec seismic zone is far from the nearest plate boundaries, which are in the centre of the Atlantic and the Caribbean, the USGS said.
The seismic zone is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths.
Recent earthquakes in Ontario and Quebec:
– June 23, 2010 – 5.0 magnitude quake. Epicentre was 61 km north of Ottawa.
– Feb. 24, 2006 – 4.5 magnitude quake. Epicentre was seven kilometres north of Thurso, Que.
– Jan. 1, 2000 – 5.2 magnitude quake. Epicentre was 70 km northeast of North Bay.
– Oct. 19, 1990 – 5.0 magnitude quake. Epicentre was nine km southwest of Mont-Laurier, Que.
– Dec. 25, 1989 – 5.9 magnitude quake in northern Quebec.
– Nov. 1, 1935 – 6.2 magnitude quake. Epicentre was about 10 km east of Temiscaming, Que.