Earthquakes in Alberta

Earthquakes in Alberta

The highest naturally occurring seismic activity in Canada is predominantly caused by the Cascadia subduction of the Juan de Fuca plate beneath British Columbia. Stresses built in the rocks due to tectonic motion are the driving force for the seismicity in this region.  This same process happens, albeit at a smaller scale, in intraplate regions such as Alberta, causing less frequent and smaller magnitude earthquakes. Pre-existing faults are apparent within the bedrock and crystalline basement of Alberta. These faults were generated in the geological past when the motions driving the rock under Alberta were significantly more active. Today, the subsurface of Alberta is still driven (relatively slowly), producing slow accumulation of stresses in the rocks, which are intermittently released as small earthquakes. The AGS catalogues this naturally occurring seismicity throughout the province in addition to induced seismicity.

a) b)
Locations of earthquakes in Alberta (a) from 1918 to September 18, 2006 (data from Earthquakes Canada online search engine) and (b) from September 18, 2006, through December 31, 2015 (data available as Digital Dataset 2013-0017).

 

While Alberta has been historically seismically quiet, the change in numbers of recorded earthquakes over time is worthy of study and attention. For example, Alberta experienced a significant increase in seismic activity from the mid- to late 70s. Between 1918 and the mid-60s, the branch of the federal government, now known as Earthquakes Canada, recorded less than a dozen minor earthquakes in Alberta. Monitoring of seismic activity within Alberta began in earnest during the mid-60s with the installation of several seismographs. Prior to 1975, these instruments recorded less than 20 earthquakes yearly. From 1977, there was a notable increase in recorded events.

Total number of earthquakes in Alberta per year from 1965 to present is shown as red histogram bars; earthquakes with a magnitude greater than 2.5 ML are shown with green. The blue line shows the number of seismic stations available for analyzing Alberta earthquakes for a given year.

 

 

The increase in recorded minor earthquakes (2.5–3.9  ML) after 1975 was not coincidental with increases in seismic station density; however, the recorded microearthquakes (events less than 2.5 ML) between 2000 and 2018 is concurrent with increasing seismic station density, reflecting establishment of several seismic networks throughout the province. With increasing station density, we expect to detect and locate more microearthquakes that were too small to be recorded by more distant stations. Earthquakes larger than 2.5 ML would be less likely to be missed, and an increase in these minor earthquakes, such as in the mid-70s and in 2013, is not likely to a result of increasing station density but reflect real increases, such as a sudden spike from 4 (2012) to 43 (2014) recorded earthquakes.

Seismic events in Alberta tend to be between micro (0 on the local magnitude [ ML] scale) and minor (3 ML) in size. Moderate earthquakes with a magnitude greater than 4 Mare rarer. The highest earthquake recorded in Alberta measured at 5.4 ML on April 14th, 2001, 40 km north east of Dawson Creek. The highest induced earthquake was measured at 4.8 ML, on January 12th, 2016, 35km west of Fox Creek.

Alberta experienced 605 earthquakes between 1985 and 2011. Although these earthquakes were very small, it is a significant figure when compared with just 41 recorded earthquakes in Saskatchewan during the same time. However, comparing Alberta with B.C. is also revealing; in southwestern B.C., there are approximately 400 earthquakes recorded each year.

Most of Alberta’s recorded earthquakes are distributed evenly along the foothills and the Rocky Mountains. These earthquakes occur within the thrust-fault systems associated with the ancient mountain-building processes that created the Rocky Mountains. There are also clusters of earthquakes detected east of the Rocky Mountain deformation belt. The clusters are the Rocky Mountain House Seismogenic Zone 30 km southwest of Rocky Mountain House, the Brazeau River Cluster 90 km northwest of Rocky Mountain House, the Cardston Earthquake Swarm 13 km north of Cardston, and the Crooked Lake Sequences approximately 30 km west of Fox Creek.