For an earthquake to occur we require two ingredients: stress in rocks due to tectonic motion and a fault line for rocks to slip on. These processes happen on a large scale, with large magnitude earthquakes, in highly active regions like the Cascadia subduction of the Juan de Fuca plate beneath British Columbia or the San Andreas Fault. As well, this same process happens, albeit at a smaller scale, in intraplate regions such as Alberta. 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), stresses continue to accumulate in rock, and these stresses are intermittently released as an earthquake. The AGS catalogues this naturally occurring seismicity throughout the province in addition to induced seismicity.
Figure 1. 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.
Figure 2. The total number of earthquakes in Alberta per year from 1965 to present is shown as green histogram bars; earthquakes with a magnitude greater than 2.5 ML are shown with red. 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 ML, red bars) after 1975 was not coincidental with increases in seismic station density; however, the recorded microearthquakes (events less than 2.5 ML, green bars) between 2000 and 2011 is concurrent with increasing seismic station density, reflecting installation of the CRANE and ATSN stations. We would expect with increasing station density 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, are not likely to a result of increasing station density but reflect real increases.
Most of Alberta’s recorded earthquakes are distributed evenly along the foothills and 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 ~30 km west of Fox Creek.
The increase in seismicity south west of Rocky Mountain House, starting in the mid-70s, prompted studies centered on data collected over a 23 day field campaign, operated by the Geological Survey of Canada and the University of Alberta. The Rocky Mountain House cluster studies documented the first evidence of induced seismicity related to hydrocarbon production in Alberta.
Figure 3. Earthquake locations that occurred up to 2010, showing the Rocky Mountain House and Brazeau River Clusters.
In a case of induced seismicity near a disposal well, an AGS study of the Brazeau River Cluster found that “it is likely that the Brazeau River Cluster events were induced by injection activities.” Specifically, it was found that seismicity started 3 years after the first injection “We find that the first statistically significant increase in seismicity lags the onset of wastewater injection (October 1991) by ~3.33 years.” “This distribution of events could suggest the presence of a pre-existing fault, which would preferentially diffuse pore pressure into the basement.”
Figure 4. Histogram of catalogued seismicity (blue bars) local to the Brazeau River Cluster alongside monthly injection rates (gray curve) at the suspected injection well.
More recently, clusters of earthquakes in previously seismically quiet areas near Cardston and Fox Creek have been tentatively linked to hydraulic fracturing operations. Currently, the Alberta Earthquake Studies Project is working to characterize and better understand induced seismicity in Alberta.
- Earthquake Monitoring
- All about Earthquakes
- Induced Seismicity
- Alberta’s Earthquakes