Earthquake Risk in Canada: What Is ‘The Big One’?

(Maiclaire Bolton, Seismologist and Sr. Product Manager, CoreLogic)

Across Canada, when thinking about earthquake risk, many commonly refer to ‘The Big One’ that will eventually strike British Columbia (B.C.). But what exactly is the definition of ‘The Big One’? Understanding the different types of earthquakes that can occur, the unique hazards they pose, and the probability that one of them will occur, all help to define ‘The Big One’.  But it’s not just about B.C., as there are also areas of eastern Canada that are prone to earthquakes, so a significant earthquake in this region could also gain the title—and related concern—of the ‘The Big One’.

The Geological Survey of Canada locates approximately 4,000 earthquakes across the country each year.¹ Most of these earthquakes are too small to cause damage, but several events are still felt each year. In addition, every few decades, a damaging earthquake impacting Canada does in fact occur, and historically, on average, every few centuries, the country experiences one of the largest earthquakes in the world as well.

Figure 1 shows earthquake activity across Canada and illustrates that nearly every province and territory has some degree of earthquake hazard. Most notably, however, the highest level of earthquake activity and resulting seismic hazard lies in B.C. where the largest and most frequent earthquakes occur. There also is an active earthquake zone in eastern Canada along the St. Lawrence River Valley and across the territories in Canada’s arctic region.

The areas of high seismic activity, and correspondingly high seismic hazard that also intersect with highly populated urban centres, are the areas of greatest concern to earthquake risk managers. They also happen to be areas populated with numerous residential, commercial and industrial properties. With this in mind, the key areas of earthquake risk in Canada are southwestern B.C. and the southern regions of Ontario and Quebec, extending from the Ottawa Valley up the St. Lawrence River.

Figure 1. Map of earthquakes with magnitude ≥3.0 in or near Canada (1627−2014)²

Source: Courtesy of Natural Resources Canada, 2016.

British Columbia

Just off the west coast of Vancouver Island lies the Cascadia subduction zone where the oceanic Juan de Fuca plate is descending, or subducting, beneath the continental North America plate. Three different types of earthquakes occur in this subduction zone environment: shallow crustal earthquakes in the overriding North America plate, deep intraslab earthquakes in the subducting Juan de Fuca plate, and very large megathrust earthquakes along the shallow interface boundary of these two plates.

Shallow crustal earthquakes can be devastating, especially if located near an urban centre. At shallow depths (less than 30 km), the ground motions are very close to the surface, and the high-frequency of shaking associated with these events can cause significant damage, especially if shaking continues for long durations. The 1946 magnitude 7.3 earthquake on Vancouver Island was the most recent large, damaging shallow crustal earthquake in southwestern B.C. Seismologists believe that in this region, shallow crustal earthquakes can reach magnitude of approximately 7.5.

Deep, intraslab earthquakes occur within the subducting Juan de Fuca plate as it descends eastward beneath the continent of North America, most commonly located beneath the Strait of Georgia and south into Puget Sound, WA. They are important because of their relatively high recurrence frequency. The most recent damaging intraslab event was the 2001, magnitude 6.8 Nisqually earthquake that occurred south of Olympia, WA. From a Canadian perspective, this event was widely felt and caused minor damage across southwestern B.C.³ These earthquakes commonly occur at depths between 50 and 80 km, but can extend down to depths of 100 km / 62 mi. Earthquakes at these depths generally cause less severe damage locally, but the resulting geographic footprint of distributed lower damage will be larger than a shallow earthquake of the same magnitude.

Historically, the Cascadia subduction zone has produced some of the largest earthquakes the world has experienced. This megathrust fault, along the plate boundary interface, extends from Brooks Peninsula on Vancouver Island to Cape Mendocino in northern California, where it terminates at the San Andreas fault. Seismologists have determined that great megathrust earthquakes have occurred on an average of every 500 years.⁴ The most recent of these events occurred on January 26, 1700 and was a magnitude 9 event. The Cascadia subduction zone has an off-shore location, so the fault rupture and greatest shaking will occur away from urban areas. Nevertheless, megathrust earthquakes with several minutes of shaking are rich in long-period (low-frequency) energy, and this type of ground motion can still be damaging to tall structures several hundred kilometers away, as observed in the 2010 magnitude 8.8 Maule, Chile and 2011 magnitude 9 Tohoku-oki Japan subduction earthquakes.

Further to the north, the Queen Charlotte fault poses a unique hazard in northern B.C. While the population exposed to this hazard is far lower, it is still present.  The largest earthquake instrumentally recorded in Canada was a magnitude 8.1 in 1949 along the Queen Charlotte fault. Damage was observed 200 km away in Prince Rupert, B.C., and the earthquake was felt as far away as Vancouver and Victoria, approximately 800 km away. More recently, a magnitude 7.7 earthquake occurred in October 2012 just to the south of the Queen Charlotte fault. While not a significantly damaging earthquake, it has been scientifically interesting for the research community and serves as a reminder that large earthquakes do occur in Canada.

Eastern Canada

Eastern Canada is located in a stable continental region, and the level of earthquake activity is much less frequent than along the west coast. However, the region has experienced large, damaging earthquakes in the past and will again in the future. The largest known earthquake in eastern Canada was the 1663 Charlevoix earthquake, which has been estimated to have a magnitude between 7.3 and 7.9.⁵ More recently, the June 2010 (magnitude 5.4), May 2013 (magnitude 5.2) and the 1988 magnitude 5.9 Saguenay earthquakes all serve as a reminder that eastern Canada is seismically active. Seismic activity in stable continental regions is generally related to the regional compressive stress field, where earthquakes most commonly occur in the areas of inherited crustal weakness that formed in ancient tectonic episodes.

Even though eastern Canada exhibits lower rates of earthquake activity, the underlying soil and rock conditions in a stable continental region are very different than in western Canada. As a result, seismic waves travel greater distances without losing their energy. As such, the ground-shaking footprints from earthquakes in eastern Canada are much larger than earthquakes in western Canada, leading to a potentially larger impacted area.

Damage Potential of ‘The Big One’

The earthquake risk in Canada is real and must be adequately prepared for, but what exactly is ‘The Big One’ in Canada? There has always been a significant amount of attention on the Cascadia subduction earthquake, but is this earthquake really ‘The Big One’? If one only considers its magnitude, yes, but will it absolutely be the most damaging event Canada will see? Likely not. Utilizing the CoreLogic® Canada Earthquake Model, and taking into consideration both the severity and frequency of potential events, estimated damage to insurable assets of Canadian exposure could be estimated at approximately $20 billion (CAD) for a magnitude 9 Cascadia megathrust event—a fraction of the total losses across the entire impacted region.

Comparatively, a shallow crustal earthquake directly beneath a major urban centre like Vancouver or Montreal, could be catastrophically worse. Modeled estimates indicate that a magnitude 6.9 earthquake located 10 km southwest of downtown Vancouver, could produce from $15 to $25 billion (CAD) in ground-up damage to insured assets. This scenario would have an impact on the national Gross Domestic Product (GDP) of 1 to 1.5 percent, similar to the impact Hurricane Katrina had on the U.S. in 2005. Furthermore, in eastern Canada, a magnitude 6 earthquake with an epicentre 15 km to the northeast of downtown Montreal, could cause $35 to $50 billion dollars in ground-up damage to insurable assets. This would have a larger impact of 2 to 3 percent of the national GDP, along the same lines as the impact of the Tohoku-oki earthquake on the Japanese economy. With a slightly larger magnitude and closer epicentre to the downtown city core in either example, these losses could be double or more, depending on the magnitude and precise location. Compared to the devastating impact of the 2011 Christchurch earthquake, which was approximately 13 percent of New Zealand’s national GDP, the two Canadian examples seem much more manageable, but it is important to consider that even though they are infrequent, low-probability events, they are not the worst-case scenarios.

These examples would also be very rare events, but so was a direct hit on the city of Christchurch, New Zealand. It is rare, but it can happen, and it is important for the industry to be aware of the potential risk. The reality is that the definition of ‘The Big One’ is relevant to one’s own perspective. Risk managers must determine what they are evaluating in terms of their own risk to adequately understand how they will be impacted. For the country of Canada, extreme catastrophic events near major urban centres, really are ‘The Big One’s to be considered.

It is not a matter of if, but when a damaging earthquake will happen in Canada. While it could be several days or several hundreds of years until the next Cascadia subduction zone earthquake ruptures, a significantly damaging earthquake could occur in Canada at any time, so it is best to be prepared.

References

1.       https://journals.lib.unb.ca/index.php/GC/article/view/15300/16404

2.       Halchuk, S, Allen, TI, Rogers, GC, Adams, J (2015): Seismic Hazard Earthquake Epicentre File (SHEEF2010) used in the Fifth Generation Seismic Hazard Maps of Canada, in, Geological Survey of Canada, Open File 7724, pp. 16, doi: 10.4095/296908.

3.       Molnar, S., J. F. Cassidy, and S. E. Dosso, 2004. Comparing intensity variation of the 2001 Nisqually earthquake to geology in Victoria, British Columbia, Bulletin of the Seismological Society of America, 94, 2229-2238.

4.       http://www.amnh.org/explore/science-bulletins/earth/documentaries/tsunami-science-reducing-the-risk/ghosts-of-tsunamis-past

5. Ebel, J. E. (June 2011), "A New Analysis of the Magnitude of the February 1663 Earthquake at Charlevoix, Quebec" (PDF), Bulletin of the Seismological Society of America, Seismological Society of America, 101 (3): 1024–1038.

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This blog post has been written by Maiclaire Bolton of CoreLogic.

Tom Larsen, Hazard Product Architect, CoreLogic will be a panelist at CatIQ’s Canadian Catastrophe Conference (C4 2017) on the CAT Models - Model & Hazard Uncertainty session during the conference.