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Canada's Earthquakes GEOSCIENCE CANADA Volume 37 Number 1 March 2010 1 ARTICLE they occurred, how they were felt, and aging earthquakes have struck Canada the effects of those earthquakes. We because, typically, they occur decades also provide a brief review of how apart, often located in offshore or earthquakes are monitored across remote, unpopulated regions. It is even Canada and some recent earthquake easier to forget that some of the hazard research. It is the results of this world’s very largest earthquakes have monitoring and research, which pro- struck within, or adjacent to, our coun- vide knowledge on earthquake hazard, try. These huge (magnitude (M) 8 or 9) that are incorporated into the National earthquakes are typically centuries Building Code of Canada. This, in apart, and are often located in remote turn, will contribute to reduced proper- areas. In this article, we summarize ty losses from future earthquakes Canada’s ‘good’, ‘bad’, and ‘ugly’ earth- Canada’s Earthquakes: across Canada. quakes. We define ‘good’ earthquakes ‘The Good, the Bad, and as those that either: the Ugly’ SOMMAIRE • have been widely felt, and there- Un bonne partie du Canada est un fore have made people more aware ‘pays de séismes’. De petits séismes of (and perhaps better prepared J.F. Cassidy1, G.C. Rogers1, M. 2 3 (que seuls les séismographes peuvent for) future earthquakes; or Lamontagne , S. Halchuk , and J. enregistrer) s’y produisent quotidien- • those that are large enough to be Adams3 1 nement. En moyenne, un séisme assez ‘scientifically useful’−they teach us Geological Survey of Canada fort pour qu’on le ressente s’y produit about the potential impact of PO Box 6000 à intervalle d’une semaine; assez fort future earthquakes in Canada. Sidney, BC, Canada, V8L 4B2 pour causer des dommages s’y produit ‘Bad’ earthquakes are those E-mail: [email protected] à intervalle de quelques années à that have caused significant damage quelques décennies; alors que l’inter- (including landslides, structural dam- 2 Geological Survey of Canada valle de récurrence des plus grands age, and other effects) in Canada, and 615 Booth Street séismes est de l’ordre des siècles. Dans ‘ugly’ are some of the world’s largest Ottawa, ON, Canada, K1A 0E9 le présent article on trouvera des earthquakes (larger than M 8). The détails sur les plus importants séismes purpose of this article is twofold: 3 Geological Survey of Canada s’étant produits sur ou à proximité du 1. To summarize the effects of Cana- 7 Observatory Crescent territoire canadien, incluant le lieu et le da’s most significant earthquakes Ottawa, ON, Canada, K1A 0Y3 moment, leurs manifestations et leurs (reminding us that destructive répercussions. On y décrit sommaire- earthquakes have struck Canada in SUMMARY ment les moyens de détection déployés the past, and will do so again in Much of Canada is ‘earthquake coun- sur le territoire canadien ainsi que the future); and try’. Tiny earthquakes (that can only be quelques-unes des recherches récentes 2. To highlight new earthquake recorded by seismographs) happen sur les risques sismiques. Ce sont les resources that constitute valuable every day. On average, earthquakes résultats des efforts de surveillance et tools for education and earthquake large enough to be felt occur every des recherches sur les tremblements de preparedness. This includes the week in Canada, damaging earthquakes terre qui ont été intégrés dans le Code new and updated ‘Earthquakes are years to decades apart, and some of national du bâtiment du Canada. Et Canada’ website the world’s largest earthquakes are typi- cela aidera à amoindrir les répercus- [http://www.earthquakescanada. cally separated by intervals of cen- sions des séismes à venir sur la pro- ca] and the updated list of signifi- turies. In this article, we provide details priété. cant Canadian earthquakes (Lam- on the most significant earthquakes ontagne et al. 2008). that have been recorded in, or near, INTRODUCTION Canada, including where and when It is easy to forget that large and dam- 2 Figure 1. Map of earthquakes with magnitude ≥2.5 in Canada (1660−2009). The red ellipses and article highlight some of the areas discussed in the text. CAUSES OF EARTHQUAKES IN An average of approximately 50 earth- types are sometimes also referred to as CANADA quakes are felt across Canada each megathrust, or great earthquakes. Just Each year in Canada, approximately year. to the north of Vancouver Island, the 4000 earthquakes are detected by seis- The largest and most frequent Pacific and North America plates slide mologists at Natural Resources Cana- earthquakes occur along the west coast, past one another along the Queen da. The earthquake distribution (Fig. 1) and most are associated with plate Charlotte Fault (Canada’s ‘San can largely be explained by tectonic motions and active faults (Fig. 2). In Andreas’). This seismically active fault setting (Fig. 2); for example, most of southwestern British Columbia (BC), zone produced Canada’s largest historic the earthquakes occur along the active the oceanic Juan de Fuca and Explorer earthquake − a M 8.1 event just west plate boundaries off the west coast. plates are subducting beneath the of the Queen Charlotte Islands in However, there is also significant activ- North American Plate at a rate of 2−4 1949. This fault extends north to the ity throughout the Cordillera (particu- cm/yr (Riddihough and Hyndman Yakutat region of Alaska, where colli- larly in the Yukon and Northwest Ter- 1991). This subduction process pro- sional tectonics (including a subduction ritories), along the Arctic margin, in duces three types of earthquakes: zone to the west that generated a M the Ottawa and St. Lawrence river val- those within the subducting plate (typi- 9.2 earthquake in 1964) again domi- leys, in the northern Appalachians, and cally at 30−60 km depth), those within nates. along the eastern offshore margin. The the North American Plate (down to 30 The seismicity in the Richard- fewest earthquakes occur within the km depth), and giant subduction earth- son and Mackenzie mountains of the stable craton (including the plains of quakes along the interface between the Yukon and Northwest Territories (Fig. Saskatchewan and Manitoba; Fig. 2). latter two plates. The last of the three 1) results from crustal stress being GEOSCIENCE CANADA Volume 37 Number 1 March 2010 3 Figure 2. Major tectonic features of Canada. Inset shows the active tectonics along Canada’s west coast, including the Queen Charlotte Fault and the subduction zone. transferred from the Yakutat collisional continent transition, and may be relat- (Adams and Basham 1991). zone to reactivate shallow thrust faults ed to the reactivation of Mesozoic rift In eastern Canada, earth- within the foreland fold and thrust belt faults. Seismicity within the Labrador quakes are believed to be primarily of the Mackenzie Mountains (Mazzotti Sea is concentrated on the extinct caused by a northeast-to-east oriented et al. 2008) and strike-slip faults within spreading ridge and associated trans- compressive stress field reactivating the Richardson Mountains (Cassidy form faults associated with the zones of crustal weakness − either and Bent 1993). The seismic zones breakup of Pangea. Concentrations of failed rifts or old fault zones (Kumara- along the eastern Arctic margin seismicity near Baffin Island and across pelli and Saull 1966; Adams and (including a M 7.2 earthquake in Baffin the Boothia and Ungava peninsulas Basham 1991). The most active zones Bay in 1933) are situated at the ocean− may be caused by postglacial rebound are located at the mouth of the St. 4 Lawrence River, near La Malbaie in Charlevoix County, in western Québec/eastern Ontario, and in the northern Appalachians. Charlevoix (Fig. 1) is the site of five large earth- quakes (M >6) since 1663, the most recent being in 1925. The offshore Atlantic margin (southeast of New- foundland) experienced a magnitude 7.2 earthquake in 1929. EARTHQUAKE MONITORING IN CANADA Earthquake monitoring began in Cana- da in the late 1800s. The first known, instrumentally detected earthquake in Canada was the March 23, 1897 M~5 Montreal-area event, recorded on a 3- component seismograph at McGill University in Montreal, Québec (QC). The first continuously operating seis- mographs in Canada were located in Toronto, Ontario (ON) (installed Sep- tember, 1897) and Victoria, BC (start- ing September 3, 1898). These were low-gain Milne seismographs (most Figure 3. The CNSN and POLARIS seismic stations operating in Canada in 2009. sensitive to large, distant earthquakes), which were a part of the global net- ning in 1991, the Canadian National These instruments are designed specifi- work established by the British Associ- Seismograph Network (CNSN) was cally to record the very strong shaking ation for the Advancement of Science. completely modernized (North and associated with large earthquakes Additional low-gain seismographs were Beverley 1994), with digital data from (when the ‘standard’ seismographs may deployed across Canada (e.g. Ottawa, approximately 80 sites being continu- be off-scale), thereby providing infor- Halifax, St. Boniface and Saskatoon) ously transmitted in real-time (via satel- mation critical for engineering purpos- during the first two decades of the lite links, dedicated phone lines, and es. As of 2009, the GSC operates 123 1900s. For a detailed description of the UHF radio links) to data processing strong-motion instruments in Canada early history of earthquake recording centres in Sidney, BC and Ottawa, ON. (Cassidy et al. 2007; Fig. 4a, b). Most in Canada, see Stevens (1980), Rogers Nearly half of the stations included of these instruments are modern (1992), and Basham and Newitt (1993). state-of-the-art three-component ‘Internet Accelerometers’ located in Significant upgrades in earth- broadband seismometers. This network the urban centres of southwestern BC quake monitoring capacity in Canada lowered the magnitude threshold in (Fig.
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