By Jeff Halashewski, Dipl Arch Tech
Flood, fire, blizzards, and ice storms are among the major natural disasters having an impact on the built environment in Canada. Among the least-discussed, and most misunderstood, are seismic events. Yes, there are regular earthquakes in this country, and, yes, they can be significant enough to have negative effects on buildings and occupants.
As a person who has been in the field of architecture for almost two decades, this author admits that until recently ‘earthquakes’ and ‘seismic’ were terms most associated with news reports of far-off places. However, once I started writing for B.C. base projects, I learned seismic activity was indeed an important consideration. Reading the Vancouver Building Code, the National Building Code of Canada (NBC), and then resources from the Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) and Architectural Institute of British Columbia (AIBC), I gained a better sense of why West Coast projects demand taking these events into account.
What is an earthquake?
An earthquake occurs when rocks break and slip along a fault in the earth. Energy is released during an earthquake in several forms, including as movement along the fault, as heat, and as seismic waves radiating out from the source in all directions, causing the ground to shake, sometimes hundreds of kilometres away.
Earthquakes are caused by the slow deformation of the outer, brittle portions of tectonic plates—the Earth’s outermost layer of crust and upper mantle. Convection resulting from the heating and cooling of the rock below these plates causes the adjacently overlying plates to move, and, under great stress, deform. The rates of this movement range from approximately 20 to 120 mm (0.8 to 4.8 in.) annually. Sometimes, tremendous energy can build up within a single plate, or between neighbouring ones. If the accumulated stress exceeds the strength of the rocks making up these brittle zones, the rocks can break suddenly, releasing the stored energy as an earthquake.
An earthquake’s magnitude or size (i.e. energy release), focal depth, faulting type, and distance are important factors in determining the amount of ground shaking that might be produced at a particular site. Where there is an extensive history of earthquake activity, these parameters can often be estimated. In general, bigger earthquakes produce ground motions with larger amplitudes, longer durations, and stronger shaking over much larger areas.
Additionally, the amplitude of ground motion decreases with increasing distance from the focus of an earthquake. The frequency of the shaking also changes with distance. Close to the epicentre, both high (rapid) and low (slow)-frequency motions are present. Farther away, low-frequency motions are dominant, a natural consequence of wave attenuation in rock. The frequency of ground motion is an important factor in determining the severity of damage to structures and which structures are affected.
Despite what Hollywood blockbusters would have you believe, earthquakes do not result in holes in the ground that open to swallow up unfortunate victims. After a strong earthquake, some cracks may be seen on the ground or in basements, but these are not faults or crevasses that will close back up—rather, they are probably simply due to soil settlement caused by the ground shaking.
Earthquakes in Canada
While earthquakes occur all over the world, almost 90 per cent happen on only 10 per cent of the planet’s surface—the active faults defining major tectonic plates. The Ring of Fire circling the Pacific Ocean, including Canada’s West Coast, is one of the most active areas in the world.
However, it is also important to keep in mind minor earthquakes have been triggered by human activities such as mining (e.g. rock bursts and cavity collapses), the filling of reservoirs behind large dams, and the injection of fluids into wells for oil recovery or waste disposal. Large dams hold back enormous quantities of water. Some of this water may penetrate into cracks in the underlying rock, triggering small earthquakes under or very near the reservoir. (These human-caused earthquakes always occur close to the site of the activity. There is no link between activities like these and earthquakes occurring hundreds or thousands of kilometres away.)
On average, the Geological Survey of Canada (GSC) records and locates more than 4000 earthquakes in Canada each year—about 11 a day! However, of these, only about 50 are generally felt. Most occur along the active plate boundaries off the B.C. coast, and along the northern Cordillera (southwestern corner of the Yukon Territory and in the Richardson Mountains and Mackenzie Valley) and arctic margins (including Nunavut and parts of northern Québec). Earthquakes also occur frequently in the Ottawa and St. Lawrence Valleys, in New Brunswick, and the offshore region to the south of Newfoundland.
The largest earthquake recorded (during historic times) in Canada was a magnitude 8.1 event that struck just off the Haida Gwaii on Canada’s Pacific coast on August 22, 1949. This earthquake (which was larger than the 1906 San Francisco earthquake) ruptured a 500-km (310-mi) long segment of the Queen Charlotte fault and was felt over almost all of British Columbia—as far north as the Yukon Territory and as far south as Oregon.