Earthquake, vibratory motion generated in the Earth by the rupture of subterranean rock subjected to elastic stresses greater than its breaking strength. The rupture gives rise to seismic waves which propagate outward from its initiation point or focus. Immediately above the focus at the surface, the epicentral region experiences the strongest shaking. At greater distances, the seismic ground motion is detectable only by sensitive instruments (eg, seismographs). There are many measures of an earthquake's size, the best known being that devised by Charles Richter in 1935. Recognizing the need to quantify earthquake size, Richter created a scale (calibrated against seismograph records) that takes into account the attenuation of seismic waves with distance from the earthquake focus.

On the Richter magnitude scale (M), earthquakes below M2 are generally not felt, even when occurring at shallow depth. Shallow M5 events cause damage only when the epicentral region supports structures built on loose soils or unconsolidated sediments. M6 earthquakes in populated areas frequently cause considerable damage. The great earthquakes of this century have had magnitudes of about 8.5. Approximately 100 earthquakes each year are of M6 strength or greater; fortunately, most occur offshore or in unpopulated areas.

The severity of surface ground shaking at a particular distance from the source is described by its intensity at that location. The Modified Mercalli Intensity Scale, used in North America to quantify the degree of surface effects, ranges from MMI (barely perceptible ground motion) to MMXII (total destruction). MMVI is the threshold of damage. The depth of an earthquake focus varies from near surface to 700 km. Most continental earthquakes occur in the crust at depths of less than 40 km. Deep-focus earthquakes (below 300 km) are restricted to parts of very active seismic belts.

In Canada earthquakes occur in the Cordillera, along the West Coast and offshore, in the Yukon and the High Arctic, in eastern Canada and along the eastern seaboard. Elsewhere in Canada, earthquakes are rare and of small magnitude. In the Cordillera, moderate earthquakes occur in the crust at minor to moderate levels of activity throughout, in a generally compressive regime. A large earthquake (M7) occurred in the vicinity of Hope, BC, or farther south in northern Washington State, in 1872. This is the largest known event in the region; all others have been M6 or less.

Along the West Coast and beneath Vancouver Island and the Strait of Georgia, earthquakes are the product of active plate subduction (see plate tectonics; geological history). The small Juan de Fuca Plate is at present being thrust beneath the Olympic Mts and southern Vancouver Island. The result is a zone of earthquakes beneath Puget Sound at depths of 70-100 km, overlain by a zone of shallower, generally smaller events. The regime extends north to Canadian territory and the southern Strait of Georgia. The smaller Explorer Plate is no longer being thrust beneath northern Vancouver Island; thus the junction of the 2 plates is a shear zone marked in the offshore by the Nootka Fault and extending deep beneath the islands. The shallower crust is occasionally subjected to large earthquakes (eg, the M7 event on Vancouver Island near Courtenay, 1946), presumably in response to the shearing stresses below.

Offshore, a transform-spreading ridge system separates the Explorer and Juan de Fuca plates from the Pacific Plate. None of the many earthquakes occurring there each year is felt onshore. Farther north the transform system intersects the Continental Shelf and extends north along the shelf as a transcurrent fault. Canada's greatest known earthquakes have occurred along this feature. In 1949 a 200 km long segment of the fault ruptured in an M8 earthquake that caused chandeliers to sway in Jasper. The thrusting of the Pacific Plate beneath Alaska generates many severe (M8) earthquakes that may affect Canadian territory. A Good Friday 1964 M8.6 earthquake generated a tsunami or seismic sea wave that caused considerable damage in Port Alberni, Vancouver Island.

There are active seismic zones in the Richardson and Mackenzie mountains, YT, and a relatively limited seismic zone in the Beaufort Sea. Numerous earthquake zones occur in the arctic islands, one extending south through the Boothia Peninsula, across the mouth of Hudson Bay and northern Québec to the Labrador Sea. Baffin Bay was the site of an M7 earthquake in 1933. An event of similar magnitude occurred in the Laurentian Channel south of Newfoundland in 1929, generating a tsunami that caused considerable damage and drowned 27 people on the Burin Peninsula.

In eastern Canada seismicity is diffuse throughout the northern Appalachians, the series of M5 events in the Miramichi region of NB in 1982 being the most recent example. More concentrated zones occur at the mouth of the St Lawrence, in the St Lawrence Valley near La Malbaie in Charlevoix County, and in western Québec. The Charlevoix zone is the site of most of the larger events in eastern Canada. One of the largest occurred in 1534-35 between the voyages of Jacques Cartier, and was reported to him by the native people of the area on his return. More recently, an M7 earthquake in 1925 was felt throughout eastern Canada and the northeastern US. In the western Québec zone, damaging earthquakes have occurred at Temiscaming (1935) and Cornwall (1944). Both events were approximately M6.

Earthquakes in Canada are monitored by the National Seismograph Network operated by the Geological Survey, Natural Resources Canada. Approximately 100 seismographs are distributed from St John's to Vancouver Island and from Alert in the High Arctic to the Niagara Peninsula. The network is capable of detecting all earthquakes greater than M3.5 anywhere in Canada and those greater than M2 in more densely populated regions. Approximately 300 events greater than M3 are located annually. Probability estimates of seismic ground motion resulting from Canadian earthquakes are incorporated into the National Building Code of Canada; structures designed according to its provisions can resist moderate earthquakes without significant damage and major earthquakes without collapse.