Perhaps you felt it, perhaps you didn’t. The seismic waves from the November 25th, 1988, earthquake passed beneath the feet of everyone reading this article, but your reaction would have depended on a number of factors, chief amongst them being your distance from Chicoutimi or Jonquiere, Quebec. That being equal, your reaction would have depended on the nature of the ground beneath you, and your "coupling" to that ground– That is, it would have varied if you were walking, riding in a car, living in a single or multistory building, in bed, in the bath, and so on.
Whatever the case, you should realize that Canada is earthquake country – particularly around its edges. The diaries of Jacques Cartier contain the earliest references to a large earthquake in Canada, thought to be an event in 1534 near La Malbaie, Quebec, and not far from the November epicentre. The following notes and the map of Canadian earthquakes of last decade 1975-85 are abstracted from a very informative Geofacts article distributed by the Geological Survey of Canada (mailing address given below).
Magnitudes of the early earthquakes have to be estimated from the historical reports of damage and ground shaking. Because the nature of the local rocks and the geological structure of a region affect the intensity of ground movement, these estimates are being continually refined in the light of better knowledge and the observation of later events. Two of the largest of these early earthquakes were an estimated magnitude 7.5, which occurred near the mouth of the Saguenay River in 1663, and an estimated magnitude 7.5 east of Vancouver in 1872.
In this century there have been eight events of magnitude 7 or 8 in Canada. In eastern Canada, a magnitude 7 earthquake occurred in 1925, again near La Malbaie in the St. Lawrence Valley. This was felt as far west as the Mississippi and as far south as Virginia. Although no one was killed or injured, most chimneys in the immediate epicentral area were destroyed and many stone buildings were damaged beyond repair. The great stone chimney of the church at Riviere-Ouelle fell through the roof and the organ pipes were thrown into the auditorium. Considerable damage was caused at Quebec City, Trois-Rivieres and Shawinigan Falls.
Only four years later, in 1929, a submarine earthquake (magnitude 7.2) beneath the Grand Banks south of Newfoundland caused an underwater landslide. This broke 12 trans-Atlantic cables in 28 places and set up a seismic sea-wave (tsunami), which drowned 27 people. The wave was 5 metres high and struck the south coast of the Burin Peninsula, sweeping away houses and causing an estimated $1 million damage. This was the most serious loss of life in any recorded Canadian earthquake.
On the west coast, magnitude 7 earthquakes occurred on Vancouver Island in 1918 and offshore in 1929. One of the best documented quakes was the 1946 magnitude 7.3 event near the east coast of Vancouver Island. At 10 a.m. on Sunday, June 23, a shock was felt over most of Vancouver Island and throughout the Vancouver and Lower mainland areas. Landslides and slumping occurred, and the shaking caused masonry to break and chimneys to fall. Inside buildings, shop goods and household objects were shaken off their shelves. Most of the serious damage was restricted to the east coast of the island. In the Courtenay area, 30 schools had to be closed because of earthquake damage. At one school a heavy chimney crashed through the roof into an empty classroom. The only person killed was a man who drowned when his boat was tipped by a wave created by subsidence along the shore of a lake.
The largest recorded earthquake in Canada, with a magnitude of 8.1, occurred off the Queen Charlotte Islands in 1949. However, because the islands are sparsely populated, the amount of damage was slight. In 1970 a magnitude 7.4 earthquake occurred just south of the islands but was too far offshore to cause any damage. Similarly, there was no recorded damage from a magnitude 7.2 earthquake in 1979 in a remote area on the Alaska-Yukon border.
One of the most spectacular results from a recent earthquake in Canada occurred on October 5, 1985. A magnitude 6.6 earthquake located near the North Nahanni River in the Northwest Territories triggered an immense rock avalanche containing an estimated 5 million to 7 million cubic metres of rock. Half a square kilometre of mountainside became detached and trees and automobile-sized blocks of rock slid 1.5 kilometres down a narrow valley. Fortunately the area is uninhabited.
Along the west coast, earthquakes are caused by the slow movement of a series of major plates forming the earth’s lithosphere (the outer cool, rigid part of the globe). In the area of the Queen Charlotte Islands, two of the largest plates on the earth – the North American Plate and the Pacific Plate – are sliding past each other at about 6 mm per year. Farther south, in California, the same action results in the San Andreas Fault. Between California and the Queen Charlotte Islands, a smaller plate – the Juan de Fuca Plate – is thrusting beneath the continent at about 4 cm per year, eventually melting to produce the Cascade Range of volcanoes, including Mount St. Helens.
By contrast, eastern Canada lies entirely within the North American Plate and far from its active boundaries in the centre of the Atlantic Ocean and along the west coast. The forces producing earthquakes here are different. It seems that the slow movement of the North American Plate away from the Mid-Atlantic Ridge may activate old zones of weakness and faults such as the St. Lawrence Valley, causing them to readjust and accommodate the continuing strain. The earthquakes of the Northwest Territories may be similar ‘adjustments’ within the folded rocks of the western mountain belts.
In the Arctic Islands and Baffin Islands, earthquakes also seem to be associated with older geological features. They may, however, also be related to stresses produced during the uplift of the land after the removal of the ice sheets of the last major glaciation of the Arctic region.
Here are a few facts about the Saguenay earthquake courtesy of Dr. Robert G. North, Head of the Seismology Program of the Geological Survey of Canada.
TIME: 18:46 EST, 25 November, 1989
PLACE: Latitude 48.12o North, Longitude 71.19o West
DEPTH: 29 km
MAGNITUDE: 6
Interesting features.
- 35 km south of the cities of Chicoutimi and Jonquiere; 150 km north of the Charlevoix-Kamouraska earthquake zone; no previously known significant earthquake activity in the Saguenay region.
- Widely felt, as expected for an earthquake of this magnitude (see map below). Felt strongly by most people within about 500 km, felt by many within 1000 km, perceptible by some persons in special circumstances beyond 1000 km; damage near epicentre, some isolated examples of damage up to 350 km; no deaths directly attributable to the earthquake, a few minor injuries.
- Largest earthquake in eastern North America since 1935. First of November 1935, near Temiscaming, Quebec, magnitude 6.2.
- Twice as deep within the earth’s crust as most of the eastern Canadian earthquakes previously studied.
- Aftershocks, relatively few and small. Up to mid-December 1988, 70 aftershocks recorded with magnitudes all less than 3.0, except for one magnitude 4.1 aftershock on 25 November at 22:38 EST.
- Foreshock on 23 November 1988, 04:11 EST, magnitude 4.7, felt widely.
- New engineering design data for eastern North America recorded at eleven sites in Quebec. First strong ground motion records (accelerograms) from a large earthquake in eastern North America; maximum horizontal and vertical accelerations on rock were 0.11g and 0.07g, respectively, recorded north of Chicoutimi at an epicentral distance of 43 km.
Dr. North was also provided a more intimate glimpse of the event, by using some seismic records at 6 stations ranging from 300 to 800 km from the earthquake. The vertical axis of the diagram gives distance from the source, in km. The horizontal axis gives reduced travel time, in seconds. "Reduced" means that the times of arrival of the P waves at each station has been adjusted by the transformation:
reduced = actual – Y/8.2 seconds
… where Y is the distance of the seismograph from the epicentre in kilometres. The constant 8.2 is the average velocity, in km/sec, of the P wave along its path outwards from the epicentre to the station. Therefore the reduced arrival times of the P waves at the various stations are all about zero.
Rumbling along behind the swift P waves on each record are the S waves and the surface waves (designated L). You should consult a text book for a description of these various wave types. S waves, like the P waves, can travel through the body of the earth (but unlike P waves not through the core). L waves must travel the longer route along the surface, and at a slower velocity. Can you show that the S wave velocity is about 4.5 km/sec, the L wave velocity about 3.5 km sec. (The arrival times of the L waves in Figure 3 were picked by an amateur – leave some room for error in this estimate!).
Notice also the size of the various arrivals. The P waves, though fast, are of low energy. Moving the ground surface rapidly up and down, they can produce a solid "thump", sometimes even a sonic boom-like effect as they are transformed into air waves, but little damage. Rather it is the S and L wave arrivals, together lasting as much as a minute and a half and shaking the ground surface from side to side, that cause most of the discomfort and destruction.
The information presented in this article was based almost entirely on material provided by and available from the Geological Survey of Canada, Geophysics Division, 1 Observatory Crescent, Ottawa, Ontario, Canada, K1A 0Y3.