Earthquakes are violent movements or trembling of the earth caused by shifting tectonic plates. The crust of the earth is composed of a thin, hard layer of rock. The crust is broken into pieces or plates (six major plates and nine smaller ones). Crustal plates move very slowly, at about the same pace as fingernails grow, but they can become jammed, causing a build-up of pressure. The sudden release of this stress results in an earthquake.
Each year, there are more than three million earthquakes that occur around the world. Fortunately, most are small and cause no damage. However, approximately 1,000 are capable of causing damage (magnitude 5 or higher on the Richter scale–see below), and 18 are classified as severe events (magnitude 7 or higher). Earthquakes can happen anywhere, but more than 90 percent occur near the boundaries where major tectonic plates meet.
There are about 1,500 earthquakes recorded in Canada each year. A few dozen are strong enough to cause damage. The strongest earthquakes occurred near the Pacific rim. Significant earthquakes have also occurred in the St. Lawrence and Ottawa river valleys, and in the Arctic. There are several Canadian communities vulnerable to earthquakes including Vancouver, Montreal, Ottawa, Victoria and Quebec City.
The Richter scale measures the strength of an earthquake from 0 (weak) to 10 (intense). For every ten-fold increase in the ground motion, the Ritcher measure increases by one number, so a magnitude 6 quake is ten times stronger than a 5. Mild quakes (magnitude of 4 or lower) may be felt by some people, but should not cause damage. Strong quakes (magnitude of 7 or more) can do major damage. Actual damage depends on a number of factors, including the depth of the epicentre, soil conditions, and how long the shaking continues.
Earthquakes can cause terrible losses. For example, in 1995, Japan experienced an earthquake causing more than C$150 billion in damage. In 1960, Chile experienced the strongest earthquake ever recorded: 9.5. Some of the world’s largest earthquakes have occurred in western Canada including a magnitude 9 subduction earthquake west of Vancouver Island in 1700, and the 8.1 event in the Queen Charlotte Islands in 1949.
The collapse of buildings, bridges and other structures is the primary cause of death due to earthquakes. Fortunately, no house in Canada is known to have collapsed during an earthquake. The excessive shaking of the ground causes objects to fall, posing the greatest threat of injury during an earthquake. The shifting of the tectonic plates can also trigger other hazards, including fires and tsunamis. Indeed, an earthquake off Newfoundland’s Burin Peninsula in 1929 generated a 7 m tsunami that drowned 28 people, the largest recorded loss of life in Canada due to an earthquake.
