What is an earthquake?
An earthquake is the rapid shaking of the earth's surface that
follows the sudden release of energy within the earth, created by
various means such as a volcanic eruption, or a movement of molten
material at depth. But most commonly, they are due to sudden fractures
along the edges of the plates which make up the earth's crust. These
plates are continuously moving against one another. Over geological
time, the earth is subjected to stresses and distortions that cause
mountains to be built and continents to drift apart. Under these
forces, rocks gradually bend or stretch. They do this until they
can resist no longer - until the stress is greater than the strength
of the rocks. Then they break (snap) into a new position. The snapping
creates vibrations or seismic waves. It is these vibrations that
we feel as an earthquake when they reach the surface.
The place at which the energy is released is called the focus of
an earthquake. The focus is underground. The point on the earth's
surface directly above the focus of the earthquake is called the
epicentre. The ground motion produced when the waves from an earthquake
reach the surface around the epicentre can vary a great deal. The
motion depends on the depth of the focus, local surface and subsurface
conditions (such as whether the ground is unstable or is firm rock)
and on the magnitude of the earthquake. The intensity of these ground
motions is most severe at the epicentre and generally decreases
with distance away from it.
The magnitude, or size, of an earthquake is measured on the Richter
Scale. The magnitude of an earthquake is the measure of the quantity
of energy released by the earthquake. Earthquakes with magnitude
of 2.0 or less are usually called micro-earthquakes and are normally
too small to be felt. Earthquakes with a magnitude of about 5 or
greater are strong enough to be recorded by seismographs all over
the world and earthquakes of 6 or greater can do damage. Great earthquakes
have a magnitude of 8 or higher.
The intensity of the effects of an earthquake at the surface of the earth is measured on the Modified Mercalli Scale. On this scale, the effects may range from I (not felt except by instruments) to XII (total damage).
What causes earthquakes in Canada?
Earthquakes along the West Coast are caused by the slow movement
of a series of major plates. In the area of the Queen Charlotte
Islands, two of the earth's largest plates, the North American Plate
and the Pacific Plate, are sliding past each other at about 6 millimetres
per year. Farther south, the Juan de Fuca Plate is thrusting beneath
the continent at about 4 centimetres per year, eventually melting
to produce the Cascade Range of volcanoes. This range includes Mount
St. Helens in Washington State, a recently active volcano.
By contrast, eastern Canada lies entirely within the North American
Plate and is far from its active boundaries, which are in the mid-Atlantic
(eastern boundary) or just off British Columbia (western boundary).
The forces producing earthquakes in eastern Canada 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.
In the Arctic Islands and on Baffin Island, 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 from the last continental glaciation
of this area.
Earthquakes that Shook Canada
Many large earthquakes have occurred in Canada’s short history.
Among the Canadian historical documents, it is in Jacques Cartier's
journal that the first mention of the occurrence of an important
earthquake is found. It probably happened around 1534 near La Malbaie,
about 100 kilometres downstream of the city of Québec. West
coast Aboriginal legends also allude to earthquakes confirming that
they regularly occur in some parts of Canada.
The magnitude of ancient earthquakes has been evaluated according
to the description of damages and ground shaking recorded in historical
documents. Two of the most important Canadian earthquakes that occurred
before measuring devices existed probably would have had a magnitude
between 7.0 and 7.5 on the Richter scale. One happened in near the
mouth of the Saguenay River in 1663; the other was in 1872, east
of Vancouver.
This map shows the location of some of the most important earthquakes
that happened since the Europeans arrived in Canada.
Descriptions for each of the major earthquakes shown on the map
are found in the document Major
Earthquakes. |
