Earthquakes are ground movements of great intensity that occur in areas where the likelihood of earthquakes are real and variable. These areas are called high seismic zones. It is impossible to predict an earthquake, so we must act on the vulnerability of our society in order to protect ourselves. The strength of an earthquake is measured by its magnitude – the energy that is released during an earthquake – using an algorithmic scale known as the Richter scale. To date, the strongest recorded earthquake reached 9.5 on the Richter scale (Chile, 1960).
During a major earthquake, seismic waves spread inside the Earth and on its surface. In general, the further away from the source of these seismic waves, the weaker ground movements are. However, in some circumstances, this statement has not been verified. These are known as site effects. Local geology greatly influences ground movements on the surface, no matter the distance from the source of the earthquake. For example, clay and sand can be the source of major site effects, increasing the amplitude and duration of seismic waves and leading to more widespread human and economic damage than for rock sites at the same distance.
Preparing and protecting yourself
Risks in Montreal
Montreal is in the Western Québec Seismic Zone, which extends from Montreal to Témiscamingue and the Laurentians east of Ontario. It is considered the second most vulnerable city for earthquakes, after Vancouver, because of its high population density and type of soil. Since seismological monitoring stations were installed in eastern Canada, it has been shown that the Montréal area is in a zone of moderate seismic hazard, where a 5-6 magnitude earthquake occurs approximately every 25 years, and every 100 years for earthquakes with a magnitude greater than 6. The strongest earthquake to hit Montréal was estimated at 5.8 on the Richter scale and happened in 1732.
As with many cities across the world, site effects are the primary aggravating factor in Montreal in terms of earthquakes. Just 20 years ago after the Saguenay earthquake in 1988, the Montreal-Est city hall suffered irreparable damages and had to be rebuilt. A detailed analysis showed that the mediocre state of the building and the large layer of clay underneath the building made it extremely vulnerable during ground movements, even though the earthquake took place more than 300 km away.
Here are some of Québec’s biggest earthquakes;
Source : Natural Resources Canada, Earthquake Zones in Eastern Canada
Risks following an earthquake
Although an earthquake is one of the most destructive hazards on earth, a number of related phenomena can worsen the situation:
When the soil is full of water, liquefaction can occur during an earthquake.
A mass of soil that slides towards the bottom of a slope is a common occurrence after an earthquake. In Montreal, the risk is low because the highest altitude in Montreal, other than Mount Royal, is the Saint-Jacques cliff, an area that is not at risk for a major earthquake.
Falling rock or debris
Weakened infrastructures can cause falling rock or debris, which can cause major accidents and affect residents’ safety.
Major industrial accidents
Ground movements can create major breakages in at-risk facilities, releasing chemical substances or causing major fires.
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A tsunami is a series of large waves often caused by the shock wave of an underwater earthquake or other phenomena. When waves approach the shore and the water becomes more shallw, the waves become higher. They can happen without warning and cause major flooding and damage to shoreline areas.
Source : Natural Resources Canada
Impact on the population and infrastructures
Earthquakes are very destructive phenomena that can impact the population. In Quebec, no recorded earthquake has been especially devastating.
- Damages to buildings and infrastructures (bridges, tunnels, electric lines, etc.)
- Falling debris that could injure passersby
- Sectioning off roads due to large amounts of debris
- Underground pipe breakage