The shock was felt six hundred miles away, including much of New England. Damage to buildings was substantial but there were no casualties.
On March 1, 1925, an earthquake of magnitude 6.7 rocked the lives of thousands of people in eastern Canada. The location was Charlevoix-Kamouraska and the depth of the quake was six miles. The shock was felt six hundred miles away. The main quake was followed by a series of aftershocks caused by the readjusting of the earth’s crust. The earthquake was widely felt and caused damage, especially to unreinforced masonry buildings, along the St. Lawrence River, near the epicenter, and at Quebec City, Trois-Rivieres, and Shawinigan.
The St. Lawrence Valley represents an enormous break in the earth’s crust. About 350 million years ago an enormous meteorite collided with this fracture zone, in the Charlevoix area, further weakening the earth’s crust. In fact, the most seismically active area in eastern Canada is the Charlevoix-Kamouraska area. In 1663 and 1925, the largest earthquakes ever recorded in Quebec were centered in this particularly vulnerable area.
There is a distinctive feature about earthquakes in eastern Canada in that the crystalline rocks of the Canadian Shield transmit seismic waves very effectively. They die out much more slowly than waves produced by earthquakes in areas such as the West Coast. Thus, damage and shaking occur at greater distances from the epicenter. This explains why, on November 25, 1988, an earthquake of magnitude 6 centered in the Saguenay, was strongly felt in Quebec City and as far away as Washington, D.C., six hundred miles from the epicenter. Objects were shaken off shelves, a power failure followed and minor damage was done to some buildings in the Lower Town of Quebec City.
For many people who had never experienced an earthquake before, it was an unsettling and even frightening event. Although the earthquake did not cause any direct casualties, uneasy feelings were widespread among the population. Numerous earthquakes have often occurred in eastern Canada. Some of them had significant geological effects such as surface faulting, liquefaction, submarine slumping, rock avalanches, rock falls, landslides, railroad embankment slides, and one tsunami. These earthquakes had a strong psychological and social impact on people, mainly due to their lack of preparedness for them.
The probability of an earthquake being centered precisely below the Quebec City area is low because very few such earthquakes have ever been recorded. But we are not immune to significant damage because if a major earthquake, like the one that occurred in 1925, were to be centered in the Charlevoix area, Quebec City would experience it. Although earthquakes of magnitude less than 5.5 do not generally do any damage, destructive effects are not related solely to the magnitude of the earthquake.
The distance between the epicenter and built-up areas, the condition and type of construction in the area, and the nature of the ground are all factors that can have a significant impact on the extent of damage. Buildings constructed on rock are more earthquake resistant. In unconsolidated sediment, earthquakes are much more intensely felt. When earth fill is not well compacted and contains a great deal of water, it behaves like jelly. Some parts of Quebec City would be at greater risk than others. For example, the valley of the Saint- Charles River is filled with clay sediment, which tends to amplify seismic vibrations.
The previous occurrence of large earthquakes suggests that this part of Canada may be the site of a large earthquake in the future. The study of these past quakes helps to calculate a seismic hazard assessment for surrounding urban areas because it is known that the 1925 quake damaged buildings 150 miles from the epicenter. The National Building Code of Canada contains guidelines for the design and construction of new buildings and for the renovation of existing structures. These guidelines are intended to ensure that buildings are more resistant to the shaking that accompanies an earthquake. Throughout Canada, buildings that are constructed in the areas most susceptible to seismic vibrations must adhere to stricter standards. The higher the seismic hazard, the more safety factors must be taken into account. For example, the seismic hazard in the Quebec City area is ranked at four on a scale of 0–6 for ground acceleration.