An earthquake occurs when rocks break and slip along a fault in the earth. Energy is released during an earthquake in several forms, including as movement along the fault, as heat, and as seismic waves that radiate out from the "source" in all directions and cause the ground to shake, sometimes hundreds of kilometers away.
Earthquakes are caused by the slow deformation of the outer, brittle portions of "tectonic plates", the earth's outermost layer of crust and upper mantle. Due to the heating and cooling of the rock below these plates, the resulting convection causes the adjacently overlying plates to move, and, under great stress, deform. The rates of plate movements range from about 2 to 12 centimeters per year. Sometimes, tremendous energy can build up within a single, or between neighbouring plates. If the accumulated stress exceeds the strength of the rocks making up these brittle zones, the rocks can break suddenly, releasing the stored energy as an earthquake.
Most earthquake damage is caused by ground shaking. The magnitude or size (energy release) of an earthquake, distance to the earthquake focus or source, focal depth, type of faulting, and type of material are important factors in determining the amount of ground shaking that might be produced at a particular site. Where there is an extensive history of earthquake activity, these parameters can often be estimated. In general, large earthquakes produce ground motions with large amplitudes and long durations. Large earthquakes also produce strong shaking over much larger areas than do smaller earthquakes. In addition, the amplitude of ground motion decreases with increasing distance from the focus of an earthquake. The frequency content of the shaking also changes with distance. Close to the epicenter, both high (rapid) and low (slow)-frequency motions are present. Farther away, low-frequency motions are dominant, a natural consequence of wave attenuation in rock. The frequency of ground motion is an important factor in determining the severity of damage to structures and which structures are affected.
No! A common misconception is that of a hole in the ground that opens during an earthquake to swallow up unfortunate victims. This has nothing to do with reality but is Hollywood's version of earthquakes. After a strong earthquake, some cracks may be seen on the ground or in basements. These are not faults, nor are they crevasses ready to close up again. Theses cracks are probably due to soil settlement caused by the ground shaking.
Earthquakes occur all over the world; however, most occur on active faults that define the major tectonic plates of the earth. 90% of the world's earthquakes occur along these plate boundaries (that represent about 10% of the surface of the earth). The "Ring of Fire" circling the Pacific Ocean, and including Canada's west coast, is one of the most active areas in the world.
The earthquake activity of numerous volcanoes is closely monitored to provide warning signs of an imminent eruption. Large volcanic eruptions, especially the explosive type, can release huge amounts of energy that can be recorded by seismographs even far from the source.
Recent volcanic activity in Canada has been experienced in BC and the Yukon. Worldwide, the majority of volcanoes and earthquakes are located in the same areas. This relationship is explained through a geological model called plate tectonics. You can find additional explanations on plate tectonics:
In Eastern and Northern Canada, earthquakes are not related to volcanic processes. Although volcanic rocks exist in many regions (sometimes as old as 2 billions years of age) and magmatic bodies can be found (the Monteregian Hills of Quebec are 60 million year old intrusives), these magmatic events are just too old to have any relationship with current earthquake occurrences. No volcanic or magmatic activity is currently underway in these parts of Canada.
For more information on volcanoes in Canada, see Volcanoes of Canada (GSC).
For several hours, or even days, after a strongly felt earthquake, it is quite possible that people may feel more shocks. This possibility always exists, but keep in mind these four facts:
It is impossible to predict either the number or the magnitude of aftershocks that might occur. These vary greatly from one region to another, according to many factors which are poorly understood.
With the present state of scientific knowledge, it is not possible to predict earthquakes and certainly not possible to specify in advance their exact date, time and location, although scientists have carried out research on a wide variety of attempted prediction methods.
However, the rates of earthquakes in particular regions, expressed in terms of probabilities, can be usefully estimated. Canada, along with other countries, is working to minimize damage and injuries through the implementation of modern earthquake-resistant standards so people will be protected whenever and wherever an earthquake occurs.
Although cold temperatures greatly affect the ground near the surface, it has no effect at greater depths. Near the surface, freeze and thaw cycles can weaken and break rock due to high water pressure. However, this is a phenomenon limited to near surface soil.
Consider a mine: the temperature inside the mine will be influenced by surface temperature only for about the first 50 m. Deeper in the mine the temperature will be influenced by the internal heat of the earth - a temperature that is relatively constant throughout the year.
The hypocentre (the place where displacement occurs along a rock fracture) of an earthquake is generally located several km below the surface (on average, between 5-30 km in Eastern Canada), where the surface temperature would have no influence. For example, the hypocentre of the 1988 Saguenay earthquake occurred at a depth of 28 km where the temperature is approximately constant at 300°C year round.
Furthermore, the principle causes of earthquakes (movement of tectonic plates, volcanoes, etc.) are large scale phenomena, unrelated to surface temperature.
However, close to lakes and rivers, when the ambient temperature drops below -20°C many little microseisms may be heard and are sometimes felt. These microseisms are not earthquakes as they are caused by cracking ice and movements of ice blocks one against another. They are cryoseisms, also known as frost quakes, and can only be felt close to the body of water from which they originate. Such ice cracks can sometimes be detected by a seismograph if it is located close to the body of water.
Seismic trace of a typical frost quake recorded on the vertical component of the seismic station in Sadowa, Ontario, near Georgian Bay (SADO), January 18, 2000 at 6:55 pm, a very cold night (12 frost quakes were recorded within 2 hours that night). A seismologist immediately recognizes the nature of such an event by the single frequency contained in the record.
No, there are no months that have more earthquakes than others. Examining the list of Canadian or global earthquakes, there isn't a season that stands out as having an increased number of earthquakes.
The explanation for this can be found by considering that the mechanisms that cause earthquakes are independent of seasonal temperature changes ( see effects of cold temperatures on earthquakes ), and independent of the changes in position of the Earth in the solar system at different times of the year. It is internal geological forces that play the most important role in generating earthquakes.
Most large earthquakes are as a result of immense continental plates, called tectonic plates, that move, one with respect to another. The driving force for this movement is found in the Earth's mantle in the form of convective currents. These currents carry the tectonic plates around the Earth generating earthquakes and volcanic eruptions. The movement of the plates creates strain which is then accumulated in faulted areas causing earthquakes. Both the movement of the plates and the accumulation of strain along faults are continual processes independent of the time of year.
Since the distance between the Earth and Sun changes throughout the year due to the elliptical trajectory of the Earth around the Sun, it seems possible that the attractive gravitational forces between the two bodies might cause extra strain in the Earth's crust. However, strain models have shown that this extra force is insignificant compared to the tectonic force present.
Since the temperature and gravitational forces are the only forces changing with the seasons, seasonal effects can be eliminated as a factor in influencing the frequency of earthquakes.
Yes! Minor earthquakes have been triggered by human activities such as mining (rockbursts and cavity collapse), the filling of reservoirs behind large dams, and the injection of fluids into wells for oil recovery or waste disposal. Large dams hold back enormous quantities of water. Some of this water may penetrate into cracks in the underlying rock, and sometimes this may trigger small earthquakes under or very near the reservoir.
Following an underground nuclear explosion, small earthquakes have often been recorded near the test site. These are due to the collapse of the cavity created by the explosion.
Man-made earthquakes always occur close to the site of the activity. There is no link between human activities like these and earthquakes occurring hundreds or thousands of kilometres away.
No, except for very rare exceptions. Every year, hundreds of earthquakes occur in Canada. Only a very tiny minority of these precede a larger earthquake.
Although a large earthquake may be preceded by a foreshock (the Saguenay earthquake of November 1988 is an example), the occurrence of a small earthquake is not in itself a typical sign. Hundreds of small earthquakes occur every year in Canada, whereas major earthquakes have occurred only a few times in this century.
A small earthquake, however, provides an ideal opportunity to offer reminders about safety measures to take before, during and after an earthquake.
Magnitude is a measure of the amount of energy released during an earthquake. It is frequently described using the Richter scale. To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicentre. Since magnitude is representative of the earthquake itself, there is only one magnitude per earthquake.
Taking the Saguenay QU earthquake of November 25, 1988 as an example, one could not therefore speak of magnitude 6 at Quebec City and magnitude 4 to 5 at Montreal. The effects (or intensities) experienced at different places were different, but the magnitude of the earthquake is unique; in this example, it was 6 on the Richter scale. Magnitude thus has more to do with the effects of the earthquake overall.
The magnitude scale is logarithmic. This means that, at the same distance, an earthquake of magnitude 6 produces vibrations with amplitudes 10 times greater than those from a magnitude 5 earthquake and 100 times greater than those from a magnitude 4 earthquake. In terms of energy, an earthquake of magnitude 6 releases about 30 times more energy than an earthquake of magnitude 5 and about 1000 times more energy than an earthquake of magnitude 4.
It is very unlikely that an earthquake of magnitude less than 5 could cause any damage.
The Intensity scale is designed to describe the effects of an earthquake, at a given place, on natural features, on industrial installations and on human beings. The intensity differs from the magnitude which is related to the energy released by an earthquake.
Without going into the seismological details, the magnitude defined by Charles Richter is the source of all magnitude scales. Over the years however, it was realized that the magnitude that Richter had defined for California (ML means local magnitude), did not apply to Eastern North America where the seismic waves attenuate differently. Otto Nuttli, a seismologist at the University of Saint-Louis in the United States, developed a magnitude formula which corresponded better to the reality of Eastern America. One of the formulas which Nuttli derived is used to measure the seisms of Eastern Canada. The formulation used is called Magnitude Nuttli or mN. In order to simplify communication with the public, Canadian seismologists will often refer to the Richter magnitude whereas strictly speaking the seisms that occur in Eastern Canada are measured according to the Nuttli magnitude. An exception exists for the very small earthquakes of the Charlevoix Region, where the Richter scale is used. Around the world other scales of magnitude exist according to the source conditions of the earthquakes (depth), the conditions of attenuation, the type of measured wave, etc. More and more, seismologists describe earthquakes according to the magnitude of the moment scale (MW or M).
No, it is not an error. As magnitude calculations are based on a logarithmic scale, a ten-fold drop in amplitude decreases the magnitude by 1. Let us assume that on a seismogram:
Naturally, a negative magnitude is found only for very small events, which are not felt by humans.
Though theoretically there is no mathematical limit with the magnitude calculation, physically there is a limit. The magnitude is related to the surface area of the blocks of rock which rub together and in doing so give rise to seismic waves. Since the tectonic plates have finite dimensions, the magnitude must therefore also reach a maximum. It is believed that the greatest earthquakes can reach magnitude 9.5, which corresponds to the magnitude of the Chilean earthquake described below.
This is difficult to answer absolutely. According to past earthquakes , one can however draw up some general information for Eastern Canada.
Though seismologists generally refer to magnitude on the Richter scale, several magnitude scales do exist.
In addition to the international networks which can detect earthquakes of magnitude 5.0 and greater, the majority of the countries have their own national network.
No, earthquakes occur at more or less at the same rate every year. For more info: USGS web site
The greatest earthquake of recent history is the Chilean earthquake of May 22, 1960, which is estimated at magnitude 9.5. According to the USGS, this earthquake caused the death of more than 2000 people in Chile, in addition to generating a tsunami which propagated around the Pacific, adding several hundreds of victims to the assessment. The greatest world earthquakes since 1900 are described on the USGS site.
On average, the Geological Survey of Canada (GSC) records and locates over 4000 earthquakes in Canada each year. That is about 11 per day! Of these 4000, only about 50 (1/week) are generally felt.
Earthquakes occur across much of Canada. Most earthquakes occur along the active plate boundaries off the British Columbia coast, and along the northern Cordillera (southwestern corner of the Yukon Territory and in the Richardson Mountains and Mackenzie Valley) and arctic margins (including Nunavut and northern Quebec). Earthquakes also occur frequently in the Ottawa and St. Lawrence Valleys, in New Brunswick, and the offshore region to the south of Newfoundland.
Yes! Some of the world's largest earthquakes have occurred here (see next question).
The largest earthquake recorded (during historic times) in Canada was a magnitude 8.1 event that struck just off the Haida Gwaii on Canada's west coast on August 22, 1949. This earthquake (larger than the 1906 San Francisco earthquake) ruptured a 500-km-long segment of the Queen Charlotte fault and was felt over almost all of British Columbia, and as far north as the Yukon Territory and as far south as Oregon State.
Although not recorded by seismographs, the largest earthquake ever to strike Canada was undoubtedly the giant megathrust (subduction zone) earthquake of 1700 off the west Coast of Vancouver Island.
Every day! Scientists at the Geological Survey of Canada office near Sidney, B.C. record and locate approximately 1000 earthquakes each year in western Canada.
Yes! Some of the world's largest earthquakes have occurred in western Canada. Click here to see the 5 most significant.
Western Canada is the most seismically active region in Canada. It consists of several discrete areas of [intense earthquake activity][eqcan 5y west eng] each corresponding to a particular plate tectonic regime. The most seismic of these regions is offshore, west of Vancouver Island. More than 100 earthquakes with a magnitude of 5 or greater have occurred here in the past 70 years. Most of the seismicity occurs in areas of fractured oceanic crust, which mark boundaries of small plates known as the Explorer and Juan de Fuca plates
Earthquake activity is also high in the Cascadia Subduction Zone. Here, the Juan de Fuca Plate dips below the easterly neighbouring North American plate. Thus, both deep (dipping plate) and shallow (overriding plate) earthquakes occur in this zone, though no earthquakes occur at the interface of the plates. Another region of high seismicity is defined by a zone of plate breakage or "faulting" immediately west of the Queen Charlotte Islands ("the Queen Charlotte fault"). Earthquakes of magnitude 7 occurred here in May of 1929 and June of 1970.
The St. Elias Mountains, southwest Yukon Territory and the extreme northwest of B.C., too, is a highly seismic region. This is because of plate margin deformation between two converging plates in the area (the "Pacific" to the west and "North American" to the east.) Finally, the Canadian Cordillera typically shows intense seismicity north of 60 degrees in a broad zone through the Mackenzie and Richardson Mountains. The largest earthquake recorded here, with magnitude of 6.9, occured in the Mackenzie Mountains in December, 1985. South of 60 N, seismicity drops off markedly away from the coast to a low level through much of the Cordillera, though it is slightly higher in the Coast Mountains from southern British Columbia to the Yukon Border.
Understanding earthquake hazards involves many types of studies: monitoring earthquakes, monitoring crustal deformation; mapping the marine environment for evidence of offshore earthquake activity; studying wave propagation; mapping earth structure; understanding local geological conditions; and looking for geological evidence of prehistoric earthquakes.
Many different types of studies are conducted by scientists at the Pacific Geoscience Centre of the Geological Survey of Canada to better improve our understanding of earthquake hazards in western Canada.
The recurrence time varies from subduction zone to subduction zone. In the Cascadia subduction zone 13 megathrust events have been identified in the last 6000 years, an average one every 500 to 600 years. However, they have not happened regularly. Some have been as close together as 200 years and some have been as far apart as 800 years. The last one was 300 years ago.
Megathrust earthquake are the world's largest earthquakes. The last Cascadia earthquake is estimated at magnitude 9. A megathrust earthquake in Chile in 1960 was magnitude 9.5, and one in Alaska in 1964 was magnitude 9.2.
The Cascadia fault, on which megathrust earthquakes occur, is located mostly offshore, west of Vancouver Island, Washington, and Oregon, although it does extend some distance beneath the Olympic Peninsula of Washington State. The large distance between the Cascadia fault and the urban centres limits the level of shaking that the urban areas are exposed to.
The sudden submergence of the outer coast when a megathrust earthquake occurs kills vegetation which can be dated. Megathrust earthquakes also cause underwater landslides off the continental shelf into the deep ocean. The landslide deposits can be recognized in core samples taken from the ocean floor.
The deformation of the crust in a predictable pattern can be detected by very careful geodetic measurements using Global Positioning Satellites, precise levelling, micro-gravity measurements and changing distance measurements using laser technology.
No. Earthquake shaking, in the frequencies that damage buildings, increases to a maximum between a magnitude 7 and 8 earthquake, then the shaking simply involves a bigger area. However, the duration of shaking for a megathrust earthquake is much longer. It can be several minutes. This long duration can result in damage to some types of buildings that might not be damaged at the same strength of shaking produced by a smaller earthquake.
The Kobe earthquake was right beneath the city and the megathrust earthquake will be about 150 kilometres from Vancouver. The damage pattern would be very different. We can get a good example of the kinds of damage Vancouver can expect to experience if we look at what happened to Anchorage, Alaska, during the 1964 magnitude 9.2 megathrust earthquake. Anchorage is about the same distance from the Alaska subduction fault. Small buildings generally had little or no damage, unless they were affected by landsliding. Almost all the damage involved large buildings or large structures such as bridges.
No. Vancouver Island is part of the North American plate. The fact that there is water between Vancouver Island and the mainland is function of the current position of sea level. However, the west coast of Vancouver Island will drop as much as a metre or two when the next megathrust earthquake occurs.
No. Inland earthquakes, which are not as big but can be much closer to our urban areas and occur much more frequently, are our biggest earthquake hazard.
The thrusting motion of megathrust earthquake causes large vertical movement on the sea floor and this displaces a large volume of water which travels away from the undersea motion as a tsunami.
No. Just the coast exposed to the open Pacific is vulnerable to damaging tsunamis waves. The areas vulnerable to tsunamis are indicated in the red-tabbed pages of the telephone books published for the coastal communities of British Columbia.
West Coast and Alaska Tsunami Warning Centre (WCATWC) Message Definitions
Data from selected NRCan seismometers are forwarded to the National (United States) Oceanic and Atmospheric Administration’s (NOAA) West Coast and Alaska Tsunami Warning Centre (WCATWC) in Palmer, Alaska. This information is integrated with other seismic, tide gauge, and deep ocean buoy system data to produce tsunami information statements, alerts, watches, or warnings for all North American coastlines (including the Atlantic and Arctic). WCATWC distributes these messages to Emergency Measures Organizations (EMO) and other clients 5 to 15 minutes after a potentially tsunamigenic earthquake has occurred and provide updates at regular intervals.
WCATWC product definitions changed to the definitions provided below on February 12, 2008. The products issued by the center are warning, watch, advisory, and information statements. Each has a distinct meaning relating to local emergency response. In summary:
|Warning||->||Inundating wave possible||->||Full evacuation suggested|
|Watch||->||Danger level not yet known||->||Stay alert for more info|
|Advisory||->||Strong currents likely||->||Stay away from the shore|
|Information||->||Minor waves at most||->||No action suggested|
Based on seismic data analysis or forecasted amplitude (dependent on whether the center has obtained sea level data), WCATWC will issue the appropriate product. Warnings and Advisories suggest that action be taken. Watches are issued to provide an early alert for areas that are distant from the wave front, but may have danger. Once the danger level is determined, the watch is upgraded to a warning or advisory, or canceled. The full definition of each message is given below and further details are available at http://wcatwc.arh.noaa.gov/, the WCATWC website.
Tsunami Warning - a tsunami warning is issued when a potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.
Tsunami Watch - a tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or advisory - or canceled - based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.
Tsunami Advisory - a tsunami advisory is issued due to the threat of a potential tsunami which may produce strong currents or waves dangerous to those in or near the water. Coastal regions historically prone to damage due to strong currents induced by tsunamis are at the greatest risk. The threat may continue for several hours after the arrival of the initial wave, but significant widespread inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.
Tsunami Information Statement - a tsunami information statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.
No. It takes many, many small earthquakes to release the amount of energy equivalent to a large earthquake. The amount of energy released increases about 40 times every time there is an increase of one unit on the magnitude scale. Thus, if we consider a small earthquake at the felt level, about magnitude 2, there would have to be 40x40x40x40x40x40x40 of these earthquakes to release the amount of energy as one magnitude 9 event. That is about one million small earthquakes a day, every day, for 500 years. That level of earthquake activity is not observed.
This Earthquakes Canada site is the authoritative source of information on Canadian eathquakes. Available here, among other things:
No casualities were ever directly related to Canadian earthquakes. In fact, Canadian earthquakes have never caused the collapse of a building. Only some injuries were caused by the fall of objects.
Although it has been reported that a yound girl was killed during the 1732 Montreal earthquake, it has never been substanciated by independent sources.
Yes! While there are differences between the recordings of an earthquake and a nuclear explosion, the same basic instrumentation and measurement techniques apply. Being geographically the second largest country in the world, Canada plays an important role in nuclear explosion monitoring.
If you live in the East or the North of Canada, the presence of faults in your area is not indicative of a higher probability earthquakes. In these areas, the faults represent very old geological movements. The Geological Survey of Canada has produced maps for certain areas of Canada. You can consult what is available in the GEOSCAN database.
Building your own seismograph is possible, but it requires time and materials. If your project is due tomorrow, forget about it! If you have a little more time here is a reference:
The 1979 article is reproduced on the Redwood City (California) Public Seismic Network site.
The vertical scale has been adjusted to a level intended to suppress most local noise and emphasize Canadian earthquakes. These displays are intended to provide qualitative information for the general public. There is no simple correspondence between amplitude on the real-time seismogram viewer and earthquake magnitude, as it depends on the distance to the earthquake and other factors.
Those requiring detailed technical information can download waveforms from our waveform archive; however, using and interpreting the data may require specialized seismological software and expertise. Some recordings which can look quite large are actually just noise such as wind or human activity close to the seismograph station. See "Interpreting Seismograms".
Yes! Engineers can, and are, designing earthquake-resistant structures.
The first seismic hazard maps for use in Canada have been in use since 1953. This initial hazard map was a subjective assessment based on historical seismicity. In 1970 the first modern maps were developed using probabilistic methods. In 1985 two maps were produced, "acceleration" - suitable for use when designing small structures, and "velocity" - suitable for use when designing large structures.
Seismologists at the Geological Survey of Canada produce seismic hazard maps for use in the National Building Code of Canada.
The safest type of structure is a modern, well-designed, and well-constructed building. Generally, wood-frame houses perform very well during an earthquake. However, even these structures are prone to damage from soil failure, chimneys may be damaged or collapse, windows may break, interior walls may crack, and those houses not securely bolted to their foundation may fail at or near ground level. For more information on your home and earthquakes, click here. For some examples of damage to typical wood-frame houses during the M=7.3 Vancouver Island earthquake of 1946, click below:
Unreinforced masonary structures (those not seismically upgraded) are generally more vulnerable to earthquake damage. For some photos of damage caused to unreinforced masonary structures during the M=7.3 Vancouver Island earthquake of 1946, click below:
Falling objects pose the greatest danger during a major earthquake. In Canada, no house has ever collapsed during an earthquake. However, many types of objects may fall and cause damage or injuries. Of prime concern, therefore, is protection from falling objects such as framed pictures, light fixtures, plaster from ceilings or the upper part of walls, or chimneys which may fall outside or through the roof into the house.
Here is what to do:
To learn more about earthquake preparedness, follow the links at Preparing for earthquakes.
For more information on earthquake preparedness and what to do during and after earthquakes, follow the links at Preparing for earthquakes.
Most earthquake damage is caused by ground shaking. The magnitude or size of an earthquake, distance to the earthquake focus or source, type of faulting, depth, and type of material are important factors in determining the amount of ground shaking that might be produced at a particular site. Where there is an extensive history of earthquake activity, these parameters can often be estimated.
The magnitude of an earthquake, for instance, influences ground shaking in several ways. Large earthquakes usually produce ground motions with large amplitudes and long durations. Large earthquakes also produce strong shaking over much larger areas than do smaller earthquakes. In addition, the amplitude of ground motion decreases with increasing distance from the focus of an earthquake. The frequency content of the shaking also changes with distance. Close to the epicenter, both high (rapid)and low (slow)-frequency motions are present. Farther away, low-frequency motions are dominant, a natural consequence of wave attenuation in rock. The frequency of ground motion is an important factor in determining the severity of damage to structures and which structures are affected.
Generally speaking, Canadian wood-frame houses are well able to withstand vibrations generated by earthquakes - even very large ones. Moreover, modern buildings must be designed according to national or provincial building code standards, which are intended to minimize the probability of building collapse in major earthquakes.
However, building codes do not prevent certain types of non-structural damage. Thus, it is possible that cracks may be seen on some walls. Unreinforced masonry (e.g. brick walls and chimneys) has little resistance to strong horizontal shaking and may collapse. Vibrations may also cause ground settlement under a house. Sometimes this may cause small cracks in the basement or warping of walls. These are indirect effects that do not indicate that a fault lies near the house.
For more on the effects of earthquakes on buildings, see section 4 above, "Seismic Hazards and Earthquake Engineering." See also How would your home stand up?
In the hour immediately following a relatively large earthquake, GSC Seismologists locate the earthquake and measure its magnitude. They use data supplied by the national seismograph network, which feeds continuous data 24 hours per day to the Ottawa and Sidney, BC offices. They pass this information on to the federal Office of Critical Infrastructure Protection and Emergency Preparedness, Provincial Emergency Program offices, to the news media - and, in Quebec, to the Quebec Provincial Police and to Hydro-Quebec.
During the following hours, the seismologists decide whether it would be feasible to conduct a field survey to learn more about the geological environment where the earthquake occurred, and to record any aftershocks that might occur in the ensuing hours and days.
In a field survey, seismologists set up portable seismographs to measure any further release of energy through small earthquakes. This information is analyzed in the weeks and months after the main earthquake and permits scientists to better understand the phenomenon of earthquakes in Canada. In the short term, this information cannot be used to predict earthquakes. In the long term, it will provide the basis for a more comprehensive understanding of seismic activity in the region.
Also, if the earthquake was large, other scientists specializing in surface deposits (clay, sand) may join the field survey team. Engineers may also come to inspect buildings to better determine the effects of the earthquake. Some of these specialists may return again after several months to gather additional data.