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Other source of information: Questions and Answers About Earthquakes in Western Canada
This page is continuously under construction.
INDEX1- Seismic Sources - Earthquakes, Nuclear Blasts, Mining EventsEarthquakesGeneral
What causes earthquakes along the boundaries of tectonic plates? What is the relationship between volcanoes and earthquakes? Will more shocks be felt after a strong earthquake? Can earthquakes be predicted? Can earthquakes be triggered by human activity? Does the rate of earthquakes increase during the cold weather? Are there certain months of the year that are more seismically active than others? What is the intensity of an earthquake? Does a small earthquake mean that a larger earthquake is coming? Magnitude
What is the difference between the "magnitude" and the "intensity" of an earthquake? What it is the difference between magnitudes ML and mN? Certain earthquakes have a negative magnitude, is this an error? Is there a maximum magnitude for an earthquake? At what magnitude do earthquakes begin to be felt? When does damage start do to be observed? Do several magnitude scales exist? Distribution and frequency of Earthquakes
How often do earthquakes occur? Where can I find information on the world's earthquakes? Are earthquakes really on the increase? What was the greatest earthquake in world history? B- Canada Where can I find information on Canadian earthquakes? What is the greatest magnitude earthquake in Canadian history? In Canada, how many casualties were caused by earthquakes? What do the acronyms stand for in the list of Canadian earthquakes? Geology (faults, landslides, etc) 2- Instruments and networks
What do seismic waves look like? Is it possible to build your own seismograph? 3- The Canadian National Earthquake Database4- Seismic Hazard5- Facing Earthquakes
What should you do after a strong earthquake? What causes damage? Can houses withstand earthquakes? 6- Seismologists7- Non-Seismology Related Questions
Please visit the Geological Survey of Canada
1- Seismic Sources - Earthquakes, Nuclear Blasts, Mining EventsEarthquakesGeneralWhat is an "earthquake"?Physically, an earthquake is the result of a sudden movement of two blocks of rock along a break (fault) deep within the earth's crust. After very strong earthquakes, this slippage may be visible on the surface. In eastern Canada, such slippage along faults has always occurred within the crust. Although many faults in eastern Canada are visible on the surface of the ground, none has ever been shown to be currently active. These faults result from geological movement occurring over millions of years. The presence of these surface faults does not indicate that future earthquakes will occur there. A common misconception is that of the hole in the ground which opens up during an earthquake to swallow up unfortunate victims. This has nothing to do with reality and it 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 ground settlement cased by vibrations.What causes earthquakes along the boundaries of tectonic plates?Earthquakes along plate boundaries ("interplate earthquakes") occur from the deformation of 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 stresses, 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. Ninety percent of the world's earthquakes occur along the boundaries of plates.What is the relationship between volcanoes and earthquakes?Earthquakes can be associated with volcanic activity. Worldwide, 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.
Worlwide, the majority of volcanoes and earthquakes are located in the same areas. This relationship is explained through a geological model called plate tectonics (see question/answer above).
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 current volcanic or magmatic activity is currently underway in these parts of Canada.
Can earthquakes be predicted?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. However, a great deal of research is being conducted to develop reliable prediction methods. 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.Does the rate of earthquakes increase during the cold weather?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. Are there certain months of the year that are more seismically active than others?No, there aren't any 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 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. What is the intensity of an earthquake?See the Modified Mercalli Intensity Scale.Can earthquakes be triggered by human activity?Human activities, such as underground nuclear explosions or the filling of reservoirs behind large dams, may sometimes trigger earthquakes, but they always occur close to the site of such activities. 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. On the other hand, 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. There is no link between human activities like these and earthquakes occurring hundreds or thousands, of kilometres away.Does a small earthquake mean that a larger earthquake is coming?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. MagnitudeWhat is the "magnitude" of an earthquake?Magnitude is a measure of the amount of energy released during an earthquake. It may be expressed 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 thus only one magnitude per earthquake. Taking the Saguenay 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) at the two places were different, but the magnitude of an earthquake is unique; in this example, it was 6 on the Richter scale. 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.What is the difference between the "magnitude" and the "intensity" of an earthquake?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. What it is the difference between magnitudes ML and mN?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 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).Certain earthquakes have a negative magnitude, is this an error?No, it is not an error. As you know, magnitude is based on a logarithmic scale, thus a ten-fold drop in amplitude decreases the magnitude by 1. Let us assume that on a seismogram: an amplitude of 20 millimetres corresponds to a magnitude 2 earthquake. 10 times less (2 millimetres) corresponds to a magnitude of 1; 100 times less (0.2 millimetres) corresponds to magnitude 0; 1000 times less (0.02 millimetres) corresponds to magnitude -1. Naturally, a negative magnitude is found only for very small events, which are not felt by humans.Is there a maximum magnitude for an earthquake?Though theoretically there is no mathematical limit with the magnitude calculation, physically there is a limit. The magnitude is connected to the surface area of the blocks of rock which rub 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 above.At what magnitude do earthquakes begin to be felt? When does damage start do to be observed?It is difficult to answer absolutely. According to past earthquakes , one can however draw up some general information for Eastern Canada.Do several magnitude scales exist?Though seismologists generally refer to magnitude on the Richter scale, several magnitude scales do exist. The United States Geological Survey (USGS) site describes several.Distribution and frequency of EarthquakesA- GlobalHow often do earthquakes occur?Where can I find information on the world's earthquakes?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. All these networks are listed on the University of Washington site.Are earthquakes really on the increase?No, earthquakes occur at more or less at the same rate every year. For more info: USGS web siteWhat was the greatest earthquake in world history?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 10 greatest world earthquakes between 1900 and 1999 are described on the USGS site.B- CanadaWhere can I find information on Canadian earthquakes?Earthquakes Canada's site contains a large volume of information on Canadian eathquakes. Available there, among other things:What is the greatest magnitude earthquake in Canadian history?The 1949 earthquake of magnitude 8.1 in the Queen Charlotte Islands archipelago was the greatest Canadian earthquake ever recorded. An even greater earthquake, would have occurred on January 26, 1700, and was felt across British-Columbia. In Canada, how many casualties were caused by earthquakes?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. In Canada, the only loss of life related to an earthquake, although indirectly, were those caused by the tsunami created by the 1929 Grand Banks earthquake. Geology (faults, landslides, etc)Where can I find information on the faults and the geology of my area?If you live in the East or the North of Canada, immediately the presence of faults in your area is not indicative of a higher probability earthquakes. In these areas of Canada, 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. If however you wish to obtain detailed geological maps of your area, you must contact your Provincial Office of Natural Resources.
2- Instruments and networksWhat is a seismograph?What do seismic waves look like?Is it possible to build your own seismograph?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:
Note: If you cannot obtain this article, we would be pleased to send a copy through the mail to you (PLEASE specify the subject of your message).
3- The Canadian National Earthquake DatabaseWhat do the acronyms stand for in the list of Canadian earthquakes?Please consult the Earthquake List Legend
4- Seismic Hazard
5- Facing EarthquakesWhat should you do during an earthquake?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: Self-Help Advice for Families and Individuals - Be Prepared, Not Scared (Public Safety and Emergency Preparedness Canada) What should you do after a strong earthquake?
To learn more about earthquake preparedness: Prepare yourself and your family Public Safety and Emergency Preparedness Canada What causes damage?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. Can houses withstand earthquakes?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.6- SeismologistsWhat is a seismologist?What do scientists do after an earthquake?During the hour immediately following a relatively large earthquake (over magnitude 4.5 for eastern Canada), Geological Survey of Canada seismologists locate the earthquake and measure its magnitude. They use data supplied by the seismograph network in eastern Canada, which is linked directly 24 hours per day to the Ottawa office. (In British Columbia, a similar network is linked to the office in Sidney, B.C.). They pass this information onto Office of Critical Infrastructure Protection and Emergency Preparedness and to the Canadian press - and, in Quebec, to the Quebec Provincial Police and to Hydro-Quebec as well. In the following hours, the seismologists decide whether it is desirable 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 scientific analysis 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.
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