Question 1. Which Are The Two Kinds Of Earthquakes?
Volcanic and Tectonic.
Question 2. How Is The Earthquake Measured?
By using the richter scale and Mercalli scale.
Question 3. What Is Richter Scale?
Richter scale is used to measure the total energy released during the earthquake.
Question 4. What Is Seismic Belt?
Some regions of the earth are prone to earthquake. These regions are called seismic belts.
Question 5. Who Devised Richter Scale?
Question 6. What Is Epicenter?
The place of origin of earthquake is known as epicenter.
Question 7. What Is Focus?
The point at which an earthquake occurs is called Focus or Epicentre.
Question 8. Which Are The Three Types Of Earthquake?
Shallow, Intermediate and deep.
Question 9. When An Earthquake Strikes Tremors Spread Towards In?
Question 10. Who First Detected The Seismic Waves?
Question 11. What Is An Earthquake? How Is It Formed?
An earthquake occurs when the structure of the earth shifts and moves. One of the causes is due to the movement of the molten rock down in the earth’s crust. The rocks become soft when they are exposed to the great heat of the interior of the earth, and his the change makes them move. Because of this movement beneath, we on the surface, feel that the ground is shaking. It is called an earthquake. The volcanic eruptions also cause earthquakes.
Question 12. What Is Mercalli Scale?
Mercalli scale (devised in 1902) is used to assess the effects of a quake at a particular place, rather than to measure the overall power.
Question 13. Which Is The Worst Earthquake Disaster Ever Recorded So Far?
In the Shensi, Shansi and honan provinces of china on February 2, 1556 which is estimated to have killed more than 830,000 people.
Question 14. What Is The Man Made Causes For Earthquake?
Filling of very large dams, direct injection of fluids into the ground, underground nuclear explosion.
Question 15. What Is An “earthquake”?
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.
Question 16. What Causes Earthquakes?
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.
Question 17. How Do Earthquakes Cause Damage?
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 is 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.
Question 18. Does The Earth Open Up During An Earthquake?
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.
Question 19. Where Do Earthquakes Occur?
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.
Question 20. What Is The Relationship Between Volcanoes And Earthquakes?
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:
- USGS – Plate tectonics
- University of Nevada – plate tectonics
- U.C. Berkeley
In Eastern and Northern Canada, earthquakes are not related to volcanic processes. Although volcanic rocks exist in many regions (sometimes as old as 2 billion years of age) and magmatic bodies can be found (the Monteregian Hills of Quebec are 60 million year old intrusive), 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.
Question 21. Will More Shocks Be Felt After A Strong Earthquake?
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 keeps in mind these four facts:
- In most cases, these shocks (called aftershocks) will be smaller; therefore, the vibrations will be weaker.
- Aftershocks do not mean that a stronger earthquake is coming.
- Aftershocks are normal; they show that the earth’s crust is readjusting after the main earthquake.
- The number of felt aftershocks is quite variable and thus cannot be predicted. There might be several per day, or only several per week.
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.
Question 22. 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, 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.
Question 23. 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 hypocenter (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 hypocenter 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.
Question 24. Are There Certain Months Of The Year That Are More Seismically Active Than Others?
- 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.
Question 25. What Is The Intensity Of An Earthquake?
See the Modified Mercalli Intensity Scale.
Question 26. Can People Cause 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.
Question 27. 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.
Question 28. What Is The “magnitude” Of 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 epicenter. 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.
Question 29. 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.
Question 30. 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 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).
Question 31. Certain Earthquakes Have A Negative Magnitude, Is This An Error?
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:
- Amplitude of 20 millimeters corresponds to a magnitude 2 earthquake.
- 10 times less (2 millimeters) corresponds to a magnitude of 1;
- 100 times less (0.2 millimeters) corresponds to magnitude 0;
- 1000 times less (0.02 millimeters) corresponds to magnitude -1.
Naturally, a negative magnitude is found only for very small events, which are not felt by humans.
Question 32. 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 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.
Question 33. At What Magnitude Do Earthquakes Begin To Be Felt? When Does Damage Start Do To Be Observed?
This is difficult to answer absolutely. According to past earthquakes, one can however draw up some general information for Eastern Canada.
Question 34. Do Several Magnitude Scales Exist?
Though seismologists generally refer to magnitude on the Richter scale, several magnitude scales do exist.
Question 35. How Often Do Earthquakes Occur?
Global Frequency of Earthquakes.
Question 36. 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.
Question 37. Are Earthquakes Really On The Increase?
No, earthquakes occur at more or less at the same rate every year.
Question 38. What 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 greatest world earthquakes since 1900 are described on the USGS site.
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