Earthquake Resistant Buildings (22606)
1. Which Are The Two Kinds Of Earthquakes?
Volcanic and Tectonic.
2. How Is The Earthquake Measured?
By using the richter scale and Mercalli scale.
3. What Is Richter Scale?
Richter scale is used to measure the total energy released during the
earthquake.
4. What Is Seismic Belt?
Some regions of the earth are prone to earthquake. These regions are
called seismic belts.
5. Who Devised Richter Scale?
C.F. Richter
6. What Is Epicenter?
The place of origin of earthquake is known as epicenter.
7. What Is Focus?
The point at which an earthquake occurs is called Focus or Epicentre.
8. Which Are The Three Types Of Earthquake?
Shallow, Intermediate and deep.
9. When An Earthquake Strikes Tremors Spread Towards In?
Seismic waves
10. Who First Detected The Seismic Waves?
R.D. Oldham.
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.
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.
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.
14. What Is The Man Made Causes For Earthquake?
Filling of very large dams, direct injection of fluids into the ground,
underground nuclear explosion.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
25. What Is The Intensity Of An Earthquake?
See the Modified Mercalli Intensity Scale.
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.
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.
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
It is very unlikely that an earthquake of magnitude less than 5 could
cause any damage.
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.
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
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.
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.
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.
34. Do Several Magnitude Scales Exist?
Though seismologists generally refer to magnitude on the Richter scale,
several magnitude scales do exist.
35. How Often Do Earthquakes Occur?
Global Frequency of Earthquakes.
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.
37. Are Earthquakes Really On The Increase?
No, earthquakes occur at more or less at the same rate every year.
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.