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Earthquake
An earthquake is a sudden vibration or trembling in the Earth. More than 150,000 tremors
strong enough to be felt by humans occur each year worldwide. Earthquake motion is caused
by the quick release of stored potential energy into the kinetic energy of motion. Most
earthquakes are produced along faults, tectonic plate boundary zones, or along the midoceanic ridges. At these areas, large masses of rock that are moving past each other can
become locked due to friction. Friction is overcome when the accumulating stress has enough
force to cause a sudden slippage of the rock masses. The magnitude of the shock wave
released into the surrounding rocks is controlled by the quantity of stress built up because of
friction, the distance the rock moved when the slippage occurred, and ability of the rock to
transmit the energy contained in the seismic waves. The San Francisco earthquake of 1906
involved a 6 meter horizontal displacement of bedrock. Sometime after the main shock wave,
aftershocks can occur because of the continued release of frictional stress. Most aftershocks
are smaller than the main earthquake, but they can still cause considerable damage to already
weakened natural and human constructed features.
Types of Earthquakes
There are two main types of earthquakes: natural and man-made. Naturally occurring
(tectonic) earthquakes occur along tectonic plate lines (fault lines) while man-made
earthquakes are always related to explosions detonated by man. Earthquakes are usually
classified on the following bases:
(a) Cause of origin;
(b) Depth of focus; and
(c) Intensity and magnitude of earthquake.
(a) Cause of Origin: On the basis of the causes of earthquake, they are classified as:- (i)
Tectonic and (ii) Non-tectonic earthquakes. The non-tectonic earthquakes are mainly of three
types due to surface causes, volcanic causes and collapse of cavity roofs. The non-tectonic
earthquakes due Volcanic to surface causes Earthquakes or Denudation earthquakes
(b) Depth of focus: As we know, the instrument designed to detect seismic waves is called
seismometer and the seismograph is a seismometer to record the earth vibration. This record
of earth vibration is known as seismogram. It has now become possible to estimate the depth
of focus of an earthquake by analyzing seismograms. On the basis of the depth of focus,
earthquakes are classified as:
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Earthquake
(i) Surface-earthquakes
(ii) Shallow-focus earthquakes or normal earthquakes.
(iii) Intermediate-focus earthquakes, and
(iv) Deep-focus earthquakes.
(i) Surface-earthquakes: Surface-earthquakes are those in which the depth of the focus is
less than 10,000 metres.
(ii) Shallow-focus earthquakes: The earthquakes with the hypocentre at a depth of 10 to 50
kms are known as shallow-focus earthquakes.
(iii) Intermediate-focus earthquakes : When the earthquake is originated at a depth of 50 to
300 Kms, it is called intermediate- earthquake.
(iv) Deep-focus earthquakes : The deep-focus earthquakes or the plutonic earthquakes are
those with hypocentres located at depths more than 300 kms. Majority of the deep focus
earthquakes originate between 500 and 700 kms.
Shallow-focus earthquakes constitute about 85 percent of all the earthquakes and the
intermediate and deep-focus earthquakes account for 12 and 3 percent respectively of all the
earthquakes. Thus it is seen that the intermediate and deep-focus earthquakes together
account for only 15 percent of the earthquakes.
(c) Intensity and Magnitude of Earthquakes
As we know, the tremors caused by earthquake may be so feeble and imperceptible that they
can only be registered by highly sensitive instruments and may be so vigorous to cause large
scale devastation. The strength of an earthquake can be measured either by its intensity or by
its magnitude.
Intensity of an earthquake is a measure of the degree of damage and destruction it can cause.
These effects can be observed without the help of any instrument. It is also a fact that
intensity of an earthquake diminishes outwards from the epicenter.
Therefore places in which the earthquake has manifested itself with equal intensity are
contoured with - a line known as 'isoseismal'. Areas with one and the same intensity of
earthquake restricted by isoseismal lines are known as isoseismal areas. There is a number of
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Earthquake
disadvantages in using intensity as a measure of the strength of a particular earthquake, some
of the important disadvantages are as follows:
(i) The strength of an earthquake decreases with distance from its epicentre. Thus different
degree of damage occurs at different distances for the same earthquake.
(ii) The degree of damage depends much on the geological characteristics of a particular area
as well as the type of construction, population-density etc.
However, two scales of intensities are in vogue viz. (i) Rossi Forrel's Scale and (ii) Mercalli
Scale.
(i) Rossi-Forrel's Scale
According to this scale, there exists ten distinct and well-defined intensities beginning from/
and ending with X of which / represents the most mild earthquake while X the most
disastrous one. The scale is as follows:
Intensity Number
I. Imperceptible
II. Feeble
III. Very Slight
IV. Slight
V. Weak
VI. Moderate
Effects :
Recorded by sensitive instruments only. Recorded by all seismographs and felt by
experienced persons only. Felt by several persons at rest. It is strong enough for the duration
and direction to be recorded. Felt by persons in motion. Moveable objects disturbed. Affects
window doors, ceilings etc. General alarm, ringing of bells, disturbance of furniture and beds
etc.
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Earthquake
General awakening of persons from sleep, stopping of clocks, visible oscillation of trees etc.
Overthrows moveable objects, general panic, fall of plaster from the walls without damage to
the building etc.
Fall of chimneys and cracks in the walls of buildings. Partial or total destruction of buildings.
General destruction of buildings rock-falls and landslides in mountai- ncous regions.
(ii) Mercalli Scale
This scale is developed by Mercalli, an Italian seismologist, after he made studies of the
intensity and regional effects of earthquakes. The scale had at first ten divisions but later on it
was modified to a scale of 12 degrees. The higher the number of intensity the greater is the
damage.
Earthquake Waves
Earthquakes are a form of wave energy that is transferred through bedrock. Motion is
transmitted from the point of sudden energy release, the earthquake focus, as spherical
seismic waves that travel in all directions outward. The point on the Earth's surface directly
above the focus is termed the epicenter.
Fig 3.1: Movement of body waves away from the focus of the earthquake
Two different types of seismic waves have been described by geologists: body waves and
surface waves shown in Fig. 3.1. Body waves are seismic waves that travel through the
lithosphere. Two kinds of body waves exist: P-waves and S-waves. Both of these waves
produce a sharp jolt or shaking. P-waves or primary waves are formed by the alternate
expansion and contraction of bedrock and cause the volume of the material they travel
through to change. They travel at a speed of about 5 to 7 kilometers per second through the
lithosphere and about 8 kilometres per second in the asthenosphere. The speed of sound is
about 0.30 kilometers per second. P-waves also have the ability to travel through solid, liquid,
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Earthquake
and gaseous materials. When some P-waves move from the ground to the lower atmosphere,
the sound wave that is produced can sometimes be heard by humans and animals.
a. S-waves or secondary waves are a second type of body wave. These waves are slower
than P-waves and can only move through solid materials. S-waves are produced by
shear stresses and move the materials they pass through in a perpendicular (up and
down or side to side) direction.
b. Surface waves travel at or near the Earth's surface. These waves produce a rolling or
swaying motion causing the Earth's surface to behave like waves on the ocean. The
velocity of these waves is slower than body waves. Despite their slow speed, these
waves are particularly destructive to human construction because they cause
considerable ground movement.
Earthquake Measurement
The strength of an earthquake can be measured by a device called a seismograph. When an
earthquake occurs this device converts the wave energy into a standard unit of measurement
like the Richter scale. In the Richter scale, units of measurement are referred to as
magnitudes. The Richter scale is logarithmic as shown in Table 3.1. Thus, each unit increase
in magnitude represents 10 times more energy released. Table 10m-1 describes the
relationship between Richter scale magnitude and energy released. The following equation
can be used to approximate the amount of energy released from an earthquake in joules when
Richter magnitude (M) is known: Energy in joules = 1.74 x 10(5 + 1.44*M)
Table 3.1: Relationship between Richter Scale magnitude and energy released
S. no.
Magnitude in
Richter Scale
Energy
in Joules
1.
2.
3.
2.0
5.0
1.3 x 108
2.8 x 1012
6.0 - 6.9
7.6 x 1013 to 1.5 x 1015
4.
5.
6.
6.7
7.0
7.7 x 1014
2.1 x 1015
7.4
7.9 x 1015
7.6
1.5 x 1016
8.3
9.3
1.6 x 1017
4.3 x 1018
9.5
8.3 x 1018
7.
8.
9.
10.
Released
Comment
Smallest earthquake detectable by people.
Energy released by the Hiroshima atomic bomb.
About 120 shallow earthquakes of this magnitude occur
each year on the Earth.
Northridge, California earthquake January 17, 1994.
Major earthquake threshold.
Turkey earthquake August 17, 1999. More than 12,000
people killed.
Deadliest earthquake in the last 100 years. Tangshan,
China, July 28, 1976. Approximately 255,000 people
perished.
San Francisco earthquake of April 18, 1906.
December 26, 2004 Sumatra earthquake.
Most powerful earthquake recorded in the last 100
years. Southern Chile on May 22, 1960. Claimed 3,000
lives.
Figures 3.2 and 3.3 describe the spatial distribution of small and large earthquakes
respectively. These maps indicate that large earthquakes have distributions that are quite
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Earthquake
different from small events. Many large earthquakes occur some distance away from a plate
boundary. Some geologists believe that these powerful earthquakes may be occurring along
ancient faults that are buried deep in the continental crust. Recent seismic studies in the
central United States have discovered one such fault located thousands of meters below the
lower Mississippi Valley. Some large earthquakes occur at particular locations along the plate
boundaries. Scientists believe that these areas represent zones along adjacent plates that have
greater frictional resistance and stress.
Fig. 3.2: Distribution of earthquakes with a magnitude less than
5on the Richter Scale.
fig 3.3: Distribution of earthquakes with a magnitude greater than
7 on the Richter Scale.
Earthquake Damage and Destruction
Earthquakes are a considerable hazard to humans. Earthquakes can cause destruction by
structurally damaging buildings and dwellings, fires, tsunamis, and mass wasting.
Earthquakes can also take human lives. The amount of damage and loss of life depends on a
number of factors. Some of the more important factors are:

Time of day. Higher losses of life tend to occur on weekdays between the hours of
9:00 AM to 4:00 PM. During this time interval many people are in large buildings
because of work or school. Large structures are often less safe than smaller homes in
an earthquake.

Magnitude of the earthquake and duration of the event.

Distance form the earthquake's focus. The strength of the shock waves diminish with
distance from the focus.

Geology of the area effected and soil type. Some rock types transmit seismic wave
energy more readily. Buildings on solid bedrock tend to receive less damage.
Unconsolidated rock and sediments have a tendency to increase the amplitude and
duration of the seismic waves increasing the potential for damage. Some soil types
when saturated become liquefied.
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Earthquake

Type of building construction. Some building materials and designs are more
susceptible to earthquake damage.

Population density. More people often means greater chance of injury and death.
The greatest loss of life because of an earthquake this century occurred in Tangshan, China in
1976 when an estimated 250,000 people died. In 1556, a large earthquake in the Shanxi
Province of China was estimated to have caused the death of about 1,000,000 people. A
common problem associated with earthquakes in urban areas is fire. Shaking and ground
displacement often causes the severing of electrical and gas lines leading to the development
of many localized fires. Response to this problem is usually not effective because shock
waves also rupture pipes carrying water. In the San Francisco earthquake of 1906, almost
90% of the damage to buildings was caused by fire.
In mountainous regions, earthquake provoked landslides can cause many deaths and severe
damage to built structures. The town of Yungay, Peru was buried by a debris flow that was
triggered by an earthquake that occurred on May 31, 1970. This disaster engulfed the town in
seconds with mud, rock, ice, and water and took the lives of about 20,000 people.
Another consequence of earthquakes is the generation of tsunamis. Tsunamis, or tidal waves,
form when an earthquake causes a sudden movement of the seafloor. This movement creates
a wave in the water body which radiates outward in concentric shells. On the open ocean,
these waves are usually no higher than one to three meters in height and travel at speed of
about 750 kilometres per hour. Tsunamis become dangerous when they approach land.
Frictional interaction of the waves with the ocean floor, as they near shore, causes the waves
to slow down and collide into each other. This amalgamation of waves then produces a super
wave that can be as tall as 65 meters in height.
Prediction
Many methods have been developed for predicting the time and place in which earthquakes
will occur. Despite considerable research efforts by seismologists, scientifically reproducible
predictions cannot yet be made to a specific day or month. However, for well-understood
faults the probability that a segment may rupture during the next few decades can be
estimated. Earthquake warning systems have been developed that can provide regional
notification of an earthquake in progress, but before the ground surface has begun to move,
potentially allowing people within the system's range to seek shelter before the earthquake's
impact is felt.
Preparedness
Lecture Delivered by: Dr. Shahid-e-Murtaza
Earthquake
The objective of earthquake engineering is to foresee the impact of earthquakes on buildings
and other structures and to design such structures to minimize the risk of damage. Existing
structures can be modified by seismic retrofitting to improve their resistance to earthquakes.
Earthquake insurance can provide building owners with financial protection against losses
resulting from earthquakes. Emergency management strategies can be employed by a
government or organization to mitigate risks and prepare for consequences.
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