Download what is an earthquake

EARTHQUAKE
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WHAT IS AN EARTHQUAKE
An earthquake is a vibration or oscillation of the surface
of the earth, ranging from a faint tremor to a wild motion
caused by a transient disturbance of the elastic or
gravitational equilibrium of the rocks at or beneath the
earth the surface.
It forms energy of wave motion transmitted through the
surface layer of the earth in widening circles from a point
of sudden energy release.
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INTERIOR OF THE EARTH
Thus, on the basis of seismic
investigation, the earth can be divided
into three major zones, these are
 Crust
 Mantle
 Core
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CAUSES OF THE EARTHQUAKE
Earthquakes are caused mainly due to disequilibrium in any
part of the crust of the earth. A number of causes have
been assigned to cause disequilibrium or isostatic
imbalance in the earth’s crust such as:
Vulcanicity
Faulting & elastic rebound theory
Hydrostatic pressure and anthropogenic causes
Plate tectonic theory
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NATURE OF EARTHQUAKE
When a stone is thrown into the pool, waves spread
through the water in all directions. Similarly, waves spread
out through the earth when there is a sudden localized
disturbance of the rock. An earthquake is one such type of
disturbance. When an earthquake occurs, the fracturing
rocks emit SEISMIC WAVES, which radiates in all directions
from the focus.
 Waves:
These are of two types: Body waves & Surface Waves
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Body Waves: The body waves are two types:
P-Waves:
Primary waves. These
are fastest,
compressional and
capable of propogating
through both, the solid
and liquid media
S-Waves:
Secondary waves. These are
transverse, comparatively slower
and pass only through solid media
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Surface Waves: The body waves are two types:
L-waves:
Shear motion in
horizontal plane.
Rayleigh waves:
Motion in vertical plane.
Surface Waves are more disastrous than the Body Waves.
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 Shocks
The earthquake vibrations shake the ground even hundreds
of kilometers away. The shaking is strongest at its source.
When the vibrations are felt in the bedrock and ground,
they are called shocks. Some earthquakes are preceded by
smaller foreshocks, and larger earthquakes are always
followed by aftershocks.
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 Focus & Epicenter
EPICENTRE
Earth’s surface
FOCUS
MANTLE
The point where the earthquakes are generated is called
focus. The point on the earth’s surface, directly above the
focus, is called the epicenter .
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CLASSIFICATION OF THE EARTHQUAKE
Each earthquake differs from the other and thus it
becomes difficult to classify all the earthquakes in certain
categories. Inspite of these limitations earthquakes are
classified on the basis of common characteristics as given
below:
1. On the basis of causative factors:
a.
Natural earthquakes: Natural earthquakes are those
which are caused by natural processes i.e. due to
endogenetic forces. These are further divided into four
subcategories
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Volcanic earthquakes
Tectonic earthquakes
Isostatic earthquakes
Plutonic earthquakes
b.
Artificial or man-induced earthquake or
anthropogenic earthquakes: Artificial or man-induced
earthquake or anthropogenic earthquakes are caused by
human activities such as pumping of water and mineral oil
from underground aquifers and oil reserves respectively,
deep underground mining, blasting of rocks by dynamites
for constructional purposes e.g. for the construction of dams and
reservoirs, roads etc., nuclear explosion, storage of huge volume of water in big
reservoirs etc.
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Examples, 1931 earthquakes of Greece due to Marathon
Dam, 1936 earthquakes of Hoover Dam (USA) due to Lake
Mead, Koyna earthquake (Maharastra, India) of 1967 due to
Koyna reservoir etc.
2. On the basis of focus:
Guttenberg has divided the world seismic centres on the
basis of the depths of their foci into 3 types viz.
Shallow focus earthquakes – foci are located at the
depths from the ground surface to 70 km,
Intermediate focus earthquakes – foci at the depths
between 70 km to 300 km, and
Deep focus earthquakes – foci at the depths between 300
km to 700 km.
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3. On the basis of human causalities:
Earthquakes are grouped into 3 categories on the basis of
their hazardous impacts in terms of human casualities.
Moderately hazardous earthquakes – when human
deaths caused by severe seismic tremors are below 50,000
mark,
Highly hazardous earthquakes causing human deaths
ranging between 51,000 to 1,00,000 mark, and
Most hazardous earthquakes causing human casualities
above 1,00,000 mark.
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INTENSITY OF THE EARTHQUAKE
It is based on the degree of damage occurred at the Earth’s
surface and on the people’s reaction to the shock. Italian
seismologists Prof. Guiseppe Mercalli proposed the
following scale to measure the intensity of an earthquake.
Mercalli Intensity
Effects
I
Instrumental
Detected only by
seismograph
II
Feeble
Noticed only by
sensitive people
Richter
Magnitude
3.5 to 4.2
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III
Slight
Like the vibrations due to 3.5 to 4.2
a passing lorry; felt by
people at rest, especially
an upper floors
IV
Moderate
Felt by people while
walking, rocking of loose
objects, including
standing vehicles
V
Rather strong
Felt generally: most
sleepers are awakened
and bells ring
VI
Strong
Trees away and all
suspended objects
swing, damage by
overturning and falling of
loose objects
4.3 to 4.8
4.9 to 5.4
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VII
Very strong
General alarm, wall
cracks, plaster falls
VIII
Destructive
Car drivers seriously
disturbed, masonry
fissured, chimneys fall,
poorly constructed
buildings damaged
IX
Ruinous
Some houses collapse
where ground begins to
crack, and pipes break
open
X
Disastrous
Ground cracks badly,
many buildings
destroyed and railways
lines bent, landslides on
steep slopes
5.5 to 6.1
6.2 to 6.9
7 to 7.3
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XI
Very disastrous
Few buildings remain
standing, bridges
destroyed, all services
(railways, pipes and
cables out of action),
great landslides and
floods
7.4 to 8.1
XII
Catastrophic
Total destruction, objects
thrown into air, ground
rises and fall in waves
>8.1
(maximum
known
8.9)
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MAGNITUDE OF THE EARTHQUAKE
It is a numerical scale of magnitudes from 0 to 9, proposed
by Prof. Charles Richter of California Institute of Technology
in 1935. The idea of magnitude was based on observed
surface movements. The magnitude of an earthquake is a
measure of the amount of energy released. Earthquake
magnitudes are based on direct measurements of the size
(amplitude) of seismic waves, made with recording
instruments (seismograph), rather than on subjective
observations of destruction. The total energy released by
an earthquake can be calculated from the amplitude of the
waves and distance from the epicenter.
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EFFECTS OF THE EARTHQUAKE
Loss of life and property
Landslides and Avalanches
Development of fissures and cracks
Disturbances of groundwater circulation and drainage
pattern
Fires
Upwarp and downwarp of the crust
TSUNAMIS
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WORLD DISTRIBUTION
Earthquakes are not randomly distributed over the globe
but they tend to occur in narrow continuous belt. The most
notable aspects of the distribution of earthquakes in the
world is their locating along a narrow belt. There are three
well-defined belts or zones of seismic activity in the world,
where most earthquakes originate are identified. These
earthquake zones are:
The Circum-Pacific Belt
The Mediterranean and Trans-Asiatic Belt
The Mid-oceanic Ridges and the African rift system Belt
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INDIA AND EARTHQUAKE
Seismic zonation map of country is guide to the earthquake vulnerability of the
region. India has been divided into four zones with respect to severity of
earthquakes. Of this zone V is seismically the most active and zone II is the least
active. Delhi comes under zone IV of seismic hazard map. Asiatic and Indian plates
where the process of mountain building is still in progress due to the Asiatic plate
is moving southward whereas the Indian plate is moving northward and hence the
northern margin of the Indian plate is being subducted below the Asian plate. The
collision of Asiatic and Indian plates and resultant subduction of Indian plate and
consequent folding and faulting and gradual rise of the Himalayas at the rate of 50
mm per year cause earthquakes of northern India, Tibet and Nepal
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Seismic Zoning Map of India
Resource Developemnt Centre, New Delhi
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PREDICTION OF AN EARTHQUAKE
The Astrologers and Mystics are the people who predict an
Earthquake, which has no scientific basis.
There is no scientific way to predict an earthquake.
However, continuous recording of small - scale earthquakes
is sometimes useful in prediction of a major earthquake.
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HOW CAN I PREPARE FOR AN EARTHQUAKE
Learn first aid procedures.
Keep emergency items on hand, including first aid kit,
supply of important medicines, flashlight, battery-powered
radio, batteries, some drinking water and nonperishable
food, tools to shut off utilities, and a fire extinguisher.
Plan to be self sufficient for a minimum of 72 hours after
an earthquake or other major disaster.
Conduct an earthquake safety check of your house
periodically
Develop a family earthquake plan and communication
plan for those first hours after an earthquake.
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Learn how to shut off all utilities in your home.
Avoid placing heavy objects on shelves or heavy pictures
on walls where they could fall during a quake.
Use bolts or straps to secure heavy items that might
topple over, such as a bookcase, china cabinet, or water
heater.
Avoid placing heavy objects on shelves or heavy pictures
on walls where they could fall onto a bed.
Keep flammable or hazardous liquids such as paints, pest
sprays, or cleaning products in cabinets or secured on
lower shelves.
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WHAT SHOULD I DO DURING AN EARTHQUAKE
If you’re
Indoors, duck under a desk or sturdy table or stand in a
corner. If not near a desk or table, move against an interior
wall, and project your head with your arms. Stay away
from windows, bookcases, file cabinets, heavy mirrors,
hanging plants, and other heavy objects that could fall.
Watch out for falling plaster or ceiling tiles. Stay under
cover until the shaking stops.
In a high-rise building, do not use the elevators. Do not
be surprised if the fire alarm or sprinkler systems activate.
Outdoors, move to a clear area, away from trees, signs,
buildings, or downed electrical wires and poles.
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On a sidewalk near buildings, and there is no safe haven
from overhead wires or other hazards, duck into a doorway
to protect yourself from falling bricks, glass, plaster, and
other debris.
Driving, pull over to the side of the road and stop. Avoid
overpasses, power lines, and other hazards. Stay inside the
vehicle until the shaking is over.
In a crowded store or other public place, do not rush for
exits. Once the shaking stops, crowds trying to rush to the
exits may be more dangerous than the earthquake itself.
In a wheelchair, stay in it. Move to cover, if possible, lock
your wheels, and protect your head with your arms.
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In the kitchen, move away from the refrigerator, stove,
and overhead cupboards.
In a stadium or theater, stay in your seat and protect your
head with your arms. Do not try to leave until the shaking
is over. Then leave in a calm, orderly manner.
In bed, hold on and stay there, protecting your head with
a pillow.
In a coastal area, move to higher ground.
In a mountainous area or near unstable slopes or cliffs,
be alert for falling rocks and other debris that could be
loosened by the earthquake.
After an earthquake, be prepared for aftershocks
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WHAT TO DO AFTER AN EARTHQUAKE
Check yourself for injuries and provide help injured or
trapped persons.
Report any fires, chemical hazards, gas leaks, or broken
water lines to your emergency personnel.
Stay out of damaged buildings and away from damaged
areas.
Wear shoes in all areas near debris and broken glass.
Stay away from beaches. Tsunamis and seiches
sometimes hit after the ground has stopped shaking.
If you're at school or work, follow the emergency plan or
the instructions of the person in charge.
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Clean up spilled medicines, bleaches, gasoline, or other
flammable liquids immediately and carefully.
Open closet and cabinet doors cautiously.
Listen to radio (or television) for updated emergency
information and instructions.
Use battery-powered lanterns or flashlights to inspect
your home.
Do not use flame producing devices/open flame
appliances if gas leak is suspected.
Do not touch downed power lines or objects touched by
them.
Use the telephone only for emergencies
Expect aftershocks.
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