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Earthquake

Introduction:

The vibration of earth that accompanies an earthquake is one of the most terrifying natural

phenomena known. From geological point of view, earthquakes provide the evidences of the

instability of the earths crust and a logical starting point for any examination of the dynamics

of the earth.

Earthquakes are largely confined to relatively narrow ones in the lithosphere. These ones of

high seismic activity are a key to identifying the boundaries of the ma!or lithospheric plates.

Earthquakes are associated with large fractures, or faults, in the earths crust or upper

mantle .

"ost earthquakes take place along faults in the upper #$ miles of the earth%s surface when

one side rapidly moves relative to the other side of the fault.

The following points should be noted in case of construction is a seismically active

region

&' Tectonic features

#' (istance from the active fault

)' "agnitude of the expected earthquake

*' +ature of the foundation material

$' (ynamic characteristics of the structure to be constructed

In case of a building structure are following points are to be taken care of to minimize

the effect of earthquake

&' ood quality construction material as per specification should be used

#' The foundation should not be on loose ground, rather it should be preferably on solid

rock

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)' (epth of foundation should be uniform

*' Foundation outline approximating a square gives more stability

$' The walls should be continuous in nature with few doors and windows. Too many

doors and windows and discontinuous walls lower the stability

6) (oors and windows should not be in vertical rows, preferably along the diagonals

-' einforced concrete should be used more commonly than brick works

/' oof should be flat and of reinforced concrete to add the lateral stiffness

0' 1ll the parts of a building should be well tied so that it moves as a single unit during

an earthquake vibration

&2' 3eight of the building should be kept uniform , which adds to the stability

&&' 4onstruction of chimneys, arches, domes , cantilevers and other extra pro!ections

should be avoided

Elastic !ebound Theory:

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5n geology , the elastic rebound theory was the first theory to satisfactorily explain

earthquakes. 6reviously it was thought that ruptures of the surface were the result of strong

ground shaking rather than the converse suggested by this theory.

"ccording to this theory an earthquake is the result of the elastic rebound of previously

stored elastic strain energy in the rocks on either side of the fault#

5n an interseismic period the earth%s plates move relative to each other except at most plate

boundaries where they are locked.

4onsider two plates moving in opposite

directions. 7ut because they are pressed together

by the weight of the overlying rock, friction locks

them together. 5nstead of slipping taking place

along the fault, the blocks are deformed in the

vicinity of the fault. 1s the rock is strained, elastic

energy is stored in it.

The deformation builds at the rate of a few cm

per year, over a time period of many years.

8hen the accumulated strain is great enough to

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overcome the frictional strength of the rocks an

earthquake occurs.

The blocks suddenly slip

at a certain point. This point

is known as the focus 9orhypocenter ' of the

earthquake.

:nce rupture is initiated, it will travel at a high speed. 5n great earthquakes, the slip ,

or offset of the blocks can be as large as &$ m.

$hat is an earthquake%

:nce the frictional bond is broken, the elastic energy, which has been slowly stored over tens

or hundreds of years, is suddenly released in the form of intense seismic vibrations which

constitute the earthquake. The seismic waves propagate large distances in all directions away

from the fault. +ear the focus the waves can have large destructive amplitudes.

The time required to build up elastic energy in the rocks, ad!acent to a fault, is enormous

compared to the time that elapses during the release of stored energy, for earthquakes lastonly a few minutes.

Terminology:

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&ocus'hypocenter: The location of an earthquake%s hypocenter is the position where the

energy stored in the strain in the rock is released

Epicenter: The epicenter is the point on the Earth%s surface that is directly above the

hypocenter or focus , the point where an earthquake or other underground explosion

originates.

(easurement of Earthquakes:

The amount of stored energy can be measured in several ways. Two most common methods

are;

&. "easuring the distortion of surveyed lines.

#. "easuring the energy of the released seismic waves.

Energy released is the most precise way of measuring the si e of an earthquake. 7ut it is a

long, complicated process to determine the fault dimensions, the slip and other factors needed

to compute it.

Thats why Richter magnitude scale is used, which is based on the amplitude of seismic

waves recorded by seismographs.

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!ichter magnitude scale:

The ichter magnitude scale, or more correctly local magnitude " < scale, assigns a single

number to quantify the amount of seismic energy released by an earthquake . 5t is a base=&2

logarithmic scale obtained by calculating the logarithm of the combined hori ontal amplitude

of the largest displacement from ero on a seismometer output. 1d!ustments are included to

compensate for the variation in the distance between the various seismographs and the

epicenter of the earthquake.

7ecause of the logarithmic basis of the scale, each whole number increase in magnitude

represents a tenfold increase in measured amplitude> in terms of energy, each whole number

increase corresponds to an increase of about )# times the amount of energy released.

?eismographs can easily detect earthquakes of magnitude less than &. Events with

magnitudes of about *.@ or greater are strong enough to be recorded by any of the

seismographs in the world. The largest earthquakes yet recorded show ichter magnitude of

about /.$.

)etermination of Earthquake (agnitude from a *eismograph !eading:

BT

am +

= log

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8here, m A magnitude

1 A maximum trace motion

a A maximum ground motion 9microns A &2 =@m'

A 1Bmagnification of seismograph

7 A correction factor that allows for the weakening of seismic waves with

increasing distance from the earthquake 9found from table using

distance'

T A duration of one oscillation Cor period of seismic wave 9sec'

*eismograph:

?eismographs are used by seismologists to measure and record the si e and force of seismic

waves . 7y studying seismic waves, geologists can map the interior of the Earth, and measure

and locate earthquakes and other ground motions.

Earthquake magnitude, effects and statistics:

The following describes the typical effects of earthquakes of various magnitudes near the

epicenter.

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This table should be taken with extreme caution, since intensity and thus ground effects

depend not only on the magnitude, but also on the distance to the epicenter, the depth of the

earthquake%s focus beneath the epicenter, and geological conditions 9certain terrains can

amplify seismic signals'.

Types of *eismic $aves:

There are mainly two types of seismic waves;

&. 7ody 8ave

#. ?urface wave

+# ody $aves:

7ody waves travel through the interior of the Earth. 7ody waves transmit the first=arriving

tremors of an earthquake, as well as many later arrivals. There are two kinds of body waves;

Primary ( P-waves ) and Secondary ( S-waves ).

http://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/S-wavehttp://en.wikipedia.org/wiki/P-wavehttp://en.wikipedia.org/wiki/S-wave

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a# - waves:

6 waves are longitudinal or compressional waves, which mean that the ground is alternately

compressed and dilated in the direction of propagation. 5n solids these waves generally travel

slightly less than twice as fast as ? waves and can travel through any type of material. 6waves are sometimes called Dprimary wavesD. 8hen generated by an earthquake they are less

destructive than the ? waves and surface waves that follow them, due to their lesser

amplitudes.

b# * waves:

? waves are transverse or shear waves, which mean that the ground is displaced

perpendicularly to the direction of propagation. ? waves can travel only through solids, asfluids 9liquids and gases' do not support shear stresses. Their speed is about @2 of that of 6

waves in a given material. ? waves are sometimes called Dsecondary wavesD, and are several

times larger in amplitude than 6 waves for earthquake sources.

.# *urface $aves:

?urface waves are analogous to water waves and travel !ust under the Earth%s surface. They

travel more slowly than body waves. 7ecause of their low frequency, long duration, and largeamplitude, they can be the most destructive type of seismic wave. There are two types of

surface waves; Rayleigh waves and Long waves .

a# !ayleigh waves:

ayleigh waves , also called ground roll, are surface waves that travel as ripples similar to

those on the surface of water. They are slower than body waves, roughly -2 of the velocity

of ? waves, and have been asserted to be visible during an earthquake in an open space like a

parking lot where the cars move up and down with the waves.

http://en.wikipedia.org/wiki/P_wavehttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/S_wavehttp://en.wikipedia.org/wiki/Transverse_wavehttp://en.wikipedia.org/wiki/Surface_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Rayleigh_waveshttp://en.wikipedia.org/wiki/P_wavehttp://en.wikipedia.org/wiki/Longitudinal_wavehttp://en.wikipedia.org/wiki/S_wavehttp://en.wikipedia.org/wiki/Transverse_wavehttp://en.wikipedia.org/wiki/Surface_wavehttp://en.wikipedia.org/wiki/Rayleigh_wavehttp://en.wikipedia.org/wiki/Love_wavehttp://en.wikipedia.org/wiki/Rayleigh_waves

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b. Long Waves:

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/ow to 0ocate the Epicenter%

The principal is quite similar to deducing the distance to a lightning bolt from the time interval

between the flash and the sound.

The lightning flash may be likened to the P waves of earthquake and the thunder to the S waves .

(ue to certain difference in wave velocity, the interval between the arrival of 6 and ? waves

increases with the distance traveled by the waves, and for each * - time interval there is

associated a definite distance to the epicenter# This is indicated in the travel time chart for 6

and ? waves in the following figure.

nowing the distance, say G 1 of an earthquake from a given station, one can only say the

earthquake lies on a circle of radius G 1 , centered on station 1. 5f however, one also knows the

distances from two additional stations 7 and 4, the three circles centered on the ) stations, with

radii G 1 9A&$22 km', G7 9A$@22 km', and G4 9A/@22 km' intersect uniquely at the point H, the

epicenter.

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/ow to )etermine the !ichter (agnitude%

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)isadvantages of the (( *cale:

The "" scale, while being directly oriented to building effects, relies on a methodology

of sub!ective comparisons> its information sources consist of observations, postcard

damage reports, and newspaper clippings, expressed in a oman numeral scale. 7esides the sub!ectivity of the "" scale, another problem is that of its age; The listing of

construction materials emphasi es masonry, and does not refer to many modern methods

of construction such as glass curtain walls, hung ceilings, or precast concrete.