VIII Semester BTech Civil Engineering

Earthquake Resistant Design Of Structures

CE832EE

Dr Sujatha Unnikrishnan
Associate Professor
Department of Civil Engineering

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session.
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● Switch off your mobile.
● Mute your mike and unmute it when want to discuss or ask
questions.
● Write lecture notes.
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Earthquake
● Earthquake is the disturbance that happens at some depth
below the ground level which causes vibrations at the ground
surface.
● These vibrations happen in all the directions and are totally
uncertain.
● These vibrations are momentary, happening for a short
while.
● Earthquakes are totally unpredictable.
● Earthquake is the shaking or trembling caused by the
sudden release of energy below the ground.

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Earthquake

● The vibration of the earth’s surface as a result of a release of

energy in the earth’s crust.

● Seismology is the study of the generation, propagation and

measurement of seismic waves through earth and the
sources that generate them.
● Seismos – earthquake, logos-science

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Interior of the earth

● Inner Core (radius ~1290km)

● Outer Core (thickness ~2200km)
● Mantle (thickness -2900km)
● Crust (thickness ~5 to 40km).

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Interior of the earth

● The Inner Core is solid and consists of heavy metals (e.g.,

nickel and iron), while the Crust consists of light materials
(e.g., basalts and granites).
● The Outer Core is liquid in form and the Mantle has the
ability to flow.
● At the Core, the temperature is ~2500°C, the pressure ~4
million (4 x 106) atmospheres and density ~ 13.5 gm/cc.
● On the surface of the Earth, the temperature is ~25°C, the
pressure ~1 atmosphere and density ~ 1.5 gm/cc

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Core temperature 25000C crust temperature 250C

Core pressure 4 x 106 atm crust pressure 1 atm
The high pressure and temperature gradients between the crust and the
core cause convection currents to develop in the viscous mantle.

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Plate tectonic theory

● The convective flows of Mantle material cause the
● Crust and some portion of the Mantle, to slide on the
● hot molten outer core.
● This sliding of Earth's mass takes place in pieces called
Tectonic Plates.
● The surface of the Earth consists of seven major tectonic
plates and many smaller ones.
● These plates move laterally and collide at each other
resulting in earthquakes.
● These plates move in different directions and in different
speeds (5 to 10 cm /year)

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Plate tectonic theory

● Plate tectonics is responsible for continental drift (two plates

move away from each other), mountain formation (front plate
is slower so rear plate collides with it), volcanic eruptions and
earthquake.

● Relative motion of crustal plates gives rise to 3 kinds of plate

boundaries – divergent, convergent and transform

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Zones of divergence

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Zones of divergence

● Plates moves apart, hot magma wells up through cracks,

solidifies and becomes part of the crust.
● Generally, spreading ridges are located beneath the oceans.
● This process is also known as sea-floor spreading.

● A few areas where the spreading occurs along the

continental mass-Eg. East African Rift Valley

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Zones of convergence

● Boundaries along which the edge of one plate overrides the

other.
● On collision, the higher density plate may bend downwards,
and descend beneath the other plate. It gets heated, melts
and form new magma. This magma rises to the surface to
produce volcanos. This process is called subduction.
● One of the areas around Indian peninsula where subduction
process is in progress is near Andaman-Sumatra region.

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Plate with higher density subducts

● \

● On collision-the two plates are pushed upwards against each other, they form
major mountain systems such as the Himalayas.

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Transform zones

● The plates slide past each other horizontally.

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● Plate tectonic theory explains well the interplate

earthquakes.
● It does not explain intraplate earthquake.

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Earthquake

● Rocks are made of elastic material, and so elastic strain

energy is stored in them during the deformations that occur
due to the gigantic tectonic plate actions that occur in the
Earth.
● The material contained in rocks is also very brittle.
● When the rocks along a weak region in the Earth's Crust
reach their strength, a sudden movement takes place there

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● Opposite sides of the fault (a crack in the rocks where

movement has taken place) suddenly slip and release the
large elastic strain energy stored in the interface rocks.

● The energy released during the 2001 Bhuj earthquake is

about 400 times (or more) that released by the 1945 Atom
Bomb dropped on Hiroshima.

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Elastic Rebound Theory

● Elastic Rebound Theory Proposed by M F Reid in 1906

When the plates collide, they may be prevented from moving

because of the frictional resistance along the plate
boundaries. This causes building up of stress and strain
along the plate edge.
● Fracture, resulting in sudden release of strain energy.
● This is the origin of an earthquake.
● Gradual accumulation and subsequent release of stress and
strain-elastic rebound.

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● Most earthquakes occur along the boundaries of the tectonic

plates – interplate earthquakes
● Within the plate – intraplate earthquake

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Earthquake epicentres around the world

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Seismic waves
● During earthquake waves spread out through the earth.
● Large strain energy released during an earthquake travels in
the form of seismic waves in all directions.
● As the waves radiate from the fault, they undergo geometric
spreading and attenuation due to loss of energy in the rocks.
Since the interior of the earth consists of heterogeneous
formations, the waves undergo multiple reflections,
retraction, dispersion and attenuation as they travel.
● The seismic waves arriving at a site on the surface of the
earth are a result of complex superposition giving rise to
irregular motion
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Seismic waves

● Mainly there are three types of waves associated with

propagation of an elastic stress wave generated by an
earthquake:
○ Primary waves (P waves)
○ Secondary waves (S waves)
○ Surface waves

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Primary waves (P waves)

● These waves propagate by longitudinal or compressive

action, which mean that the ground is alternately
compressed and dilated in the direction of propagation.
● P waves are the fastest (8 to 13 km/s)
● Therefore, when an earthquake occurs, these are the first
waves to reach any seismic station and hence the first to be
recorded.
● The P waves resemble sound waves because these too are
compressional or longitudinal waves in nature.
● P waves are capable of traveling through solids, liquids and
gases.
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Primary waves (P waves)

𝐸 1−ν
Vp =
ρ 1+ν 1−2ν
where E is the Young’s modulus
ν is the Poisson’s ratio
ρ is the density.

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S waves

● Particles oscillate at right angles to the direction of

propagation of the wave and cause shearing deformation as
they travel through a material.
● Compared to P waves, these are relatively slow (5 to 7
km/s).
● In nature, these are like light waves.

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● These waves are capable of traveling only through solids.

● The shear wave velocity is given by
𝐸
Vs = = 𝐺/ρ
2ρ 1+ν

𝐸
where G = is the shear modulus.
2 1+ν

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● Velocity of S waves is directly proportional to the shear

strength of the material through which they pass.
● S waves do not travel through liquids as fluids have no
shearing stiffness.
● They travel at the rate of 5 to 7 km per second. For this
reason these waves are always recorded after P waves in a
seismic station.

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Surface waves

● When the vibratory wave energy is propogating near the

surface of the earth, waves known as Rayleigh waves and
Love waves can be identified.
● These are called surface waves because their journey is
confined to the surface layers of the earth only. Surface
waves travel through the earth crust and does not propagate
into the interior of earth.

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● Surface waves are the slowest among the seismic waves.

Therefore, these are the last to be recorded in the seismic
station at the time of occurrence of the earthquake.
● They travel at the rate of 4 to 5 km per second.
● Complex and elliptical particle motion is characteristic of
these waves.
● These waves are capable of travelling through solids and
liquids.

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Rayleigh waves

● The Rayleigh waves are tension-compression waves similar

to the P-waves except that their amplitude diminishes with
distance below the surface of the ground.

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Love waves

● Love waves are similar to “S” waves; they are shear waves
that diminishes rapidly with distance below surface.

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● The damage and destruction associated with earthquakes

can be mainly attributed to surface waves.
● This damage potential and the strength of the surface waves
reduce with increase in depth of earthquakes.

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Terminology

● Focus-point of generation of an earthquake

● Epicentre - point on the earth’s surface directly above the
focus
● Focal depth-depth of focus from epicentre (Most of the
damaging earthquakes have shallow focus with focal
depths less than about 70km).
● Epicentral distance – distance from epicentre to any point of
interest
● homoseismal line – line joining locations at which the shock
arrives simultaneously

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Terminology

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Magnitude & Intensity

● Magnitude is a quantitative measure of the actual size of the

earthquake.
● Professor Charles Richter - Richter Scale
● Intensity is a qualitative measure of the actual shaking at a
location during earthquake
● The intensity scales are based on three features of shaking -
perception by people and animals,
performance of buildings, and
changes to natural surroundings.

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● Modified Mercalli Intensity (MMI) Scale.

● Another intensity scale is Mendvedev-Spoonheuer-Karnik

scale (MSK 64). This scale is more comprehensive and
describes the intensity of earthquake more precisely. Indian
seismic zones were categorized on the basis of MSK 64
scale.

● Intensity scale ( I – XII) (imperceptible shaking –

catastrophic destruction)

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Intensity scale (MMI Scale)

● I - Not felt except by a very few under specially favourable

circumstances
● II - Felt only by a few persons at rest, specially on upper
floors of buildings; and delicately suspended objects may
swing.
● III - Felt quite noticeably indoors, specially on upper floors of
buildings but many people do not recognise it as an
earthquake; standing motor cars may rock slightly; and
vibrations may be felt like the passing of a truck.

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Intensity scale

● IV - During the day felt indoors by many, outdoors by a few,

at night some awakened; dishes, windows, doors disturbed;
walls make creaking sound, sensation like heavy truck
striking the building; and standing motor cars rock
noticeably.
● V - Felt by nearly everyone; many awakened; some dishes,
windows, etc, broken; a few instances of cracked plaster;
unstable objects overturned; disturbance of trees, poles and
other tall objects noticed sometimes; and pendulum clocks
may stop.

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Intensity scale

● VI - Felt by all, many frightened and run outdoors; some

heavy furniture moved; a few instances of fallen plaster or
damaged chimneys; and damage slight.

● VII Everybody runs outdoors, damage negligible in buildings

of good design and construction; slight to moderate in well
built ordinary structures; and some chimneys broken, noticed
by persons driving motor cars.

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Intensity scale

● VIII - Damage slight in specially designed structures;

considerable in ordinary but substantial buildings with partial
collapse; very heavy in poorly built structures; panel walls
thrown out of framed structures; falling of a chimney, factory
stacks, columns, monuments, and walls; heavy furniture
overturned, sand and mud eject in small amounts; changes
in well water; and disturbs persons driving motor cars.

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Intensity scale

● IX - Damage considerable in specially designed structures;

well designed framed structures thrown out of plumb; very
heavy in substantial buildings with partial collapse; building
shifted off foundations; ground cracked conspicuously; and
underground pipes broken.

● X - Some well built wooden structures destroyed; most

masonry and framed structures with foundations destroyed;
ground badly cracked; rails bent; landslides considerable
from river banks and steep slopes; shifted sand and mud;
and water splashed over banks.
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Intensity scale

● XI - Few, if any, masonry structures remain standing; bridges

destroyed; broad fissures in ground, underground pipelines
completely out of service; earth slumps and landslips in soft
ground; and rails bent greatly.

● XII - Total damage; waves seen on ground surfaces; lines of

sight and levels distorted; and objects thrown upward into
the air.

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Difference between Magnitude and Intensity

● Magnitude of an earthquake is a measure of its size.
● The size of an earthquake can be measured by the amount
of strain energy released by the fault rupture.
● The magnitude of the earthquake is a single value for a
given earthquake.
● Intensity is an indicator of the severity of shaking generated
at a given location.
● The severity of shaking is much higher near the epicenter
than farther away.
● Thus, during the same earthquake of a certain magnitude,
different locations experience different levels of intensity.

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Difference between Magnitude and Intensity

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Magnitude

● The magnitude of an earthquake is related to the amount of

energy released by the geological rupture causing it, and is
therefore a measure of the absolute size of the earthquake,
without reference to distance from the epicenter.
● While earthquake intensity is depicted in Roman numerals
and is always a whole number, magnitude is depicted in
Arabic numerals and need not be a whole number.
● A number of approaches for measurement of magnitude of
an earthquake have come into existence.

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Richter Magnitude, ML

● Richter defined the earthquake magnitude as the logarithm

to the base 10 of the largest displacement of a standard
seismograph situated 100 km from the focus.
● M=log10A
● where A denotes the amplitude in micron (10-6m) recorded
by the instrument located at an epicentral distance of 100
km; and M is the magnitude of the earthquake.

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● When the distance from the epicenter at which an

observation is obtained other than 100 km, a correction is
introduced to the equation as follows:
100
● M = M∆ - 1.73 log10 ( )
∆
● where M is the magnitude of the earthquake;
● ∆=distance from epicenter (km),
● M∆= magnitude of the earthquake calculated for earthquake
using the values measured at a distance ∆ from the
epicenter.

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Isoseismal line – line joining locations experiencing equal

earthquake intensity

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● Shallow focus earthquake- Upto a depth of 70km from the

surface of the earth
● Intermediate focus earthquake – occur between 70 and 300
km
● Deep focus earthquake – focal depth of more than 300km

● Seismic energy from a source deeper than 70km gets largely

dissipated by the time it reaches the surface.
● The main consideration in the design of earthquake resistant
structures is shallow focus earthquakes.

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Effects of earthquake – direct effects

● Damage structures
● In regions consisting of hills and slopes – earthquake may cause
landslide- which might damage buildings and lead to loss of life
● Liquefaction- when earthquake shakes wet sandy soil, soil particles
move apart, allowing water to seep in between them. There is a
sudden reduction of shear resistance and the soil will behave like a
fluid.
○ Buildings will lean or topple
○ After earthquake, soil consolidates-further damage to buildings
○ Displacement of retaining walls

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Effects of earthquake – indirect effect

● If epicentre of an earthquake is under the sea, one side of

the ocean floor drops suddenly, sliding under the other plate.
The violent movement of the sea floor results in series of sea
waves with extremely long time periods. These waves are
called tsunamis.
● Sieches – similar to small tsunamis- reservoirs, lakes etc.
during earthquake.
● Earthquake can cause fire by damaging gas lines and
snapping electric wires
● Earthquake can damage dams-causing floods- resulting in
damage to structures and loss of life.
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Severity of an earthquake is assessed by:

● Magnitude – in terms of the energy released (constant)

○ Magnitude of an earthquake is a single value

● Intensity – destructive effect on people, structures and

natural features. (vary with location)
○ During the same earthquake, different locations
experience different

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Measuring Instruments
● The instrument that measures earthquake shaking, a seismograph,
has three components - the sensor, the recorder and the timer.
● The principle on which it works - a pen attached at the tip of an
oscillating simple pendulum (a mass hung by a string from a support)
marks on a chart paper that is held on a drum rotating at a constant
speed.
● A magnet around the string provides required damping to control the
amplitude of oscillations.
● The pendulum mass, string, magnet and support together constitute
the sensor; the drum, pen and chart paper constitute the recorder; and
the motor that rotates the drum at constant speed forms the timer.

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● 3 components-sensor, recorder, timer

● Sensor consists of- pendulum mass, string, magnet and
support
● Recorder consists of – drum, pen and chart paper
● Timer consists of – motor that rotates the drum at constant
speed.
● When the supporting frame is shaken by the earthquake
waves, the inertia of the mass causes it to lag behind the
motion of the frame. This relative motion is recorded by pen
on the paper. The earthquake records is called
seismograms.
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● One instrument each to measure along the two orthogonal

horizontal directions.
● For measuring vertical oscillations, the string pendulum is
replaced with a spring pendulum oscillating about a fulcrum.

● In modern seismographs, the relative motion between the

pendulum and the frame produces an electric signal which is
electronically magnified (1000 X) before it is used to drive an
electric stylus to produce the seismogram depicting even
very weak seismic waves.

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● By varying the characteristics of equipment one could record

displacement, velocity or acceleration during an earthquake.
● The devises that measure the ground accelerations are
called accelerometer.
● The accelerometers register the accelerations of the soil
and the record obtained is called an accelerogram.

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Classification of seismographs

● Displacement seismograph - if the natural period of the

pendulum is long relative to the period of ground motion and
if an appropriate damping coefficient for the pendulum is
chosen, the displacement x of the pendulum is proportional
to the ground motion xg. Recorded displacement can be
expressed in terms of ground motion times a constant.
● Velocity seismograph
● Acceleration seismograph

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Strong Ground Motion

● Motion that affects living beings and their environment is of

interest for engineers and is termed as strong ground
motion.
● Ground motion at a particular instant of time can be defined
by 3 orthogonal components of translation.
● Ground motion is random in nature, its amplitude and
direction varying randomly with time.

● Strong motions that can possibly damage structures are of

interest.

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Characteristics of ground motion

● Ground motion can be described in terms of displacement,

velocity or acceleration.
● Variation of ground acceleration with time, recorded at a
point on the ground during an earthquake is called
accelerogram.
● Ground motion records are called time histories -
acceleration, velocity and displacement time histories.
● For structural engineering purpose, acceleration gives the
best measure of an earthquake’s intensity.

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● The nature of accelerograms may vary depending on energy

released at source, type of slip at fault rupture, geology
along the travel path from fault rupture to the Earth's surface,
and local soil.
● They carry distinct information regarding ground shaking;
peak amplitude, duration of strong shaking, frequency
content (e.g., amplitude of shaking associated with each
frequency) and energy content (i.e., energy carried by
ground shaking at each frequency) are often used to
distinguish them.

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Peak ground acceleration (PGA)

● The PGA is a measure of maximum amplitude of motion and
is defined as the largest absolute value of acceleration time
history.
● A horizontal PGA value of 0.6g (= 0.6 times the acceleration
due to gravity) suggests that the movement of the ground
can cause a maximum horizontal force on a rigid structure
equal to 60% of its weight.
● In a rigid structure, all points in it move with the ground by
the same amount, and hence experience the same
maximum acceleration of PGA.

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● Generally, the maximum amplitudes of horizontal motions in

the two orthogonal directions are about the same.
● However, the maximum amplitude in the vertical direction is
usually less than that in the horizontal direction.
● In design codes, the vertical design acceleration is taken as
1/2 to 2/3 of the horizontal design acceleration.
● The maximum horizontal and vertical ground accelerations in
the vicinity of the fault rupture do not seem to have such a
correlation.

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Classification of earthquakes

● Classification based on ground motion

● Classification based on magnitude
● Classification based on earthquake effect

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Classification based on ground motion

1. Practically a single shock – occurs only at short distances
from epicentre, only on firm ground, only for shallow
earthquakes. When these conditions are not fulfilled, multiple
wave reflections change the nature of motion.
2. Moderately long extremely irregular motion – occurs at
moderate distance, only on firm ground. They are of almost
equal severity in all directions.
3. Long period ground motion exhibiting pronounced prevailing
periods of vibration
4. Ground motion involving large scale permanent deformations
of the ground.
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Classification based on magnitude

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Classification based on earthquake effect

ke mitude
● M< 2.5, Usually not felt, but can be recorded
● M 2.5 – 5.4, Often felt, but only cause minor damage

● M 5.5 – 6, Slight damage to structures

● M 6.1 – 6.9, May cause lot of damage in populated areas

● M 7 – 7.9, Serious damages

●M ≥ 8, Catastrophic, can even destroy communities near the

epicentre

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Question

● Discuss briefly classification of earthquakes.

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Seismic zoning

● It is not possible to predict exactly when and where

earthquakes will occur, how strong they will be and what
characteristics the ground motion will have.
● To estimate the ground shaking, engineers depend on
seismic zone map.
● Seismic zoning is prepared based on isoseismal maps.
● Isoseismal is a contour bounding areas of equal intensity
and different isoseismals when plotted for a particular
earthquake constitute an isoseismal map.

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● India lies at the northwestern end of the Indo-Australian

Plate, which encompasses India, Australia, a major portion
of the Indian Ocean and other smaller countries.
● This plate is colliding against the huge Eurasian Plate and
going under the Eurasian Plate.
● This process of one tectonic plate getting under another is
called subduction.

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Tectonic Plate Boundaries at India

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Some Past Earthquakes

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● Four Great earthquakes (M>8) occurred in a span of 53 years from

1897 to 1950.
● January 2001 Bhuj earthquake (M7.7) is almost as large.
● Each of these caused disasters, but also allowed us to learn about
earthquakes and to advance earthquake engineering.

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● 1819 Cutch Earthquake produced an ~3m high uplift of the

ground over 100 km.
● The 1897 Assam Earthquake caused severe damage up to
500km radial distances; the type of damage sustained led to
improvements in the intensity scale from I-X to I-XII.
● Extensive liquefaction of the ground took place over a length
of 300km during 1934 Bihar-Nepal earthquake in which
many structures went afloat.

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Seismic Zones of India

● The varying geology at different locations in the country

implies that the likelihood of damaging earthquakes taking
place at different locations is different.
● Thus, a seismic zone map is required to identify these
regions.
● Based on the levels of intensities sustained during damaging
past earthquakes, the 1970 version of the zone map
subdivided India into five zones - I, II, III, IV and V.

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● The maximum Modified Mercalli (MM) intensity of seismic

shaking expected in these zones were V or less, VI, VII, VIII,
and IX and higher, respectively.
● Parts of Himalayan boundary in the north and northeast, and
the Kachchh area in the west were classified as zone V.

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● The seismic zone maps are revised from time to time as

more understanding is gained on the geology, the
seismotectonic and the seismic activity in the country.
● The Indian Standards provided the first seismic zone map in
1962, which was later revised in 1967 and again in 1970.
● The map has been revised again in 2002 and it now has only
four seismic zones - II, III, IV and V.

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● The areas falling in seismic zone I in the 1970 version of the

map are merged with those of seismic zone II.
● Also, the seismic zone map in the peninsular region has
been modified.

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Indian Seismic Zone Map as per IS: 1893 (Part 1)-2002

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Deemed to be University

Seismic zoning map of India (IS 1893-2002)

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Deemed to be University

Seismic zoning map of India: IS 1893 (Part 1) : 2016

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Deemed to be University

MAP OF INDIA SHOWING EPICENTRES OF PAST EARTHQUAKES IN INDIA: IS 1893

(Part 1) : 2016

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Deemed to be University

References

1. S. K. Duggal, “Earthquake Resistant Design of Structures”, Oxford

University Press.
2. Pankaj Agarwal & Manish Shrikhande, “Earthquake Resistant Design of
Structures”, PHI Learning Private Limited

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