Dr. N. VENKATANATHAN
venkatanathan@eee.sastra.edu
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Earthquakes are natural ground motions
caused as the Earth releases energy.
One of the first attempts at the scientific
study of earthquakes followed the 1755
Lisbon earthquake.
Seismology is derived from Greek words
“seismos” (earthquake) and “logos” (study).
The scientific study of earthquakes and the
propagation of elastic waves through the
Earth.
The events that generate seismic waves
are called seismic sources.
 The most common seismic sources are
plate tectonics and volcanoes.
 The field also includes studies of
earthquake effects, such as tsunamis as
well as diverse seismic sources such as
volcanic, tectonic, oceanic, atmospheric,
and artificial processes.
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S.
Date
Region
Mag
Death
NO:
1
January 23, 1556
Shaanxi, China
8.0
toll
830, 000
2
July 27, 1976
Tangshan, China
7.5
255, 000
3
October 11, 1138
Aleppo, Syria
-
230, 000
4
December 26, 2004 Sumatra, Indonesia
9.1
228, 000
5
January 12, 2010
Haiti
7.0
316, 000
6
December 22, 856
Damghan, Iran
8.0
200, 000
7
December 16, 1920 Haiyuan, Ningxia, China
7.8
200, 000
As tectonic plates move relative to each
other, elastic strain energy builds up
along their edges in the rocks along fault
planes.
• Since fault planes are not usually very
smooth, great amounts of energy can be
stored as movement is restricted due to
interlock along the fault.
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When the shearing stresses induced in the
rocks on the fault planes exceed the shear
strength of the rock, rupture occurs.
This theory was discovered by making
measurements at a number of points across a
fault.
Prior to an earthquake it was noted that the
rocks adjacent to the fault were bending.
These bends disappeared after an
earthquake suggesting that the energy
stored in bending the rocks was
suddenly
released
during
the
earthquake.
 A vibration of the Earth produced by a
rapid release of energy, reason for the
earthquake.
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The source of an earthquake is called the
focus or hypocenter, which is an exact
location within the Earth were seismic
waves are generated by sudden release
of stored elastic energy.
 The point on Earth's surface directly
above the hypocenter is called the
epicenter.
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Seismic waves are produced due to sudden
breaking of rocks either due to natural cause
or due to explosion.
These waves propagate in the form of energy
that travels through the earth and is recorded
on seismographs.
Seismic waves move in different ways with
different speeds, based on its movement
they can be categorized into different kinds.
Body Waves
(Pass through the interior of
the earth)
P – Waves
(Primary Waves)
Propagate in the form of
compressions and dilations
S – Waves
(Secondary Waves)
Propagate in the form of
Crests and Troughs
Seismic Waves
Surface Waves
(Pass through the surface
of the earth)
Love Waves
Propagate in the form of
Crests and Troughs parallel
to the ground
Rayleigh Waves
Propagate in the form of
ocean waves and roll the
ground up and down and
also side to side
P
wave or Pressure waves or Primary
waves are longitudinal waves that
travel at maximum velocity.
 The first wave originates as body wave
from the focus of the earthquake.
 This is the fastest kind of seismic wave
and recorded at a seismic station first.
The P wave can move through solid rock and the
liquid layers of the earth.
 It propagates as a longitudinal wave, so it pushes
and pulls the rock as it moves through them.
 Due to their pushing and pulling nature, P waves
are also known as compressional waves.
 When P waves are propagating through rocks,
they move the particles of the rocks in the same
direction of wave propagation.
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These waves travel in water slowly with the
approximate speed of 1 km/s.
At the base of mantle they are travelling at high
speed, which is approximately 14 km/s.
P wave velocity depends on a material's "plane
wave modulus (M)" [It is defined as the ratio of
axial stress to axial strain in a uniaxial strain state.
One of the elastic moduli available to describe
isotropic homogeneous materials. Otherwise called
as the P-wave modulus (or) longitudinal modulus]
and its density.
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Where, “λ” is Lamé's constant, “μ” is shear
modulus, “K” is bulk modulus, and “” is
density. Notice that density is in the
denominator, so denser rocks should be
slower.
Although the density of rock in the Earth
generally increases with depth.
 The rigidity - increases even more rapidly
with depth.
 Hence, P wave velocity generally increases
with increasing depth.
 Since solids, liquids and gasses have a finite
bulk modulus;
 It can travel through any of these medium.
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Another type of body wave originated from an
earthquake, shear waves or secondary wave or S
wave.
 These waves are travels slower than the P wave.
 S wave can only move through solid rock, not
through any liquid medium like water since, shear
waves do not exist in fluids.
 S waves propagate in the form of transverse wave,
so it moves rock particles up and down or side-toside.
 So the particle displacement is perpendicular to the
direction of wave propagation.
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•
S-wave velocity depends on a material's shear modulus (μ), and density (),
•Comparing the velocity expressions, you can see that VP > VS for any material.
•Depending upon soil type "P" wave travels about 1.68 to 1.75 times faster than destructive "S" wave.
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S-wave velocity depends on a material's
shear modulus (μ), and density (),
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Comparing the velocity expressions, you can
see that VP > VS for any material.
Depending upon soil type "P" wave travels
about 1.68 to 1.75 times faster than
destructive "S" wave.
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These waves propagate only through the crust.
Compared to body waves these waves are low
frequency waves.
As these waves propagate at low velocity, they
are responsible for the damage and destruction.
Since surface waves are propagating only
through the surface of the earth’s crust, the
strength of the surface waves are more in
shallow earthquakes than the deep seated
earthquakes.
• Solid lines marked P are
compressional waves
• dashed lines marked S
are shear waves.
• S waves do not travel
through the core but
may be converted to
compressional
waves
(marked K) on entering
the core (PKP, SKS).
Waves may be reflected
at the surface (PP, PPP,
SS).
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It is named after A.E.H. Love, a British
mathematician given the mathematical
model for this wave in 1911.
It fastest of all surface wave and
propagates in the ground from side-toside.
Since these waves are confined to the
surface of the crust, they produce entirely
horizontal motion.
The amplitude
of the waves get
reduced with the
depth, this gives
differential
particle motion
at various
heights. This is
the one of the
reason for the
collapsing of
buildings.
Rayleigh waves are mathematically
predicted by John William Strutt Lord
Rayleigh in 1885.
 Like an ocean waves, Rayleigh wave rolls
along the ground just like a wave rolls
across a lake or an ocean.
 The shaking produced by these waves is
much larger than the other type seismic
waves.
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Since it rolls, it moves the ground up and
down and side-to-side in the same
direction of wave propagation, most of
the shaking felt from an earthquake is
due to the Rayleigh wave.
 The up and down rolling get decreased
with the depth. These waves also
confined to the surface only.
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Earthquakes occur along plate edges
and along faults.
The earth's crust (the outer layer of the
planet) is made up of several pieces,
called plates.
The plates under the oceans are called
oceanic plates and the rest are
continental plates.
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The plates are moved around by the motion of a
deeper part of the earth (the mantle) that lies
underneath the crust.
These plates are always bump into each other,
pull away each other, or sliding past each other.
The plates usually move at about the same
speed that your fingernails grow.
Earthquakes usually occur where two plates are
running into each other or sliding past each
other.
This theory was the outcome of the
hypothesis of continental drift proposed
by Alfred Wegener in 1912.
• In his hypothesis, he suggested that the
present continents once formed a single
land mass which had drifted apart thus
formed separate the continents.
• which is much similar to the "icebergs“
floating on a sea.
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1.
2.
3.
4.
5.
6.
7.
continental crust
oceanic crust
upper mantle
lower mantle
outer core
inner core
A: Mohorovičić discontinuity - B: Gutenberg
Discontinuity - C: Lehmann discontinuity
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Based on chemical properties the
earth can be divided into three parts,
namely,
Core,
mantle and
crust.
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Since the earthquake generation mechanism
mostly depends on the physical behaviour of
the earth, the structure of the earth can be
divided into four parts.
The inner part of the earth (i.e.) the core can
be divided into two, namely, inner core and
outer core.
The outer part of the earth’s interior can be
divided into two parts, lithosphere and
asthenosphere.
The division of the outer part of the earth into
lithosphere and asthenosphere is based on
mechanical differences and in the ways that
heat is transferred.
 The lithosphere is cooler and more rigid, whilst
the asthenosphere is hotter and mechanically
weaker.
 Also, the lithosphere loses heat by conduction
whereas the asthenosphere also transfers heat
by convection and has a nearly adiabatic
temperature gradient.
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 The
lithosphere contains both crust
and some mantle.
 A given piece of mantle may be part
of the lithosphere or the
asthenosphere at different times,
depending on its temperature,
pressure and shear strength.
The key principle of plate tectonics is
that the lithosphere exists as separate
and distinct tectonic plates, which ride
on the fluid-like (visco-elastic solid)
asthenosphere.
 Plate motions are at different range, it
varies from typical 10 - 40 mm yr-1 to
about 160 mm yr-1.
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The plates are around 100 km thick and
consist of lithospheric mantle overlain
by either of two types of crustal
material: oceanic crust and continental
crust.
 The continental crust is thicker than
oceanic crust, 50 km and 5 km
respectively.
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The distinction between continental crust
and oceanic crust is based on the density of
constituent materials.
Oceanic crust is denser than continental crust
because of presence of large amount silicon.
Oceanic crust is denser because it has less
silicon and heavier elements.
As a result, oceanic crust generally lies below
sea level, while the continental crust is above
the sea level.
The interface between one plate and
another plate is called as plate boundary.
 Most of the earthquakes are occur along the
plate boundaries
 And most of the topographic features like
mountains, volcanoes and oceanic trenches
are present near the plate boundaries.
 The best example is Pacific Plate's Ring of
Fire which is most active and widely known.
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 All
the plates appear to be moving at
different
relative
speeds
and
independent of each other, but in
reality the plates are interconnected.
 No single plate can move without
affecting others, and the activity of one
can influence another thousands of
miles away.
 For
example, as the Atlantic Ocean
grows wider with the spreading of
the African Plate away from the
South American Plate, the Pacific
sea floor is being consumed in deep
subduction trenches over ten
thousand miles away.
The average rates of plate separations can range
widely.
 There are several methods in measuring the rate of
motion of plates.
 Evidence of past rates of plate movement can be
obtained from geologic mapping studies.
 If a rock formation of known age -- with distinctive
composition, structure, or fossils - mapped on one
side of a plate boundary can be matched with the
same formation on the other side of the boundary,
then measuring the distance that the formation has
been offset can give an estimate of the average rate
of plate motion.
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Current plate movement can be tracked directly by
means of ground-based or space-based geodetic
measurements.
 Using laser instruments the plate movement can be
measured in very precise manner.
 Using space geodesy precise, repeated measurements
of carefully chosen points on the Earth's surface
separated by hundreds to thousands of kilometers.
 The three most commonly used space-geodetic
techniques are very long baseline interferometry
(VLBI), satellite laser ranging (SLR), and the Global
Positioning System (GPS)
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African Plate, Antarctic Plate, Arabian Plate,
Australian Plate, Caribbean Plate, Cocos Plate,
Eurasian Plate, Indian Plate, Juan de Fuca Plate,
Nazca Plate, North American Plate, Pacific
Plate, Philippine Plate, Scotia Plate and South
American Plate
Aegean Sea Plate, Altiplano Plate, Anatolian
Plate, Banda Sea Plate, Burma Plate, Caroline
Plate, Conway Reef Plate, Easter Plate, Futuna
Plate and Galapagos Plate,