Transcript Earthquake.ppsx

Earthquake waves, construction and working of seismograph,
Earthquake zones of India, elastic rebound theory Preventive measures
for structures constructed in Earthquake prone areas
Teaching Aids Service by
KRRC Information Section
Earthquake
 An earthquake (also known as a quake, tremor or
temblor) is the result of a sudden release of energy in the
Earth’s Crust that creates seismic waves. The seismicity or
seismic activity of an area refers to the frequency, type
and size of earthquakes experienced over a period of time.
At the Earth's surface, earthquakes manifest themselves by
shaking and sometimes displacement of the ground. When
the epicentre of a large earthquake is located offshore, the
seabed may be displaced sufficiently to cause a tsunami.
Earthquakes can also trigger landslides, and occasionally
volcanic activity.
Terminology
 Seismology: The science dealing with the study of
earthquake in all their aspects is called Seismology.
 Epicenter: The epicentre is the point on the Earth’s
surface that is directly above the hypocenter or focus, the
point where an earthquake or underground explosion
originates.
 In the case of earthquakes, the epicentre is directly above
the point where the fault begins to rupture, and in most
cases, it is the area of greatest damage. However, in larger
events, the length of the fault rupture is much longer, and
damage can be spread across the rupture zone. For
example, in the magnitude 7.9, 2002 Denali earthquake in
Alaska, the epicentre was at the western end of the rupture,
but the greatest damage occurred about 330 km away at the
eastern end of the rupture zone.
Focus: Focus refers to the site
of an earthquake or a nuclear
explosion, it is the point of
origin of earthquake below the
surface of the earth.
Epicenter: The epicentre is
the point on the earth's surface
that is directly above the
hypocentre or focus, the point
where an earthquake or
underground
explosion
originates.
Seismic Waves
wave type
body waves
surface waves
particle motion
longitudinal
transverse
horizontal
transverse
vertical elliptical
name
P wave
S wave
Love wave
Rayleigh wave
Isoseismals:
These are hypothetical lines
passing through values of same intensity in a particular
earthquake record.
Types of Waves
 Compression wave
 Transverse Wave
 Seismic Wave
 Body Waves
 Primary or p-wave


Compression wave
Secondary or s-wave

Transverse wave
 Surface
 Love wave
 Rayleigh wave
Seismic Wave
 Seismic waves are the waves of energy caused by
the sudden breaking of rock within the earth or an
explosion. They are the energy that travels through
the earth and is recorded on seismographs.
 There are several different kinds of seismic waves,
and they all move in different ways. The two main
types of waves are body waves and surface waves.
Body Waves
 P Waves (compression wave)
 The first kind of body wave is the P wave or primary wave. This is the
fastest kind of seismic wave. The P wave can move through solid rock
and fluids, like water or the liquid layers of the earth. It pushes and
pulls the rock it moves through just like sound waves push and pull the
air.
Stop and Think
 Have you ever heard a big clap of thunder and heard the
windows rattle at the same time?
 The windows rattle because the sound waves were
pushing and pulling on the window glass much like P
waves push and pull on rock. Sometimes animals can
hear the P waves of an earthquake. Usually we only feel
the bump and rattle of these waves.
Body Waves
 S wave (transverse wave)
 The second type of body wave is the S wave or secondary
wave, which is the second wave you feel in an earthquake.
An S wave is slower than a P wave and can only move
through solid rock. This wave moves rock up and down, or
side-to-side.
Surface Waves
 Love Waves
 The first kind of surface wave is called a Love wave, named
after A.E.H. Love, a British mathematician who worked out
the mathematical model for this kind of wave in 1911. It's
the fastest surface wave and moves the ground from sideto-side.
Surface Waves
 Rayleigh Waves
 The other kind of surface wave is the Rayleigh wave, named for John William
Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind
of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls
across a lake or an ocean. Because it rolls, it moves the ground up and down,
and side-to-side in the same direction that the wave is moving. Most of the
shaking felt from an earthquake is due to the Rayleigh wave, which can be
much larger than the other waves.
Most Destructive
S waves are more dangerous than P-waves because
they have greater amplitude and produce vertical
and horizontal motion of the ground surface.
Love and Rayleigh waves both produce ground
shaking at earth’s surface but very little motion deep
in the earth. Because the amplitude of the surface
waves diminishes less rapidly with distance than the
amplitude of P or S wave. Surface waves are often
the most important component of ground shaking far
from the earthquake source, thus can be the most
destructive.
Classification of Earthquakes
Man Made
Water
Reservoir
Mining
Explosion
s
Natural
Tectonic
Non
Tectonic
Volcanic
Causes of Earthquake
 Tectonic Causes: Most earthquakes are causally related to compressional or tensional
stresses built up at the margins of the huge moving lithospheric
plates that make up the earth's surface. The immediate cause of
most shallow earthquakes is the sudden release of stress along a
fault, or fracture in the earth's crust, resulting in movement of the
opposing blocks of rock past one another. These movements cause
vibrations to pass through and around the earth in wave form, just
as ripples are generated when a pebble is dropped into water.
Volcanic eruptions, rockfalls, landslides, and explosions can also
cause a quake, but most of these are of only local extent. Shock
waves from a powerful earthquake can trigger smaller earthquakes
in a distant location hundreds of miles away if the geologic
conditions
are
favourable.
Plate Tectonic Theory
 The outer layer of the earth is divided into many sections known as
plates, which are floating on the molten magma beneath the earth's
crust. The movement of these plates is determined by the convection
currents in the molten magma.
 The heat makes these plates rise on the earth's surface or take them
into deep magma. Therefore, after intervals there are plates that get
submerged in the molten magma and there are plates that rise
upwards. At times, even a new crust is formed from the molten magma,
which in turn forms a new plate until it connects itself with the already
existing one.
 These plates can be pushed up to form mountains and hills, and the
movement is so slow that it is really hard to comprehend that there is
any movement at all. However, the results of these movements are
sudden.
Plate Tectonic Theory
 These plates are the base on which the continents stand
and when these plates move, the continents also move.
 Most earthquakes occur on the edges of the plates, where a
plate is under one or across one. This movement disrupts
the balance and position of all plates, which leads to
tremors of earthquakes.
 Due to the continuous motion of the plates, excess strain is
developed on the rocks which exerts a huge amount of
pressure on them. This strain is released when the fault
planes drift apart and take a new position. The energy
released results in shock waves.
Non Tectonic Causes
 Excessive exploitation of the earth's resources for building dams to
store large volumes of water, and blasting rocks and mountains to build
bridges and roads are also considered some of the reasons behind such
natural disruptions. Though earthquakes and disasters related to it
cannot be predicted, we can at least stay safe by using some
precautionary methods like building earthquake-resistant houses and
training ourselves in what to do when an earthquake occurs.
 Reservoir Induced Seismicity (RIS): It is well established (although
little known by the general public) that large dams can trigger
earthquakes. The first observation of possible reservoir–induced
seismicity (RIS) was noted for Algeria’s Quedd Fodda Dam in 1932; the
first extensive study of the correlation between increased earthquake
activity and variations in reservoir depth was made in the 1940s for
Hoover Dam.
 The Koyana earthquake of India (10th Dec 1967) occurred in an area that
stood classified as most stable and aseismic. Reservoirs are believed to
have induced five out of the nine earthquakes on the Indian peninsula
in the 1980s which were strong enough to cause damage.
 The most widely accepted explanation of how dams cause earthquakes
is related to the extra water pressure created in the microcracks and
fissures in the ground under and near a reservoir. When the pressure of
the water in the rocks increases, it acts to lubricate faults which are
already under tectonic strain, but are prevented from slipping by the
friction of the rock surfaces.
Volcanic Eruptions:
 Volcanic earthquakes are not very common. The molten magma under
the earth's crust is under enormous pressure. To release that pressure,
it looks for an opening and exerts pressure on the weaker part of the
crust.
 A volcano erupts from a weak section, and lava or magma is thrown
out. As soon as the lava comes in contact with the outer atmosphere, it
solidifies which may result in the blocking of the volcanic mouth or the
crater.
 Tremendous amount of pressure builds up and results in a massive
explosion. This in turn results in an earthquake of a high magnitude.
 A place which is the seat of an active volcano is often prone to
earthquakes. This is because the pressure that is exerted by the magma
exceeds its limit causing the plates to move and cause earthquakes.
 Earthquakes may also be caused after a volcanic eruption, since the
eruption also leads to a disturbance in the position of plates, which
either move further or resettle.
Magnitude of an Earthquake
 A Richter Magnitude Scale is one of a number of ways to assign a single
number to quantify the energy contained in an earthquake.
 In all cases, the magnitude is a base-10 logarithmic scale obtained by
calculating the logarithm of the amplitude of waves measured by a
seismogram. An earthquake that measures 5.0 on the Richter scale has a
shaking amplitude 10 times larger and corresponds to an energy release of
√1000 ≈ 31.6 times greater than one that measures 4.0.
 Mercalli intensity scale
 The Mercalli intensity scale is a seismic scale used for measuring the
intensity of an earthquake. It measures the effects of an earthquake, and is
distinct from the moment magnitude usually reported for an earthquake
(sometimes described as the obsolete Richter magnitude), which is a measure
of the energy released. The intensity of an earthquake is not totally determined
by its magnitude. The scale quantifies the effects of an earthquake on the
Earth's surface, humans, objects of nature, and man-made structures on a scale
from I (not felt) to XII (total destruction). Values depend upon the distance to
the earthquake
Recording of an Earthquake
 Seismometers are instruments that measure motions of the ground,
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


including those of seismic waves generated by earthquakes, volcanic
eruptions, and other seismic sources. Records of seismic waves allow
seismologists to map the interior of the Earth, and locate and measure
the size of these different sources.
Basic Principle:The physics behind the sensor is Newton's Law of
Inertia:
"A body in motion tends to stay in motion unless acted upon by a force,
and a body at rest tends to remain at rest unless acted upon by another
force.“
Inertial seismometers have levers in them that keep rhythmic motion
A weight, usually called the internal mass, that can move relative to the
instrument frame, but is attached to it by a system (such as a spring)
that will hold it fixed relative to the frame if there is no motion, and
also damp out any motions once the motion of the frame stops.
Recording of an Earthquake
 A means of recording the motion of the mass relative to the frame, or
the force needed to keep it from moving.
 Any motion of the ground moves the frame. The mass tends not to
move because of its inertia, and by measuring the motion between the
frame and the mass, the motion of the ground can be determined.
 Seismographs, which generally consist of two parts, a sensor of ground
motion which we call a seismometer, and a seismic recording
system. Modern seismometers are sensitive electromechanical devices
but the basic idea behind measuring ground movement can be
illustrated using a simpler physical system that is actually quite similar
to some of the earliest seismograph systems
Recording of an Earthquake
 Seismogram—A real-time record of earthquake ground
motion recorded by a seismograph at a specific location.
Seismograms come in many forms, on "smoked" paper,
photographic paper, common ink recordings on standard
paper, and in digital format (on computers, tapes, CD
ROMs).
 Seismograms are the records (paper copy or computer
image) used to calculate the location and magnitude of an
earthquake..
 Seismograph—an instrument that records vibrations of
the Earth, especially earthquakes. Seismograph generally
refers to the seismometer and a recording device as a single
unit..
Earthquake zones of India
Travel Time vs. Epicentre Distance
P-wave
S-wave
L-wave
20.0
18.0
Travel Time (min.)
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
2000
4000
6000
Epicentre Distance (km)
8000
10000
Locating an earthquake epicentre
 S-P intervals from at least three properly located
seismographic stations that have recorded the same
earthquake are required.
 With these stations as centers and the S-P interval time
distance as radii and a convenient scale, three circles are
drawn on a globe.
 If these stations are properly located thee circles will
intersect at point, that point is the epicenter of an
earthquake.
Engineering Considerations
The role of a Civil Engineer is –
 Seismic History: To know the seismic history of the area.
 Assessment of Seismic Risk: To assess the magnitude
and probable loss or damage in quantity and quality due
to likely seismic shock in the life period of the structure.
 Aseismic Designing: To introduce suitable factors of
safety in the new construction and if possible to safeguard
the existing structures against the likely shocks.
Thank You