EARTHQUAKE
Sarada Mandal
Associate Professor in Geography
Teacher-in-Charge
PRABHU JAGATBANDHU COLLEGE
Andul, Howrah
EARTHQUAKE
Definition of Earthquake
 An earthquake is the sudden, sometimes violent
movement of the earth's surface from the release
of energy in the earth's crust. Earthquakes occur
when energy stored within the Earth, usually in
the form of strain in rocks, suddenly releases. This
energy is transmitted to the surface of the Earth
by earthquake waves. The study of earthquakes
and the waves they create is called seismology
(from the Greek seismos, “to shake”). Scientists
who study earthquakes are called seismologists.
FOCUS AND EPICENTRE
 The point within the Earth along the rupturing geological fault
where an earthquake originates is called the focus, or
hypocenter.
 The point on the Earth’s surface directly above the focus is
called the epicenter.
 Earthquake waves begin to radiate out from the focus and
subsequently form along the fault rupture. If the focus is near
the surface—between 0 and 70 km (0 and 40 mi) deep—
shallow-focus earthquakes are produced.
 If it is intermediate or deep below the crust—between 70 and
700 km (40 and 400 mi) deep—a deep-focus earthquake will
be produced.
 Shallow-focus earthquakes tend to be larger, and therefore
more damaging, earthquakes. This is because they are closer
to the surface where the rocks are stronger and build up
more strain.
Earthquake Waves
Earthquake waves
 There are four kinds of wave produced by an earthquake. Two
move over the surface, causing damage, and two pass
through the Earth itself: body waves.
 By studying body waves, scientists have worked out what the
Earth is like inside:
 Body waves tell us a great deal about what we cannot see
inside the planet.
 P-waves: Primary waves are longitudinal waves that
push and pull the earth. They are the fastest body wave,
averaging speeds of about 6 km/s and so arrive first.
 S-waves: Secondary waves are transverse waves,
which make the earth shake from side to side. Slower than P
waves, they average about 4 km/s and so arrive second.
 Both S and P waves travel throughout the body of the earth,
and can be picked up by seismometers - machines that
record earthquakes - anywhere in the world.
 However, it turns out that S waves cannot travel through the
core, and only P waves are recorded in some places:
Earthquake waves
Causes of Earthquake
 Plate tectonics
 Elastic Rebound theory – Propounded
by Harry Fielding Reid in 1911
 Occurrence of Fault
 Volcanism
 Actions of Man
PLATE TECTONICS
 Most Earthquakes are caused by Plate Tectonics. The
earth's crust consists of a number of sections or plates
that float on the molten rock of the mantle. These
plates move on convection currents caused by heat
rising from the center of the earth. The hot magma
rises and spreads out on the surface, creating new
crust. The crust spreads out forming a new plate until it
meets another plate. One of the plates will be pushed
down into the interior of the earth and reabsorbed into
the mantle. Plates can also be compressed to push up
mountains when they collide or move sideways along
transform faults.
PLATE TECTONICS
PLATE TECTONICS
FAULTS
 Most earthquakes are caused by the sudden slip along
geologic faults. The faults slip because of movement of
the Earth’s tectonic plates. This concept is called the
elastic rebound theory. The rocky tectonic plates move
very slowly, floating on top of a weaker rocky layer. As
the plates collide with each other or slide past each
other, pressure builds up within the rocky crust.
Earthquakes occur when pressure within the crust
increases slowly over hundreds of years and finally
exceeds the strength of the rocks. Earthquakes also
occur when human activities, such as the filling of
reservoirs, increase stress in the Earth’s crust.
FAULTS
Elastic Rebound Theory

In 1911 American seismologist Harry Fielding Reid studied the
effects of the April 1906 California earthquake. He proposed the
elastic rebound theory to explain the generation of certain
earthquakes that scientists now know occur in tectonic areas,
usually near plate boundaries. This theory states that during an
earthquake, the rocks under strain suddenly break, creating a
fracture along a fault. When a fault slips, movement in the crustal
rock causes vibrations. The slip changes the local strain out into the
surrounding rock. The change in strain leads to aftershocks (smaller
earthquakes that occur after the initial earthquake), which are
produced by further slips of the main fault or adjacent faults in the
strained region. The slip begins at the focus and travels along the
plane of the fault, radiating waves out along the rupture surface. On
each side of the fault, the rock shifts in opposite directions. The fault
rupture travels in irregular steps along the fault; these sudden stops
and starts of the moving rupture give rise to the vibrations that
propagate as seismic waves. After the earthquake, strain begins to
build again until it is greater than the forces holding the rocks
together, then the fault snaps again and causes another earthquake.
Human Activities
 Fault rupture is not the only cause of earthquakes; human
activities can also be the direct or indirect cause of
significant earthquakes. Injecting fluid into deep wells for
waste disposal, filling reservoirs with water, and firing
underground nuclear test blasts can, in limited
circumstances, lead to earthquakes. These activities increase
the strain within the rock near the location of the activity so
that rock slips and slides along pre-existing faults more
easily. While earthquakes caused by human activities may be
harmful, they can also provide useful information. Prior to the
Nuclear Test Ban treaty, scientists were able to analyze the
travel and arrival times of P waves from known earthquakes
caused by underground nuclear test blasts. Scientists used
this information to study earthquake waves and determine
the interior structure of the Earth.
Volcanic Earthquakes
 Volcanic earthquakes occur near active volcanoes
but have the same fault slip mechanism as tectonic
earthquakes. Volcanic earthquakes are caused by
the upward movement of magma under the
volcano, which strains the rock locally and leads to
an earthquake. As the fluid magma rises to the
surface of the volcano, it moves and fractures rock
masses and causes continuous tremors that can
last up to several hours or days. Volcanic
earthquakes occur in areas that are associated
with volcanic eruptions, such as in the Cascade
Mountain Range of the Pacific Northwest, Japan,
Iceland, and at isolated hot spots such as Hawaii.
Results of Earthquake





Landslide
Soil liquefaction
Fire
Tsunami waves and flooding
Diseases
Major belts of Earthquake
of the World
Average number of
Earthquakes per year
LEVEL
RICHTER
MAGNITUDE
FREQUENCY/YEAR
GREAT
+8.0
1
MAJOR
7.0-7.9
18
LARGE/DESTRUCTIV
E
6.0-6.9
120
MODERATE/
DAMAGING
5.0-5.9
1000
MINOR
4.0-4.9
6000
GENERALLY FELT
3.0-3.9
49000
POTENTIALLY
PERCEPTIBLE
2.0-2.9
300,000
IMPERCEPTIBLE
<2.0
+600,000
Seismograph
 A seismometer records the vibrations from
earthquakes. Mechanical versions work by way of a
large mass, freely suspended.
 In the example on the right, a rotating drum records a
red line on a sheet of paper. If the earth moves (in this
case from left to right) the whole machine will vibrate
too.
 However, the large mass tends to stay still, so the
drum shakes beneath the pen, recording a squiggle.
 The confiner prevents the mass from bouncing around
all over the place.
 Incidentally, a seismograph is the graph that a
seismometer draws
Seismograph
Magnitude of Earthquake
FIGURE 1 - CHARLES RICHTER STUDYING A SEISMOGRAM
Magnitude of Earthquake


One of Dr. Charles F. Richter's most valuable contributions was to
recognize that the seismic waves radiated by all earthquakes can
provide good estimates of their magnitudes. . He collected the
recordings of seismic waves from a large number of earthquakes,
and developed a calibrated system of measuring them for
magnitude.
Richter showed that, the larger the intrinsic energy of the
earthquake, the larger the amplitude of ground motion at a given
distance. He calibrated his scale of magnitudes using measured
maximum amplitudes of shear waves on seismometers particularly
sensitive to shear waves with periods of about one second. The
records had to be obtained from a specific kind of instrument, called
a Wood-Anderson seismograph. Although his work was originally
calibrated only for these specific seismometers, and only for
earthquakes in southern California, seismologists have developed
scale factors to extend Richter's magnitude scale to many other
types of measurements on all types of seismometers, all over the
world. In fact, magnitude estimates have been made for thousands
of Moon-quakes and for two quakes on Mars.
Richter Magnitude of
Earthquake
 The equation for Richter Magnitude is:
ML = logA(mm) + (Distance correction factor)
Here A is the amplitude, in millimeters, measured directly
from the photographic paper record of the Wood-Anderson
seismometer, a special type of instrument. The distance
factor comes from a table that can be found in Richter's
(1958) book Elementary Seismology.
In the 'Richter scale'. An increase of one unit represents a
thirty-fold increase in energy, so an earthquake like the one
that ruined Kobe in Japan in 1995 (magnitude nearly 7) was
about 900 times as powerful as the earthquake felt in England
and Wales in 1990 (magnitude about 5).
Disadvantage of Richter
scale
 Over-dependence on instrumental
reading
 No scope for physiographic and tectonic
parameters
Seismic Moment


Seismologists have more recently developed a standard magnitude
scale that is completely independent of the type of instrument. It is
called the moment magnitude, and it comes from the seismic moment.
To get an idea of the seismic moment, we go back to the elementary
physics concept of torque. A torque is a force that changes the angular
momentum of a system. It is defined as the force times the distance
from the center of rotation. Earthquakes are caused by internal torques,
from the interactions of different blocks of the earth on opposite sides
of faults. After some rather complicated mathematics, it can be shown
that the moment of an earthquake is simply expressed by:
Seismic Moment
The formula above, for the moment of an earthquake, is fundamental to seismologists'
understanding of how dangerous faults of a certain size can be.
Next let's take the energy we found for the Double Spring Flat earthquake and estimate
its magnitude:
Seismic Energy
Both the magnitude and the seismic moment are related to the amount of
energy that is radiated by an earthquake. Richter, working with Dr. Beno
Gutenberg, early on developed a relationship between magnitude and
energy. Their relationship is:
 logES = 11.8 + 1.5M
giving the energy ES in ergs from the magnitude M. Note that ES is not
the total ``intrinsic'' energy of the earthquake, transferred from sources
such as gravitational energy or to sinks such as heat energy. It is only the
amount radiated from the earthquake as seismic waves, which ought to
be a small fraction of the total energy transfered during the earthquake
process.
More recently, Dr. Hiroo Kanamori came up with a relationship between
seismic moment and seismic wave energy. It gives:
Energy = (Moment)/20,000
Earthquake Intensity
GIUSEPPE MERCALLI
Evolution of the Mercalli
scale
The Mercalli scale originated with the widely used simple tendegree Rossi-Forel scale, which was revised by Italian
volcanologist Giuseppe Mercalli in 1883 and 1902. The terms
Mercalli intensity scale or Mercalli scale should not be used
unless one really means the original ten-degree scale of 1902.
In 1902 the ten-degree Mercalli scale was expanded to twelve
degrees by Italian physicist Adolfo Cancani. It was later
completely re-written by German geophysicist August Heinrich
Sieberg and became known as the Mercalli-Cancani-Sieberg
(MCS) scale. The Mercalli-Cancani-Sieberg scale was later
modified and published in English by Harry O. Wood and Frank
Neumann in 1931 as the Mercalli-Wood-Neuman (MWN) scale. It
was later improved by Charles Richter, the father of the Richter
magnitude scale. The scale is known today as the Modified
Mercalli Scale and commonly abbreviated MM.
Modified Mercalli
The lower degrees of the MM scale generally
deal with the manner in which the earthquake
is felt by people. The higher numbers of the
scale are based on observed structural
damage. The table below is a rough guide to
the degrees of the Modified Mercalli Scale. The
colors and descriptive names shown here differ
from those used on certain shake maps in
other articles.
Modified Mercalli
I. Instrumental Not felt except by a very few
under especially favorable conditions
II. Feeble Felt only by a few persons at rest,
especially on upper floors of buildings.
Delicately suspended objects may swing.
III.Slight Felt quite noticeably by persons
indoors, especially on the upper floors of
buildings. Many do not recognize it as an
earthquake. Standing motor cars may rock
slightly. Vibration similar to the passing of
a truck. Duration estimated.
Modified Mercalli
iv.
V.
VI.
Moderate Felt indoors by many, outdoors by
few during the day. At night, some awakened.
Dishes, windows, doors disturbed; walls make
cracking sound. Sensation like heavy truck
striking building. Standing motor cars rocked
noticeably. Dishes and windows rattle
alarmingly.
Rather Strong Felt by nearly everyone; many
awakened. Some dishes and windows broken.
Unstable objects overturned. Clocks may stop.
Strong Felt by all; many frightened and run
outdoors, walk unsteadily. Windows, dishes,
glassware broken; books off shelves; some
heavy furniture moved or overturned; a few
instances of fallen plaster. Damage slight.
Modified Mercalli
VII. Very Strong Difficult to stand; furniture broken;
damage negligible in building of good design and
construction; slight to moderate in well-built
ordinary structures; considerable damage in poorly
built or badly designed structures; some chimneys
broken. Noticed by persons driving motor cars
VIII. Destructive Damage slight in specially designed
structures; considerable in ordinary substantial
buildings with partial collapse. Damage great in
poorly built structures. Fall of chimneys, factory
stacks, columns, monuments, walls. Heavy
furniture moved.
IX. Ruinous General panic; damage considerable in
specially designed structures, well designed frame
structures thrown out of plumb. Damage great in
substantial buildings, with partial collapse.
Buildings shifted off foundations
Modified Mercalli
X. Disastrous Some well built wooden
structures destroyed; most masonry and
frame structures destroyed with foundation.
Rails bent.
XI. Very Disastrous Few, if any masonry
structures remain standing. Bridges
destroyed. Rails bent greatly.
XII. Catastrophic Total damage - Almost
everything is destroyed. Lines of sight and
level distorted. Objects thrown into the air.
The ground moves in waves or ripples.
Large amounts of rock may move.
EXAMPLES OF OTHER
INTENSITY-SCALE
 MSK-64(Medvedev-Sponheur-Karnik
scale)
 MSK-81
 EMS (European Macro-seismic) Scale
Disadvantages of intensity
scale




Depth of focus
Under-lying structures
Nature of buildings
Perception of the affected people
Relationship between
magnitude and intensity
Richter Magnitude
Energy in erg
Frequency / year
Intensity according to
MM Scale
3.0 -- 3.9
9.5×1015 -4×1017
49,000
II - III
4.0 -- 4.9
6×1017 – 8.8×1018
6,000
IV - V
5.0 – 5.9
9.5×1018 -4×1020
800
VI -- VII
6.0 – 6.9
6×1020 – 8.8×1021
120
VII -- VIII
7.0 – 7.9
9.5×1022 -4×1023
18
IX -- X
8.0 – 8.9
6×1023 – 8.8×1024
1
XI -- XII
Mapping of Earthquake
 Mapping for several earthquakes
e.g. superimposition of Plate Margins and
locations of the epicenters of 100 major
earthquakes all over the World
Or showing the locations, magnitudes,
depth and time for several earthquakes
by different symbols
Or loss estimation for several earthquakes.
Mapping of Earthquake
 Mapping for a single earthquake
 Mapping by drawing isoseismal lines,
identification of intensity zones and
epicenters
 Mapping by showing loss and damage
for different intensity zones
Mapping of Earthquake
 Mapping by showing level of earthquakeproneness of different regions of a
country (e.g. India)
Mapping by showing level of
earthquake-proneness of different
regions of a country (e.g. India)
 Collect the data for the locations and
magnitudes of earthquakes in Andaman and
Nicobar islands for the time period of last 100
years
 Divide the area by grids of latitude and
longitude ( say by 10 minutes )
 Identify the highest magnitude as the
representative value for a particular year
( say1910 )
Mapping by showing level of
earthquake-proneness of different
regions of a country (e.g. India)
 Identify the representative values for
each year in the total time period
 Calculate the number of representative
values in each grid
 Consider two types of variables in each
grid : a) the number of years
b) the number of highest
magnitudes of earthquakes
Mapping by showing level of
earthquake-proneness of different
regions of a country (e.g. India)
 Do time series analysis of those variables
for each grid by Least Square Method
Σy=Na + b Σ x
Σxy = aΣ x + bΣ x2
b=M1=maximum observed magnitude on
regression relation
Draw isopleth lines by these b values
Mapping by showing level of
earthquake-proneness of different
regions of a country (e.g. India)
 Calculate a/b values for each grid
 a/b= M2 = Mmax = Expected upper
magnitude level of earthquakes
 Draw isopleth lines by these values and
identify the zones