EARTHQUAKE

1. Introduction: Earthquake is the sudden movement of the ground caused by the release of stress
along a fracture (or fault) within the lithosphere. Earthquakes usually occur along plate boundaries,
although intraplate earthquakes do occur and probably represent the release of excess stress.

1.1 Cause of Earthquake: The earth’s crust is a rocky layer of varying thickness ranging from a depth
of about 10 kilometers under the sea to 65 kilometers under the continents. The crust is not one piece but
consists of portions called ‘plates’ which vary in size from a few hundred to thousands of kilometers. The
‘theory of plate tectonics’ holds that the plates ride up on the more mobile mantle, and are driven by some
yet unconfirmed mechanisms, perhaps therm thermal
al convection currents. When these plates contact each other,
stress arises in the crust.
• All these movements are associated with earthquakes. The areas of stress at plate boundaries which
release accumulated energy by slipping or rupturing are known as 'faults’. The theory of 'elasticity' says that
the crust is continuously stressed by the movement of the tectonic plates; it eventually reaches a point of
maximum supportable strain. A rupture then occurs along the fault and the rock rebounds under its own
elastic stresses until the strain is relieved. The fault rupture generates vibration called seismic
1.2 Earth Interior: The Earth is formed of four concentric layers that have very different physical and
chemical properties: inner core, outer core, mantle and crust.

Fig. Divisions in the Earth's Interior (Adapted from, Beatty, 1990.)

• The centre or inner core is the hottest part of planet Earth. A solid mass of iron and nickel this centre
has temperatures up to 5,500 degrees Celsius. With this extraordinary heat the inner core functions
as engine for the planet:
• Surrounding the outer core is the outer core, a liquid layer of iron and nickel, extremely hot, this
layer has similar temperatures to the inner core:
• The widest section of the Earth is the mantle with an approximate diameter of 2,900 km. The mantle
is composed of semi-molten
molten rock called magma. The upper parts of the mantle are hard rock but
deeper within the rock is softer and beginning to melt:
• The outer layer of the planet is known as the crust. It is a solid rock layer which averages about 60
kilometers in thickness.
 • The crust and the upper mantle form a cold, strong layer known as the "lithosphere" floating on the
inner mantle. The lithosphere is fragmented into a dozen of huge, irregularly shaped pieces, called
tectonic plates, which are in constant motion and slide over, under and past each other on top of the
partly molten inner layer. There are two kinds of plates: oceanic crust (i.e. the plates under the
ocean) and continental crust.

• There are 7 large and many small moving plates. The plates have a depth of 50 miles and on average
they move only a few inches a year relative to one another. Coastlines and plate boundaries do not
always align.
• Although the upper mantle is solid rock, it slowly flows in a convection current because of heat
dissipating from the core. Convection currents in the mantle cause the plates to move several
centimeters a year in different directions.

• The convection current have the characteristics such: as the hotter Mantle rises cooler material sinks;
the material rises and spreads out while the crust splits and diverges; the plates converge and subduct
as the material sinks.

• Stress is exerted the plates as they move and those around them. They may collide, sink, or pull apart
as the plates scrape boundaries creating stress which results in strain setting energy free.
• Subduction occurs when plates collide and one is drawn beneath another. This process can take
thousands of years. The collision of plates creates mountains as rock layers are forced upward. As
the plates diverge lava pushes through the mantle, cools and forms a new section of crust. Plates
moving slowly along side each other create friction and intense heat as they slide resulting in
volcanic activity if the rock melts. Earthquakes can occur if the plate slips sliding away or towards
each other.
• In some cases this is a gradual movement. Sometimes the plates lock together unable to release the
energy accumulated which builds up in the rock.
• When this energy elevates to the elastic limit of the rocks, they will break free causing the ground to
shake. This usually occurs when two plates either ride over, or slide against each other, and the
material at the edge of the tectonic plates deforms and ruptures at its weakest point. Thus the strain
energy stored within the plate is released in the form of vibrations.
1.3 Fault Lines: Earthquakes can also occur far from the edges of tectonic plates, along faults. Faults
are cracks in the lithosphere caused by the stresses created as sections of a plate (or two plates) are moving in
different directions. In this case, the earthquake event is called a slip. There are a number of different types
of faults, but most can be divided into three categories: strike-slip faults, normal faults, and thrust faults.

1.3.1 Strike-slip fault / Transform Fault: A strike slip fault occurs in an area where two plates are sliding
past each other. In relation to the ground surface the slip involves sideways movement.

1.3.2 Normal faults: Normal faults cracks where one mass of rock slides downward and pulls away from
another mass of rock. As these plates are slowly splitting apart and pulling away from each other the normal
faults are formed in this way. Normal faults are associated with downward movement on a sloping fault as
the two plates move apart. The stretching of the Earth's crust is indicative of this type of event. Deep sea
ridges in the Atlantic and Pacific are where the largest normal faults are formed along these ridges. Thrust
faults are caused by plates pulling apart and colliding with continental plates.
1.3.3 Reverse or Thrust faults: A thrust fault happens when plates are being pushed together. This involves
upward movement as the two plates collide and buckle upwards. This kind of event signifies a compression
of the Earth's crust.

see

Transform/ Strike Fault Normal Fault

Reverse or Thrust fault

Fig. Type of faults

1.4 Seismology: Seismology is the scientific study of earthquakes and the propagation of elastic waves
through the Earth or through other planet-like bodies.
• A recording of earth motion as a function of time is called a seismograph. A seismologist is a
scientist who does research in seismology.
• Seismic waves can be classified into two basic types: body waves which travel through the Earth
and surface waves, which travel along the Earth's surface. Those waves that are the most destructive
are the surface waves which generally have the strongest vibration.

1.4.1 Body waves: Body waves are of two types: compressional or primary (P) waves and shear or
secondary (S) waves.

P- and S- waves are called "body waves" because they can travel through the interior of a
body such as the Earth's inner layers, from the focus of an earthquake to distant points on the
surface. The Earth's molten core can only be traveled through by compressional waves.
P-waves travel fastest, at speeds between 4-8 km/sec (14,000-28,000 km/h) in the Earth's
crust. S-waves travel more slowly, usually at 2.5-4 km/sec (9,000-14,000 km/h). Sound waves are
usually called P-waves and are heard but not often felt. Except in the most powerful earthquakes
they generally do not cause much damage. P-waves shake the ground in the direction they are
propagating, while S-waves shake perpendicularly or transverse to the direction of propagation (i.e.
they displace material at right angles to their path).
 Fig. P and S wave

The P-wave is the first to arrive at a location, as it is the fastest. The P wave, or compressional
wave, ultimately compresses and expands material in the same direction it is travelling. The next to
arrive is the S wave which causes particles to oscillate. S waves can travel through solid material but
not through liquid or gas.

1.4.2 Surface waves: Surface waves, in contrast to body waves can only move along the surface. They
arrive after the main P and S waves and are confined to the outer layers of the Earth. They cause the most
surface destruction. Earthquake surface waves are divided into two different categories: Love and
Rayleigh.
Love waves have a particle motion, which, like the S-wave, is transverse to the direction of
propagation but with no vertical motion. Their side-to-side motion (like a snake wriggling) causes
the ground to twist from side to side, that's why Love waves cause the most damage to structures.
Rayleigh waves create a rolling, up and down motion with an elliptical and retrograde particle
motion confined to the vertical plane in the direction of propagation. Surface waves are generally not
generated by deep earthquakes.

Fig. L and R wave

1.5 Plate Boundaries: Plate Boundaries are categorized into three types:

1.5.1 Divergent boundaries (also called spreading zones): There are places on earth where two
plates are separating or spreading apart, such as at oceanic ridges. Rift valleys and faults occur when the
lithosphere is under tensional stress. At spreading zones, new magma comes up from the mantle, pushing
two plates apart and adding new material at their edges. Spreading zones are usually found in oceans along
with mid-ocean ridges.
As the new material flows out of the ridge, it pushes the existing ground floor out, until it eventually
sinks under another plate, which leads us into a different type of boundary. Earthquakes with low Richter
magnitudes along boundaries with normal fault motion tend to be shallow focus. These quakes can have
focal depths of less than 20km.This indicates the brittle lithosphere must be thin along the diverging plate
boundaries.

1.5.2. Transform boundaries (also called transform faults): These are found where plates slide past
one another. The San Andreas Fault is an example of a transform -fault plate boundary along the north
western Mexican and California coast. Earthquakes along transform boundaries show strike-slip motion on
the faults, they form fairly straight linear patterns and tend to be shallow focus earthquakes with depths
usually less than about 100 km. Richter magnitudes could be large.

1.5.3. Convergent boundaries (also called transform faults): Convergent boundaries are the place
where two tectonic plates converge (i.e. two plates move toward each other). These zones tend to be where
compressional stresses are active and this result in thrust or reverse faults being common. Converging plate
boundaries are of two types:

1.5.3.1 Subduction boundaries occur where oceanic lithosphere is pushed beneath continental or
oceanic lithospheres. Where two plates converge at an oceanic trench a subduction boundary is formed as
cold oceanic lithospheres are pushed back down into the mantle. This happens because the oceanic plate is
denser than the continental plate so, as they move together, the oceanic plate is forced underneath the
continental plate. In this case, one plate overrides, or "subducts" the other, pushing it slowly downward into
the mantle where it melts to form magma. A subducted lithosphere remains cold and brittle as it descends
and can fracture under compressional stress. These fractures generate earthquakes that define a zone of
quakes at increasing focal depth under the overriding plate. This zone is called the Benioff Zone. Depths of
up to 700km are reached in the Benioff Zone. Examples of subduction zones are found along the northwest
coast of the United States, Mexico, western Canada, southern Alaska, South America, Central America,
Japan, Philippines, Caribbean Islands and the Aleutian Islands.

1.5.3.2 Collisional boundaries where two plates of continental lithosphere collide result in fold-
thrust mountain belts. The continental crust is squashed together as the plates push together and is forced
upwards. This is called folding. Fold mountains are created by this process of folding. Where two
continental plates converge and push towards each other fold mountains can also be formed. This is how
mountain ranges such as the Himalayas and the Alps were formed. Earthquakes occur due to the thrust
faulting and range in depth from shallow to about 200 km. Examples are found along the Himalayan Belt
into China, along the Northern edge of the Mediterranean Sea through Black Sea and Caspian Sea into Iraq
and Iran.

Convergent boundary zones are characterized by deep-ocean trenches, shallow to deep earthquakes,
and mountain ranges containing active volcanoes.
 Fig. Plate Boundaries

Fig. Plate Boundaries with the fault(source :

https://allanawheeler.wordpress.com/2011/05/23/faults/
https://allanawheeler.wordpress.com/2011/05/23/faults/)
1.6 Earthquake Terminology::
The motion of plates results in stress buildup along plate boundaries as well as in interior domain of
the plate. Depending on the state of buildup of stress and amount of resistance offered by the fault strata,
rupture is initiated as stress exceeds the capacity of the strata.
• Generally, the rupture causing earthquakes initiates from a point, termed as hypocenter or focus, focus
which subsequently spreads over to a large area area.. Depending on the characteristics of strata where rupture
occurs, the shape of the ruptured area could be highly irregular and the amount of interface slip along the
ruptured surface could also vary. Several terms associated with earthquake rupture/propagation
rupture/propag are discussed
given below:

Fig. Various distance measurements associated with earthquake.

earthquake

• The place of origin of the earthquake in the interior of the earth is known as focus or origin or
centre or hypocenter. The place on the earth's surface, which lies exactly above the centre of the
earthquake, is known as the 'epicenter'
'epicenter'.
• For obvious reasons, the destruction caused by the earthquake at this place will always be maximum
and with an increasing distance from this point, the in intensity of destruction also decreases. The point on
earth's surface diametrically opposite to the epicenter is called the anti-center.
• An imaginary line which joins the points at which the earthquake waves have arrived at the earth's
ime is called a 'co-seismal'.. In homogeneous grounds with plain surfaces, the iso-
surface at the same time
seismals and coseismal coincide. Of course, in many cases due to surface and subsurface irregularities,
irregularities such
coincidence may not occur.

1.6.1 Determination of Hypocenter or Earthquake Focus: Seismologists use the elapsed time between the
arrival of a P-waves and S-waves at a given site to assist them in estimating the distance from the site to the
center of energy release. The distance of focus from the observation station is determined by the relative
arrival times of the P and S waves. The distance from hypocenter to observation point is given by:
by

1 1

where, T=difference in time of arrival of P and S waves at an observation point; S=distance from
hypocenter to observation
bservation point; and Vp and Vs are the velocity of P and S waves, respectively. The time T
 can be taken as the time of duration of the initial tremor to it built-up while Vp and Vs are geological
properties for a given locations.
• Thus, the distance from the hypocenter to the observation point is approximately proportional to the
time of duration of the initial tremor; the coefficient of proportionality is about 8 km/sec. When S has been
determined for each of three observation points the hypocenter is located as the point of intersection of
these spheres.
1.6.2 Determination of epicenter of an earthquake: The distance of a seismic station from the epicenter of
an earthquake known as the epicentral distance may be expressed in kilometers ∆ along the surface, or by
∆
the angle ∆° ( )( ) subtended at the Earth’s center (R is the radius of the Earth).
1.6.3 Foreshocks and aftershocks: In many instances the accumulating strain is partially released locally as
small earthquakes, or foreshocks. This is an indicator that strain energy is building up to the rupture level
and is sometimes a premonition (bad feeling)that a larger earthquake is about to happen.
• When an earthquake occurs, most of the stored energy is released in the main shock. However, for
weeks or months after a large earthquake there may be numerous lesser shocks, known as aftershocks, some
of which can be comparable in size to the main earthquake. Structures that have been weakened by the main
event often collapse in large aftershocks, which can cause physical damage as severe as the main shock. The
death toll from aftershocks is likely to be less, because people have evacuated damaged structures.
1.6.4 Elastic rebound Theory: Our understanding of the processes that lead to earthquakes derives to a
large extent from observations of seismic events on the San Andreas Fault in California. However, the
elastic rebound model, proposed by H. F. Reid after the 1906 San Francisco quake, is a useful guide to
how an earthquake may occur.
• The model is illustrated in Fig. below by the changes to five parallel lines, drawn normal to the trace
of the fault in the unstrained state and intersecting it at the points A–E.

Fig. Elastic rebound model of the origin of earthquakes: (a) unstrained state of a fault segment, (b)
accumulation of strain close to the fault due to relative motion of adjacent crustal blocks, and (c) “rebound” of
strained segment as an earthquake with accompanying release of seismic energy.
 • Strain due to relative motion of the blocks adjacent to the fault accumulates over several years. Far
from the trace of the fault the five lines remain straight and parallel, but close to it they are bent. When the
breaking point of the crustal rocks at C is exceeded, rupture occurs and there is a violent displacement on
the fault-plane. The relative displacement that has been taking place progressively between the adjacent
plates during years or decades is achieved on the fault-plane in a few seconds. The strained rocks adjacent
to the fault “rebound” suddenly. The accumulated strain energy is released with the seismic speed of the
ruptured rocks, which are several kilometers per second.
• The segments BC and CID undergo compression, while CD and BC experience dilatation. The
points A and E do not move; the stored strain energy at these points is not released. The entire length of the
fault-plane is not displaced, only the region in which the breaking point has been exceeded. The greater the
length of the fault-plane that is activated the larger is the ensuing earthquake.
• Although the earthquake involves a part of the fault-plane measuring many square kilometers in
area, from the point of view of an observer at a distance of hundreds or even thousands of kilometers the
earthquake appears to happen at a point.
1.6.5 Types of Earthquakes based on the focal depth: Earthquakes can be of three types based on the focal
depth:
Deep:- 300 to 700 kms from the earth surface
Medium:- 60 to 300 kms
Shallow: less than 60 kms
The deep focus earthquakes are rarely destructive because by the time the waves reach the surface the impact
reduces. Shallow focus earthquakes are more common and are extremely damaging because of their
proximity to the surface.
1.6.6 Characteristics of earthquake: The typical characteristics of earthquake depend on:
Stress drop during the slip
Total fault displacement
Size of slipped area
Roughness of the slipping process
Fault shape( Normal fault, Reverse fault, Strike slip fault)
Proximity of the slipped area to the ground surface
Soil condition
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.
1.6.7 Secondary effects of earthquakes: There are some secondary effects that can accompany large
earthquakes: landslides, seismic sea waves (tsunamis) and fire/conflagrations. A major hazard associated
with large earthquakes in mountainous areas is the activation of landslides, which can cause destruction far
from the epicenter.
• When a large earthquake occurs under the ocean, it can activate a seismic sea wave known as a
tsunami, which in Japanese means “harbor wave.” This special type of sea wave is a long-wavelength
disturbance of the ocean surface that can be triggered by large-scale collapse or uplift of part of the ocean
floor, by an underwater landslide, or by submarine volcanism.
• A tsunami propagates throughout an ocean basin as a wave with period T of around 15–30 min. The
entire water column participates in the motion. As a result, the velocity of the wave, v, is dependent on the
water depth, d, and the acceleration due to gravity, g, and is given by:
v=
1.7 Size of Earthquakes: The size of earthquake could be related to the damage caused or parameters like
magnitude. These two useful definitions of the size of earthquakes are sometimes confused.
1.7.1 Intensity of Earthquakes: The intensity of an earthquake refers to the degree of destruction caused by
it. In other words, intensity of an earthquake is a measure of severity of the shaking of ground and its
attendant damage.
• This, of course, is empirical to some extent because the extent of destruction or damage that takes
place to a construction at a given place depends on many factors. Some of these factors are: (i) distance from
the epicenter, (ii) compactness of the underlying ground, (iii) type of construction (iv) magnitude of the
earthquake (v) duration of the earthquake and (vi) depth of the focus. Intensity is the oldest measure of
earthquake.
• The seismic intensity scale consists of a series of certain key responses such as people awakening,
movement of furniture, damage to chimneys, and finally – total destruction. Numerous intensity scales have
been developed over the last several hundred years to evaluate the effects of earthquakes, the most popular is
the Modified Mercalli Intensity (MMI) Scale. This scale, composed of 12 increasing levels of intensity
that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does
not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. The lower
numbers of the intensity 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. An abbreviated version of the MMI
scale is given in Table below as per IS-1893:1984.
• 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.
• Some of the other intensity scales used are Rossi-Forel (RF) scale, Japanese Meteorological Agency
(JMA) intensity scale, etc.
• An imaginary line joining the points of same intensity of the earthquake is called an 'iso-seismal'. In
plan, the different iso-seismals will appear more or less as concentric circles over a plain, homogeneous
ground if the focus of the earthquake is a point. On the other hand, if the focus happens to be a linear tract,
the iso-seismals will occur elongated. Naturally, the areas or zones enclosed by any two successive iso-
seismals would have suffered the same extent of destruction. Over the years, researchers have tried to
develop more quantitative ways for estimating earthquake intensity. One of such relationships correlating
earthquake intensity to peak ground velocity is given by
14 !
2
Where, Vg is the peak ground velocity in cm/sec.
Another such relation reported by Wald et.al, (1999) based on Californian earthquake database is
3.47 log) ! * + 2.35
In addition to peak ground velocity, empirical relationships correlating peak ground acceleration to MMI has
also been reported.
MMI 3.47 log((Peak Ground Acceleration in cm/sec/sec) – 1.66 )
Table: Modified Mercalli Intensity Scale (IS-1893:1984).
MMI Intensity Remarks
I Not felt except by a very few under specially favorable circumstances
Felt only by a few persons at rest, especially on upper floors of buildings; and delicately
II
suspended objects may swing.
Felt quite noticeably indoors, especially on upper floors of buildings but many people
III do not recognize it as an earthquake; standing motor cars may rock slightly; and
vibrations may be felt like the passing of a truck.
During the day felt indoors by many, outdoors by a few, at night some awakened;
IV dishes, windows, doors disturbed; walls make creaking sound, sensation like heavy
truck striking the building; and standing motor cars rock noticeably.
Felt by nearly everyone; many awakened; some dishes, windows, etc, broken; a few
V instances of cracked plaster; unstable objects overturned; disturbance of trees, poles and
other tall objects noticed sometimes; and pendulum clocks may stop.
Felt by all, many frightened and run outdoors; some heavy furniture moved; a few
VI instances of fallen plaster or damaged chimneys; and damage slight.
Everybody runs outdoors, damage negligible in buildings of good design and
VII construction; slight to moderate in well built ordinary structures; and some chimneys
broken, noticed by persons driving motor cars.
Damage slight in specially designed structures; considerable in ordinary but
substantial buildings with partial collapse; very heavy in poorly built structures; panel
VIII 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
Damage considerable in specially designed structures; well designed framed structures
IX thrown out of plumb; very heavy in substantial buildings with partial collapse; building
shifted off foundations; ground cracked conspicuously; and underground pipes broken.
Some well built wooden structures destroyed; most masonry and framed structures with
X foundations destroyed; ground badly cracked; rails bent; landslides considerable from
river banks and steep slopes; shifted sand and mud; and water splashed over banks.
Few, if any, masonry structures remain standing; bridges destroyed; broad fissures in
XI ground, underground pipelines completely out of service; earth slumps and landslips in
soft ground; and rails bent greatly.
Total damage; waves seen on ground surfaces; lines of sight and levels distorted; and
XII
objects thrown upward into the air.

1.7.2 Magnitude of Earthquake: 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. Similar to intensity scales, over the years, a number of approaches for measurement of
magnitude of an earthquake have come into existence.
1.7.2.1 Richter Magnitude, ML: A workable definition of magnitude was first proposed by C.F. Richter.
He based on the data from Californian earthquakes, defined the earthquake magnitude as the logarithm to the
base 10 of the largest displacement of a standard seismograph (called Wood-Anderson Seismograph with
properties T=0.8 sec; m=2800; and damping nearly critical ≈ 0.8) situated 100 km from the focus.

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. 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
∆ 1.73
∆

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.

1.7.2.2 Moment magnitude: Over the years, scientists observed that different magnitude scales had
saturation points and the magnitudes estimated by different approaches did not point to a unique value of
earthquake size The Richter magnitude saturates at about 6.8 and the surface wave magnitude at about 7.8.
In addition, these magnitude estimates did not have a linear relation with the energy released due to
earthquake rupture. The moment magnitude is given by:
2
B ( 9.1)
3
Where, Mw is the moment magnitude, M0 is the seismic moment in N-m.

1.7.3 Predictability: Although some scientists claim ability to predict earthquakes, the methods are
controversial. Accurate and exact predictions of such sudden incidents are still not possible.

1.8 Energy of an Earthquake: An approximate relationship between surface wave magnitude, Ms, and the
energy released by an earthquake, E, is given by
log E = 4.8 +1.5Ms
10

where, E is measured in joules.

1.9 Comparison of Magnitude and Intensity: Comparisons between magnitude and intensity are fraught
with difficulty. Firstly, intensity varies with distance from the epicenter. Secondly, a large earthquake may
occur away from inhabited areas and therefore cause little apparent damage.
Focal depth, ground conditions and quality of building construction can have a considerable effect
on subjective assessments of damage. Magnitude-intensity relationships are not favored for engineering
purposes. However, intensity could be the only information available for large historical earthquakes and the
inputs from intensity measurements would be necessary in estimating the maximum earthquake potential of
the region.
In 1956, Richter proposed a simple relationship between magnitude and epicentral intensity given
by:
2
D +1
3

The equation was derived by comparison of magnitude and epicentral intensity data of Californian
earthquakes.
Seismology

2.1 Seismic zoning: Seismic Zoning can be defined as a process by which areas are subdivided into
seismic zones based on expected of ground motion, which is expressed in terms of peak horizontal ground
acceleration (PGA) or peak ground velocity (PGV). On the other way, seismic Zonation demarcates equal
hazard with respect to a characteristic of strong ground shaking and of site or structural response.
• A seismic zoning map differs from seismicity map by the fact that it specifies the levels of force or
ground motions for earthquake resistance design, where as seismicity map provides location and
characteristics of earthquakes. Seismic zonation can be done at any scale, national, regional, local, or site.
• The term Zoning implies that the parameter or parameters that characterize the hazard have a
constant value in each zone. If, for example, for practical reasons, the number of zones is reduced (from five
as is the case in large majority of national codes), we obtain a rather simplified representation of the hazard,
which in reality has continuous variation.
• A seismic zone is a region in which the rate of seismic activity remains fairly consistent. This may
mean that seismic activity is incredibly rare, or that it is extremely common. Some people often use the term
“seismic zone” to talk about an area with an increased risk of seismic activity, while others prefer to talk
about “seismic hazard zones” when discussing areas where seismic activity is more frequent.
• Many nations have government agencies concerned with seismic activity. These agencies use the
data they collect about seismic activity to divide the nation into various seismic zones. A number of different
zoning systems are used, from numerical zones to colored zones, with each number or color representing a
different level of seismic activity.
• A seismic zoning map for engineering use is a map that specifies the levels of force or ground
motions for earthquake-resistant design, and thus it differs from a seismicity map, which provides only
the occurrence of earthquake information. The task of seismic zoning is multidisciplinary and involves
the best of input from geologist, seismologist, geotechnical, earthquake and structural engineers.

2.2 Need for Seismic Zonation:

• These maps identify the regions of a country or province in which various intensities of ground
shaking may have occurred or may be anticipated.
• Maps of probabilistic hazard give an idea of the underlying statistical uncertainty, as is
done in calculating insurance rates. These maps give, for example, the odds at which specified
earthquake intensity would be exceeded at a site of interest within a given time span.
• Seismic zoning is used to reduce the human and economic losses caused by earthquakes, thereby
enhancing Economic development and Political stability.
• New probabilistic maps have been developed as the basis of seismic design provisions for
building practice. These usually give the expected intensity of ground shaking in terms of peak
acceleration. The peak acceleration can be thought of as the maximum acceleration in earthquakes on
firm ground at the frequencies that affect sizable structures.
• The losses due to damaging earthquakes can be mitigated through a comprehensive assessment
of seismic hazard and risk. Seismic zonation of vulnerable areas for bedrock motion thus becomes
important so that the planners and administrators can make use of it after applying appropriate
amplification factors to take into account the local soil conditions, for better land use planning and safe
development.
2.3 Types of Zonation:
2.3.1 Macrozonation: After the adoption of some over-simplified approaches in the fifties using
micro tremor analysis (Kanai, 1957), in the sixties Medvedev (1977), proposed a zonation method based
on an empirical correlation between the seismic impedance ratio and the variation of macro seismic
intensity.
• For larger area like, zonation of country or continent macro level is adopted. Macrozonation are
carried out considering the seismicity, geology in lager scales without considering geotechnical aspects.
• Seismic macrozonation consists of dividing a national territory into several areas indicating
progressive levels of expected seismic intensity for different return periods. These zones can be
described in terms of expected intensity, peak ground accelerations, or any other strong motion
parameter. The number of zones into which a country is divided is fairly arbitrary.
2.3.2 Microzonation is a process that involves incorporation of geologic, seismologic and
geotechnical concerns into economically, sociologically and politically justifiable and defensible land-
use planning for earthquake effects so that architects and engineers can site and design structures that
will be less susceptible to damage during major earthquakes.
• Microzonation should provide general guidelines for the types of new structures that are most
suited to an area, and it should also provide information on the relative damage potential of the existing
structures in a region.
• It consists of mapping in detail all possible earthquake and earthquake induced hazards.
Microzonation is subdivision of a region into zones that have relatively similar exposure to various
earthquake related effects.
2.3.3 Nanozonation map provides dominant period at any site within the city. The design spectrum at
each site is constructed using five parameters, all of which are derived from the dominant period value.
• Appealing as the “nanozonation” is, this approach is currently not feasible at most cities. In
addition, this approach cannot be envisaged when more than one factor contributes significantly to site
amplification.
2.4 Seismic zonation scales:
2.4.1 Macrozonation- Earthquake hazard and the associated risks are usually mapped at the sub regional
level because geoscientists usually work at very small scales, covering large areas, whereas
development planners work at large scales covering small areas. The most elegant approach would,
therefore, be to precede from whole to part hence scales for macrozonation usually used are more than
1:1,000,000
2.4.2 Microzonation - shall be graded based on the scale of the investigation and details of the studies
carried out shown in Table 10.1. The technical committee on earthquake geotechnical engineering of the
International society of soil mechanics and foundation engineering states that:
• The first grade (Level I) map can be prepared with scale of 1:1,000,000 – 1:50,000 and the
ground motion were assessed based on the historical earthquakes and existing information of geological
and geomorphological maps.
• If the scale of the mapping is 1:100,000-1:10,000 and ground motion is assessed based on the
microtremor and simplified geotechnical studies then it is called second grade (Level II) map.
• In the third grade (Level III) map ground motion has been assessed based on the complete
geotechnical investigations and ground response analysis with a scale of 1:25,000-1:5,000.
2.4.3 Nanozonation – it may be termed as rigorous zonation, where a very high and very detailed level
of zonation is required, for examples scales less than 1:5000, additional site investigation data will be
needed, specific to the site in question. The findings from such investigation may be incorporated into
computer aided analysis of seismic ground response, slope instability behavior, or liquefaction potential.
• This level of zonation, requiring detailed site specific information, is generally expensive, but
for sites where hazard potential is considered very high, or existing or proposed development is
regarded as critical or of high value, this level of investment may be warranted
2.5 Why Seismic Zonation is Necessary in India: Seismic zonation is a process, which provided
information about any decision making for urban regional planning or for earthquake design in
earthquake areas.
• In principle, seismic zoning map is the main source for zoning, which is displaying quantities
related to the expected frequency and intensity of shaking caused by earthquakes. The task of seismic
zoning is multidisciplinary and involves the best of inputs from geologists, geotechnical’s,
seismologists, earthquake and structural engineers. The rapid urbanization due to population outburst,
coming up of mega cities in potential seismic zones is the main reason for seismic hazard in India.
2.4.4 Seismic Zonation in India by BIS: (IS: 1893-1962 To Is: 1893-1984): The first national seismic
zoning map of India was compiled by the Geological Survey of India (GSI) in 1935 (Auden, 1959) after
the great Bihar – Nepal earthquake of 15 January 1934 with magnitude 8.4. Tandon (1956) first came up
with the seismic zoning map of India.
• It consists of three zones (Severe, Light and Minor hazard) based on the broad concept of space–
time earthquake statistics and the prevailing understanding of geotectonic. Then in 1962, the Bureau of
Indian Standards (BIS) (earlier, Indian Standards Institution) published the seismic zonation map of
India (IS: 1893"1962).
• It is made based on earthquake epicenters and the isoseismic map published by the GSI in 1935,
where India was divided into seven zones ranging from 0 (no damage) to VI (extensive damage). The
Deccan Plateau was considered more or less a safe zone and the hazard level was assigned 0 while the
northeastern India was assigned VI.
• The zoning was reviewed in 1966 (IS: 1893"1966) and additional information like the geology
and the tectonic features were taken into account for modifying the zones.The zonation map underwent
major revision in 1970 after the 1967 Koyna earthquake.
• The magnitude of the earthquake was 6.5 and occurred at the Deccan Plateau, which was
previously assigned the zone 0 in the earlier maps. There arose the need to utilize both geological and
geophysical data to review the zoning.
• This zonation map consists of five zones such as I, II, III, IV and V on the basis of MMI scale
with CIS – 64 scale such as I, II, III, IV and V. The major change was the removal of the zone 0 as it
was not appropriate scientifically to consider a region with zero possibility of earthquake shock.
Another addition was the merging of the zones V and VI (IS: 1893"1970).
• In 1984, the zonation map was modified (IS: 1893"1984) where the regions of different
seismogenic potential were identified on the basis of past earthquakes and the regional tectonic features.
However, the map does not show the seismic hazard at different locations and failed to assess.
 Fig.. Upgraded seismic zoning map of India (BIS, 2004)

3. Adverse effects of earthquake:

3.1 Physical damage: Damage occurs to human settlement, buildings, structures and infrastructure,
especially bridges, elevated roads, railways, water towers, pipelines, electrical generating facilities.
Aftershocks of an earthquake can cause much greater damage to already weakened structures.
• Secondary effects include fires, dam failure and landslides which may bblocklock water ways and also
cause flooding. Damage may occur to facilities using or manufacturing dangerous materials resulting in
possible chemical spills. There may also be a breakdown of communication facilities.
• The effect of an earthquake is diverse. Th There
ere are large number of casualties because of the poor
engineering design of the buildings and close proximity of the people. About 95 per cent of the people who
are killed or who are affected by the earthquake is because of the building collapse.
• There iss also a huge loss to the public health system, transport and communication and water supply
in the affected areas.
4. Earthquake control
• Earthquakes constitute a serious natural environmental hazard. Despite great efforts by scientists in
various countries, successful prediction is not yet generally possible.
• Consequently, the protection of people against seismic hazard depends currently on the identification
of especially perilous areas (such as active faults), the avoidance of these as the sites of constructions, the
development and enforcement of appropriate building codes, and the education and training of the
population in emergency procedures to be followed during and in the aftermath of a shock. Unfortunately,
many densely populated regions are subject to high seismic risk. It is impossible to prevent the accumulation
of strain in a region subject to tectonic earthquakes; the efforts of the human race are not likely to have much
effect on the processes of plate tectonics!
• However, it may be possible to influence the manner in which the strain energy is released. The
catastrophic earthquakes are those in which a huge amount of strain energy that has accumulated over a long
period of time is suddenly released in a single event. If the energy could be released progressively over a
longer period of time in many smaller shakes, the violence and disastrous consequences of a major
earthquake might be avoided. The intriguing possibility of this type of earthquake control has been
investigated in special situations.

3. Possible risk reduction measures:

3.1 Community preparedness: Community preparedness is vital for mitigating earthquake impact. The
most effective way to save you even in a slightest shaking is 'DROP, COVER and HOLD'.
3.2 Planning: The Bureau of Indian Standards has published building codes and guidelines for safe
construction of buildings against earthquakes. Before the buildings are constructed the building plans
have to be checked by the Municipality, according to the laid down bylaws. Many existing lifeline
buildings such as hospitals, schools and fire stations may not be built with earthquake safety measures.
Their earthquake safety needs to be upgraded by retrofitting techniques.
3.3 Public education is educating the public on causes and characteristics of an Effect of Soil type on
ground shaking Essential requirements in a Masonry building earthquake and preparedness measures. It
can be created through sensitization and training programme for community, architects, engineers,
builders, masons, teachers, government functionaries teachers and students.
3.4 Engineered structures: Buildings need to be designed and constructed as per the building by laws to
withstand ground shaking. Architectural and engineering inputs need to be put together to improve
building design and construction practices. The soil type needs to be analyzed before construction.
Building structures on soft soil should be avoided. Buildings on soft soil are more likely to get damaged
even if the magnitude of the earthquake is not strong. Similar problems persist in the buildings
constructed on the river banks which have alluvial soil.

5 Earthquake prediction
Predictions of earthquakes can be divided into two classes:
(i) Earthquake hazard assessment: Hazard assessment provides an estimate of probability that an
earthquake of a specified magnitude will occur in a specified region in a specified time interval. Hazard
assessments are based on a numberof observations including the number of smaller earthquakes that
occur in a region. Reasonably accurate hazard assessments are available. No reliable short-term
predictions of actual earthquakes have been documented.
(ii) Prediction of a particular earthquake: The problem of earthquake prediction is extremely difficult
and is associated with sundry other problems of a sociological nature. To predict an earthquake correctly
means deciding, as far in advance as possible, exactly where and when it will occur. It is also necessary
to judge how strong it will be, which means realistically that people want to know what the likely
damage will be, a feature expressed in the earthquake intensity. In fact the geophysicist is almost
helpless in this respect, because at best an estimate of the predicted magnitude can be made.
As seen above, even if it is possible to predict accurately the magnitude, the intensity depends on many
factors (e.g., local geology, construction standards, secondary effects like fires and floods) which are largely
outside the influence of the seismologist who is asked to presage the seriousness of the event. The problem
of prediction rapidly assumes sociological and political proportions.
Even if the approximate time and place of a major earthquake can be predicted with reasonable certainty, the
question then remains of what to do about the situation.
Properly, the threatened area should be evacuated, but this would entail economic consequences of possibly
enormous dimension.
The difficulties are illustrated by the following possible scenario. Suppose that seismologists conclude that a
very large earthquake, with probable magnitude 7 or greater, will take place sometime in a known month of a
given year under a specific urban center.