CE202

ENGINEERING GEOLOGY

Module 2
Internal Processes of the earth: -
1. a) Earthquakes- Plate Tectonics,
2. Origin of earthquakes, Seismic waves,
3. Rating of earthquakes, types of earthquakes,
4. Seismic zones of India. Basics of seismic safety factor,
5. Interior of the earth as revealed by propagation of seismic waves.

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 2.1 EARTH QUAKE:
Definition: An earthquake is an intense shaking of Earth’s surface which is caused by
movements in Earth’s outermost layer

✓ caused by the passage of seismic waves through Earth’s rocks.

✓ Seismic waves are produced when some form of energy stored in Earth’s crust is
suddenly released,
o usually when masses of rock straining against one another suddenly
fracture and “slip.”
o which triggers waves that travel in all directions.
✓ All natural earthquakes occur in the lithosphere.
✓ An earthquake is, simply, shaking of the earth’s crust.
✓ The emanation of energy occurs along a fault.
✓ A fault is a sharp break in the crustal rocks.
✓ Rocks along a fault generally move in opposing directions.

Rock fault Earth’s crust

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2.1.1 IMPORTANT TERMS

a) Fault:

✓ a planar fracture or discontinuity in a volume of rock

✓ narrow zones where rock masses move in relation to one another.
✓ The major fault lines of the world are located at the fringes of the huge tectonic
plates that make up Earth’s crust.

b) Focus:

✓ a point at some depth in the earth from where radiation of seismic waves starts

c) Epicentre:

✓ is the point on the Earth's surface directly above a hypocentre or focus,

✓ point on earth surface
✓ the point where an earthquake or an underground explosion originates.

d) Fault scrap:

✓ A fault scarp is a small step or offset on the ground surface where one side of a
fault has moved vertically with respect to the other.
✓ a cliff or escarpment directly resulting from an uplift along one side of a fault
✓ It is the topographic expression of faulting attributed to the displacement of the
land surface by movement along faults.

e) Wave fronts:

✓ A wavefront is a line or surface in the path of wave motion on which the

disturbances at every point have the same phase.
✓ Wavefronts can be of three types depending on the source of light as follows:
1. Cylindrical wavefront
2. Spherical wavefront
3. Plane wavefront

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Spherical Wavefront
When a point source in an isotropic (has same physical property in different directions)
medium,
→it sends out waves in three dimensions,
→ the wavefronts are spheres centred on the source.
Cylindrical wavefront
When the source of light is linear,
➔ all the points equidistant from the linear source lie on the surface of a cylinder.
Plane wavefront
When a fraction of a spherical or cylindrical wavefront originates from a distant source
like infinity then the wavefront which is obtained is known as a plane wavefront.

2.1.3 EFFECTS
1. Shaking of ground
2. The disparity in ground settlement
3. Natural disasters like Tsunami, landslide, mudslides, and avalanches
4. Soil liquefaction
5. Ground lurching and displacement
6. Floods and fires
7. Infrastructure collapse

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 2.2 PLATE TACTONICS
✓ It is a generally accepted scientific theory that considers the Earth's lithosphere to
comprise a number of large tectonic plates which have been slowly moving since
about 3.4 billion years ago
✓ It is the theory of Movement and interaction of earths plates
✓ Plate tectonics is a scientific theory that explains how major landforms are created
as a result of Earth’s subterranean movements
2.2.1 TECTONIC PLATES OF EARTH

Earth's lithosphere, which is the rigid outermost shell of a planet (the crust and upper
mantle), is broken into seven or eight major plates and many minor plates or "platelets".
The plates have a depth about 100km and move a few centimetres a year relative to one
another
✓ Where the plates meet is called the plate boundary
✓ Types of plate boundary:
o convergent,
o divergent,
o transform.
✓ Earthquakes, volcanic activity, mountain-building, and oceanic trench formation
occur along these plate boundaries (or faults).
✓ The relative movement of the plates typically ranges from zero to 10 cm annually

Divergent Convergent

Transform

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 Divergent boundaries (constructive boundaries or extensional boundaries)
✓ occur where two plates slide apart from each other.
✓ At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor
spreading, allowing for the formation of new ocean basin.
✓ e.g., the Mid-Atlantic Ridge and East Pacific Rise), and continent-to-
continent rifting (such as Africa's East African Rift and Valley and the Red
Sea)

Convergent boundary
Convergent boundaries (destructive boundaries or active margins)
✓ occur where two plates slide toward each other to form either a subduction
zone (one plate moving underneath the other) or a continental collision.
✓ At zones of ocean-to-continent subduction (e.g. the Andes mountain
range in South America, and the Cascade Mountains in Western United
States),

Transform boundary
Transform boundaries (conservative boundaries or strike-slip boundaries)
✓ occur where two lithospheric plates slide, or perhaps more accurately,
grind past each other along transform faults, where plates are neither
created nor destroyed.
✓ The relative motion of the two plates is either sinistral (left side toward
the observer) or dextral (right side toward the observer).
✓ Transform faults occur across a spreading center.
✓ Strong earthquakes can occur along a fault.
✓ The San Andreas Fault in California is an example of a transform
boundary exhibiting dextral motion.
Other plate boundary zones occur where the effects of the interactions are unclear,
and the boundaries, usually occurring along a broad belt, are not well defined and may
show various types of movements in different episodes.

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2.2.2 DRIVING FORCES OF TECTONIC PLATES
1. mantle dynamics related:
✓ Convection in the Mantle (heat driven)
✓ says the driving force behind tectonic plate motions envisaged large scale
convection currents in the upper mantle which are transmitted through the
asthenosphere.
2. gravity related (mostly secondary forces)
o Gravitational sliding away from a spreading ridge: says plate motion is
driven by the higher elevation of plates at ocean ridges.
o As oceanic lithosphere is formed at spreading ridges from hot mantle
material, it gradually cools and thickens with age (and thus adds distance
from the ridge).
o Cool oceanic lithosphere is significantly denser than the hot mantle
material from which it is derived and so with increasing thickness it
gradually subsides into the mantle to compensate the greater load.
o The result is a slight lateral incline with increased distance from the ridge
axis.
3. Earth rotation related.

2.2.3 PRINCIPLES OF PLATE TECTONICS

✓ Earth’s surface layer, 50 to 100 km (30 to 60 miles) thick, is rigid and is composed
of a set of large and small plates.
✓ Together, these plates constitute the lithosphere
✓ The lithosphere rests on and slides over an underlying partially molten (and thus
weaker but generally denser) layer of plastic partially molten rock known as the
asthenosphere,
✓ Plate movement is possible because the lithosphere-asthenosphere boundary is a
zone of detachment.
✓ As the lithospheric plates move across Earth’s surface,
✓ driven by forces as yet not fully understood, they interact along their boundaries,
diverging, converging, or slipping past each other.

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 ✓ While the interiors of the plates are presumed to remain essentially undeformed,
plate boundaries are the sites of many of the principal processes that shape the
terrestrial surface, including earthquakes, volcanism, and orogeny (that is,
formation of mountain ranges).

2.2 ORIGIN OF EARTHQUAKE

a) Fault:

✓ a planar fracture or discontinuity in a volume of rock

✓ narrow zones where rock masses move in relation to one another.
✓ The major fault lines of the world are located at the fringes of the huge tectonic
plates that make up Earth’s crust.

b) Focus:

✓ a point at some depth in the earth from where radiation of seismic waves starts

c) Epicentre:

✓ is the point on the Earth's surface directly above a hypocentre or focus,

✓ point on earth surface

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 ✓ the point where an earthquake or an underground explosion originates.

d) Fault scrap:

✓ A fault scarp is a small step or offset on the ground surface where one side of a
fault has moved vertically with respect to the other.
✓ a cliff or escarpment directly resulting from an uplift along one side of a fault
✓ It is the topographic expression of faulting attributed to the displacement of the
land surface by movement along faults.

e) Wave fronts:

✓ A wavefront is a line or surface in the path of wave motion on which the

disturbances at every point have the same phase.
✓ Wavefronts can be of three types depending on the source of light as follows:
4. Cylindrical wavefront
5. Spherical wavefront
6. Plane wavefront
Spherical Wavefront
When a point source in an isotropic (has same physical property in different directions)
medium,
→it sends out waves in three dimensions,
→ the wavefronts are spheres centred on the source.
Cylindrical wavefront
When the source of light is linear,
➔ all the points equidistant from the linear source lie on the surface of a cylinder.
Plane wavefront
When a fraction of a spherical or cylindrical wavefront originates from a distant source
like infinity then the wavefront which is obtained is known as a plane wavefront.
2.1.2 CAUSES
1. Volcanic Eruptions
✓ Such type of earthquakes occurs in areas, with frequent volcanic activities.
✓ When boiling lava tries to break through the surface of the earth, with the
increased pressure of gases, certain movements caused in the earth’s crust.
✓ Movement of lava beneath the surface of the earth can also cause certain
disruptions.
✓ This sends shockwaves through the earth, causing damage.
✓ These earthquakes are mild.
✓ Their range is also limited.

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 ✓ However, there have been certain exceptions, with volcanic earthquakes bring
havoc and death to thousands of people.

2. Tectonic Movements
✓ The surface of the earth consists of some plates, comprising of the upper mantle.
✓ These plates are always moving, thus affecting the earth’s crust.
✓ These movements categorized into three types:
a. constructive,
b. destructive, and
c. conservative.
i. Constructive is when two plates move away from each other,
they correspond to mild earthquakes.
ii. When two plates move towards each other and collide, this is
known as destructive plate boundaries. This is very destructive.
iii. Conservative corresponds to passing by of plates of crust.
Earthquakes of this type have varying intensities.

3. Geological Faults
✓ A geological fault is known as the displacement of plates of their original plane.

✓ The plane can be horizontal or vertical.

✓ These planes are not formed suddenly but slowly develop over a long period.

✓ The movement of rocks along these planes brings about tectonic earthquakes.

✓ These faults occur due to the impact of geological forces.

✓ The displacement of plates creates the fracturing of rocks, which releases a lot of
energy.

✓ This type of earthquake can be disastrous.

4. Man-Made
✓ The interference of man with nature can also become a cause of the earthquake.
✓ The disturbance of crustal balance due to heavy clubbing of water in dams can
cause earthquakes.
✓ Nuclear bombing can send specific types of shockwaves throughout the surface
of the earth, which can disturb the natural alignment of tectonic plates.
✓ Mining can also cause disturbance due to the extensive removal of rocks from
different areas.

5. Minor Causes
✓ Some minor causes such as landslides, avalanches, the collapse of heavy rocks,
etc. can also cause minor shockwaves.

✓ The gases beneath the surface of earth contract and expand, giving rise to
movements in plates beneath the crust.

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 ✓ The plutonic earthquake occurs because of adjustments in rock beds in the
interior of the earth’s crust.

✓ All these factors correspond to minor earthquakes, but sometimes these can also
vary to moderate earthquakes.

2.3 SEISMIC WAVES

✓ an elastic wave produced in earth causing earthquake

✓ are mechanical waves travelling through rock layers, water etc.
✓ Starts at inner point, FOCUS, travels outward to the outer surface → touches the
surface at EPICENTER
✓ the elastic waves are formed because of the rupturing that takes place deep
underground and continues to grow at a very fast pace.
✓ The speed of this growth depends on their nature and the properties of the earth.
✓ as we go deeper the seismic waves found there are of higher density, pressure,
and velocity.

2.3.1 Types of seismic waves

1. BODY WAVES: Under the earth surface

✓ Travels through the earth
a. P wave
b. S wave
2. SURFACE WAVE: On earth surface
✓ Destruct Huge buildings
a. Rayleigh Wave
b. Love wave

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BODY WAVE:
They are the fastest waves and as a result, the first waves that seismographs
can record.
Body waves can move through all states of matter including rocks and
molten lava

1. P-wave (primary wave/ pressure wave)

✓ P waves are the fastest seismic waves of all and are thus called
Primary ones.
✓ P waves grow or travel at a speed of 5 kilometres per sec through
the earth’s crust.
✓ Non destructive
✓ travel more quickly through the Earth's crust than the destructive
secondary and Rayleigh waves.
✓ P waves are the first ones to reach any particular location or point
when an earthquake occurs.
✓ Advance earthquake warning is possible by detecting the non-
destructive primary waves
✓ The waves have a tendency to flow through all three i.e., solids,
liquids, and gases.
✓ The materials that they flow through experience a force or energy
that slightly pulls them apart and pushes them together.
✓ The same energy is experienced by a building when an
earthquake occurs.

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 Animated figure: refer wordfile for seeing the animation

2. S-wave (a.k.a Secondary wave/ Shear wave)

✓ S waves are the second-fastest seismic waves and are thus called
Secondary.
✓ S waves are transverse waves (the direction of particle motion of a S
wave is perpendicular to the direction of wave propagation)
✓ the main restoring force comes from shear stress.
✓ Therefore, S waves cannot propagate in liquids with zero (or very
low) viscosity;
✓ however, they may propagate in liquids with high viscosity
✓ The speed at which the S waves travel is almost half the speed of
Primary Waves.
✓ S waves are the ones to reach any location after the primary waves
when an earthquake occurs.
✓ Unlike Primary Waves, Secondary Waves make the material go
through an up and down shaking movement from the sides when it
flows through them.
✓ Unlike P waves, S waves can travel through rocks only-SOLID Only
✓ S waves cannot travel through the molten outer core of the Earth, and
this causes a shadow zone for S waves opposite to their origin.
✓ They can still propagate through the solid inner core:
✓ when a P wave strikes the boundary of molten and solid cores at an
oblique angle, S waves will form and propagate in the solid medium.
When these S waves hit the boundary again at an oblique angle, they
will in turn create P waves that propagate through the liquid
medium.
✓ This property allows seismologists to determine some physical
properties of the Earth's inner core.

✓
Animated figure: Refer word file for seeing the animation

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SURFACE WAVE:

✓ Travels parallel to the surface

✓ Surface waves travel more slowly through Earth material at the
planet’s surface
✓ predominantly lower frequency than body waves
✓ Dispersive in nature
✓ Largest in the surface → decreases with depth: Shallow
earthquakes produce stronger surface waves; the strength of the
surface waves are reduced in deeper earthquakes.

3. Rayleigh Wave

Animated figure: refer the wordfile

✓ Both longitudinal and transverse motion
✓ Although Rayleigh waves appear to roll like waves on an ocean,
the particle motion is opposite of ocean waves.

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 ✓ Because it rolls, it moves the ground up and down, and forward
and backward in the 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.
✓ Like Love waves, the amplitude of the wave decreases
dramatically with depth
✓ Produced by localised impact, piezo-electric transduction
✓ Travels near solid surfaces
✓ In isotropic medium, movement of particle is elliptical with the
major axis as vertical and parallel to the direction of propagation

✓
✓ Speed: in metal=2-5 km/s
✓ In soil: for shallow waves(<100m) = 50-300m/s,

Depth > 1km= 1.5-4 km/s

4. Love wave (Q-wave)

✓ Produces entirely horizontal motion
✓ Amplitude is huge at the surface → decreases with depth

Animation: refer word file

✓ is a result of the interference of many shear waves (S-waves)
o guided by an elastic layer, which is welded to an elastic
half space on one side while bordering a vacuum on the
other side.
✓ are surface seismic waves that cause horizontal shifting of the
Earth during an earthquake.
✓ Love waves travel with a lower velocity than P- or S- waves,
✓ but faster than Rayleigh waves.

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 ✓ These waves are observed only when there is a low velocity layer
overlying a high velocity layer/ sub–layers.
✓ The particle motion of a Love wave forms a horizontal line
perpendicular to the direction of propagation (i.e., are transverse
waves).
✓ The amplitude, or maximum particle motion, often decreases
rapidly with depth.
✓ Large earthquakes may generate Love waves that travel around
the Earth several times before dissipating
✓ They are what most people feel directly during an earthquake.

✓ In the past, it was often thought that animals like cats and dogs
could predict an earthquake before it happened. However, they
are simply more sensitive to ground vibrations than humans and
able to detect the subtler body waves that precede Love waves,
like the P-waves and the S-waves

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 2.4 RATING OF EARTHQUAKE

2.4.1 MEASUREMENT of EARTHQUAKE

Seismograph

✓ A seismograph, or seismometer is known as an instrument which is used to record

and detect earthquakes.
✓ Generally, we can say that it consists of a mass which is attached to a fixed base.
✓ During an earthquake, the base moves and the mass does not.
✓ The motion of the base with respect to the mass is commonly said to be
transformed into an electrical voltage.
✓ The electrical voltage is recorded on paper, and magnetic tape, or another
recording medium.
✓ This record is said to be proportional to the motion of the seismometer mass
relative to the earth.
✓ But it can be mathematically also converted to a record of the absolute of the
motion of the ground.
✓ The term or device seismograph generally refers to the seismometer and its
recording device as a single unit.

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2.4.2 RATING/ RANKING of EARTHQUAKE

We use different scales like Richter Scale, Mercalli scale, Earthquake magnitude scale
etc. to rate/ rank earthquake

i. Richter scale
✓ The Richter magnitude scale is the most common standard of measurement for
earthquakes.
✓ It was invented in 1935 by Charles F. Richter of the California Institute of
Technology as a mathematical device to compare the size of earthquakes.
✓ The Richter scale is used to rate the magnitude of an earthquake, that is the amount
of energy released during an earthquake.
Magnitude of earthquake = Amount of energy released during the earthquake
✓ Richter scale does not measure the magnitude of damage

✓ Principle/ or/ Measurement/ or working:

o The Richter magnitude involves measuring the amplitude (height) of the
largest recorded wave at a specific distance from the seismic source.
o Adjustments are included for the variation in the distance between the
various seismographs and the epicentre of the earthquakes.

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 o The Richter scale is a base-10 logarithmic scale, (meaning that each order
of magnitude is 10 times more intensive than the last one).
o In other words, a two is 10 times more intense than a one and a three is
100 times greater.
o In the case of the Richter scale, the increase is in wave amplitude.
o That is, the wave amplitude in a level 6 earthquake is 10 times greater than
in a level 5 earthquake
o The amount of energy released increases 31.7 times between whole
number values.

✓ A better measure of the size of an earthquake is the amount of energy released by

the earthquake, which is related to the Richter Scale by the following equation:
Log E = 11.8 + 1.5 M
(Where Log =log10
E = energy released in ergs
and M= the Richter magnitude).

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ii) Earthquake Magnitude Scale

✓ Magnitude scales can be used to describe earthquakes so small that they are
expressed in negative numbers.
✓ The scale also has no upper limit

iii) Mercalli Scale

✓ Richter ratings only give you a rough idea of the actual impact of an earthquake.
but, an earthquake's destructive power varies depending on the composition of
the ground in an area and the design and placement of man-made structures.

✓ The extent of damage is rated on the Mercalli Intensity Scale.

✓ Mercalli ratings, which are given as Roman numerals, are based on largely
subjective interpretations.
✓ A low intensity earthquake, one in which only some people feel the vibration and
there is no significant property damage, is rated as a II.
✓ The highest rating, a XII, is applied to earthquakes in which structures are
destroyed, the ground is cracked and other natural disasters, such as landslides or
tsunamis, are initiated.

✓ Richter scale ratings are determined soon after an earthquake, once scientists can
compare the data from different seismograph stations.

✓ Mercalli ratings, on the other hand, can't be determined until investigators have
had time to talk to many eyewitnesses to find out what occurred during the
earthquake. Once they have a good idea of the range of damage, they use the
Mercalli criteria to decide on an appropriate rating.

✓ The Mercalli Intensity Scale measures the intensity of an earthquake by observing

its effect on people, the environment and the earth’s surface.

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 ✓ Mercalli scale is linear and the Richter scale is logarithmic Scale.

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 2.5. TYPES OF EARTHQUAKES

1. Tectonic Earthquake
✓ Due to the movement of tectonic plates
✓ And due to the movement of rock along the faults
✓ The resulting energy released from this fracture is a combination of
cracking rock, frictional heating, and radiated seismic waves caused by the
elastic strain.
✓ Larger than Volcanic earthquake

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 2. Volcanic earthquake
✓ Due to the seismic waves produced by Volcanic eruption/ molten magma
✓ The place where tectonic plates meet give access to the molten mantle.
✓ As a result, these fissures are a major source of volcanic activity.
✓ Volcanoes are openings in the Earth’s crust where pressure build ups are
released along with molten materials, preventing the planet from
overheating and exploding.

✓ They’re often seen at the edge of a tectonic plate.

✓ Volcanic earthquakes are a form of tectonic earthquake where volcanic

activity coincides with tectonic forces.

✓ There are actually two forms of volcanic earthquake:

o 1. volcano-tectonic earthquake, which occurs beneath a volcano.

▪ magma begins pushing upwards towards the lava tubes and
cone of the volcano.
▪ As it does, chunks of rock are broken off and sink to the
bottom to fill the void left by the rising magma.
▪ These tremors are so faint they’re only observable with
special seismic equipment.
o 2. long-period volcanic earthquakes
▪ caused when the magma forces its way into the surrounding
rock.
▪ This results in a pressure change that’s easily measured and
has a significant effect on the ecosystem.
▪ Long-period earthquakes signal a pending eruption, as
(unlike volcanic-tectonic quakes) the magma has reached a
level where it begins actively trying to vent out.
▪ Detecting a long-period earthquake allows local disaster
relief to enact preventative measures that can save
numerous lives when the eruption comes.

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 3. Explosive earthquake
✓ Purely manmade
✓ They’re created from the detonation of nuclear or chemical weapons.
✓ First observed during nuclear weapons tests, the amount of energy
released upon detonation causes massive seismic upheaval.
✓ In the case of the two atomic bombs dropped on Hiroshima and Nagasaki,
the tremors were felt or detected all around the world.

4. Collapse Earthquake
✓ These may be man-made or natural
✓ tend to occur in underground areas such as caves and mines.
✓ Breaking of rock from an explosion or settling can cause a seismic
disruption that results in cave-ins.

✓ Collapse earthquakes tend to be very low magnitude,

✓ but may still result in significant property damage when the collapsed
cavities are underneath structures

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 2.6 SEISMIC ZONES OF INDIA

✓ India had number of devastating earthquakes in the history

✓ Major reason for this is that Indian plate is driving into Eurasian plate at a rate of
47 mm/ year

✓ 54% of Indian land is vulnerable to earthquakes

✓ A seismic zone map shows various zones of different seismic danger
o Based on:
▪ the seismic evidence and historical records
▪ scientific inputs related to the Seismicity or the Frequency of
Earthquakes in a Region
▪ Earthquakes That Have Hit the Country in the Past
o It is reviewed and revised periodically based on additional data
✓ It is expected that the level of risk involved and collapse of the structure will be
minimized, by the use of seismic zoning and zone factor

Seismic zone factor (Z factor)

✓ Representation of maximum effective peak acceleration (EPA)
✓ Z factor= EPA/ g

✓ Zone factors are given on the basis of expected intensity of the earthquake in
different zones.
✓ In IS Code, it is given based on the Maximum Considered Earthquake
(MCE) and service life of the structure in a zone
✓ It is a rating of the potential intensity of earthquakes in a seismic zone
✓ It is representing the peak ground acceleration in studied site as a percentage
of gravity acceleration g (9.81 m/sec2 ).

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 ✓

Seismic zones in India

✓ According to IS 1893(part1)-2002 there is 4 levels of seismicity in India

o Zone II (lowest)
o Zone III
o Zone IV
o Zone V (highest)

1. Zone II:

o Classified as low damage risk zone

o the least seismically active zone (areas that fall under these zones in
India have a low chance of having an earthquake)
o Zone II covers 41% of India
o Liable to MSK VI or less earthquake intensity
▪ MSK= Medvedev- Sponhueuer- Karnic scale (it is a scale like
Richter, Mercalli etc.)
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 o Zone factor= 0.10 (maximum horizontal acceleration during maximum
consideration earthquake(MCE) =10% of g, g= acceleration due to
gravity)
o U.P, Bihar, West Bengal

2.Zone III:

o Classified as moderate-damage risk zone

o Earthquake of intensity MSK VII and magnitude 7.8
o Zone factor 0.16
o covers 30% of India.
o Andaman & Nicobar Islands, some parts of Kashmir, western Himalayas

3.Zone IV:

o High damage risk zone

o Earthquake intensity levels of MSK VIII
o Zone factor =0.24
o , 18% of the total area of the country belongs to Zone IV.
o Delhi, Indo-Gangetic basin, some parts of Jammu-Kashmir, Maharashtra,
Patan

4.Zone V:

o Very high damage risk zone

o Highest level of risk
o Earthquake intensity = MSK IX or greater
o Zone factor= 0.36
o Around 11% of India falls under Zone V
o State of Kashmir, western and central Himalayas, North-east Indian
region, Rann of Kutch

There are no cities in India which fall under Seismic Zone I

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 2.7 BASICS OF SEISMIC SAFETY FACTORS

✓ Seismic safety factor= the ratio between the resisting forces or moments in a
location and the driving forces or moments that may cause a massive failure/
damage during an earthquake or other seismic event such as an explosion

Driving forces and acceleration:

Earthquake forces are called lateral forces because their predominant effect is
to apply horizontal loads toa building.

o Although earthquake waves do impart a vertical component of force to

buildings, the weight of the building normally provides
sufficient resistance.
o Therefore, vertical earth-quake forces are usually only accounted for in
special cases.
o total lateral earthquake force to be applied to a building, F = ma.

o force (F)
o mass (m)
o horizontal acceleration (a),

o The acceleration is expressed as a fraction of the acceleration due to gravity,

commonly called “g.”

Resisting forces:

o inertia forces.
o When any object, such as a building, experiences acceleration, inertia force
is generated when its mass resists the acceleration
o structure should have sufficient strength against vertical loads together with
adequate stiffness (the extent to which an object resists deformation in
response to an applied force.) to resist lateral loads.

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 ✓ Formulation of seismic safety factor:

Basics of safety factor: includes the factors affecting:

a) resisting forces
b) driving forces

a) Seismic Design Factors /or/ Factors affecting resisting forces

The following factors affect and are affected by the design of the building. It is
important that the design team understands these factors and deal with them prudently
in the design phase.

1. Torsion: Uneven mass distribution will position the center of mass outside of
the geometric center causing "torsion" generating stress concentrations.
o A certain amount of torsion is unavoidable in every building
design. Symmetrical arrangement of masses, however, will result
in balanced stiffness against either direction and keep torsion
within a manageable range.
2. Damping: Buildings in general are poor resonators to dynamic shock and
dissipate vibration by absorbing it.
o Damping is a rate at which natural vibration is absorbed.

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 3. Ductility: Ductility is the characteristic of a material (such as steel) to bend, flex,
or move, but fails only after considerable deformation has occurred.
o Non-ductile materials (such as poorly reinforced concrete) fail
abruptly by crumbling.
o Good ductility can be achieved with carefully detailed joints.
4. Strength and Stiffness: Strength is a property of a material to resist and bear
applied forces within a safe limit.
o Stiffness of a material is a degree of resistance to deflection or drift (drift
being a horizontal story-to-story relative displacement).
5. Building Configuration: This term defines a building's size and shape, and
structural and non-structural elements.
o Building configuration determines the way seismic forces are distributed
within the structure, their relative magnitude, and problematic design
concerns.

b) factors affecting driving forces:

1. The magnitude of the earthquake:

o it is the measure of size of earthquake /or/ measure of energy released

during earthquake (measured in the term of amplitude)
o in general, the larger the quake, the stronger the shaking and the larger
the area affected.
o
2. The distance from the earthquake:

o the closer to the source of the earthquake, the greater the shaking
3. The type of ground material beneath the structure:
o soils may amplify or de-amplify the shaking relative to hard bedrock.
o A given Richter reading will produce vastly different amounts of damage
in different parts of the world. Even the same quake can have very
different effects in neighbouring areas.

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 2.8. INTERIOR OF THE EARTH AS REVEALED BY PROPAGATION OF
SEISMIC WAVES.

✓ Seismologists study shock waves, as they travel through earth’s interior

✓ Knowing how the waves behave as they move through different materials
enables us to learn about the layers that makes up the earth
✓ Seismic waves tell:
o Earth’s interior consists of a series of concentric shells
▪ With a thin outer crust
▪ A mantle
▪ A liquid outer core
▪ And a solid inner core

✓ When an earthquake occurs the seismic body waves (P and S waves) spread out
in all directions through the Earth's interior.
✓ Seismic stations located at increasing distances from the earthquake epicenter
will record seismic waves that have travelled through increasing depths in the
Earth.
✓ Seismic velocities depend on the material properties such as composition,
mineral phase and packing structure, temperature, and pressure of the media
through which seismic waves pass.
✓ Seismic waves travel more quickly through denser materials and therefore
generally travel more quickly with depth.
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 ✓ Anomalously hot areas slow down seismic waves.
✓ Seismic waves move more slowly through a liquid than a solid.
✓ Molten areas within the Earth slow down P waves and stop S waves because
their shearing motion cannot be transmitted through a liquid.
✓ Partially molten areas may slow down the P waves and attenuate or weaken S
waves.
✓ When seismic waves pass between geologic layers with contrasting seismic
velocities (when any wave passes through media with distinctly differing
velocities) reflections, refraction (bending), and the production of new wave
phases (e.g., an S wave produced from a P wave) often result.
✓ Sudden jumps in seismic velocities across a boundary are known as seismic
discontinuities.
✓ This seismic discontinuity is now known as the Moho (much easier than
"Mohorovicic seismic discontinuity")
✓ MOHO:
o It is the boundary between the felsic/mafic crust with seismic velocity
around 6 km/sec and the denser ultramafic mantle with seismic velocity
around 8 km/sec. The depth to the Moho beneath the continents averages
around 35 km but ranges from around 20 km to 70 km. The Moho
beneath the oceans is usually about 7 km below the seafloor (i.e., ocean
crust is about 7 km thick).

All this knowledge about how seismic waves behave regard to different
materials helps to interpret the interior structure of earth

INTERIOR OF THE EARTH AS REVEALED BY PROPAGATION OF

SEISMIC WAVES:

1. Crust:
✓ The Earth's crust ranges from 5–70 kilometres
✓ in depth and is the outermost layer. The thin parts are the oceanic crust,
which underlie the ocean basins (5–10 km) and are composed of dense
(mafic) iron magnesium silicate igneous rocks, like basalt.
✓ The thicker crust is continental crust, which is less dense and composed
of (felsic) sodium potassium aluminium silicate rocks, like granite.

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 ✓ Continental Crust

o Depth to Moho: 20 to 70 km, average 30 to 40 km

o Composition: felsic, intermediate, and mafic igneous,
sedimentary, and metamorphic rocks
o Age: 0 to 4 billion years

o Summary: thicker, less dense, heterogeneous, old

✓ Oceanic Crust

o Depth to Moho: ~7 km
o Composition: mafic igneous rock (basalt & gabbro) with thin
layer of sediments on top
o Age: 0 to 200 million years

o Summary: thin, more dense, homogeneous, young

2. Mantle:
✓ Earth's mantle extends to a depth of 2900 km, making it the planet's
thickest layer.
✓ The mantle is divided into upper and lower mantle separated by a
transition zone
✓ The upper mantle’s temperatures range from 500 to 900 degrees
Celsius
✓ The lower mantle temperature, in contrast, reaches over 4,000
degrees Celsius.
✓ The viscosity of the upper mantle is greater than the viscosity of the
lower mantle.

✓ Low Velocity Zone

o Seismic velocities tend to gradually increase with depth in the
mantle due to the increasing pressure, and therefore density,
with depth.
o both the P and S waves travel more slowly,
o the S waves are attenuated or weakened.
o This is interpreted to be a zone that is partially molten, and
greater than 99 percent solid
o Partially molten area may simply represent a zone where the
mantle is very close to its melting point for that depth and
pressure that it is very "soft."
o this represents a zone of weakness in the upper mantle. This
zone is called the asthenosphere or "weak sphere."
o The asthenosphere separates the strong, solid rock of the
uppermost mantle and crust above from the remainder of the
strong, solid mantle below.

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 ✓ The mantle is composed of silicate rocks richer in iron and
magnesium than the overlying crust
✓ The combination of uppermost mantle and crust above the
asthenosphere is called the lithosphere.
✓ The lithosphere is free to move (glide) over the weak asthenosphere.
✓ The tectonic plates are the lithospheric plates.

✓ 670 km Seismic Discontinuity

Below the low velocity zone are a couple of seismic

discontinuities at which seismic velocities increase.

The 670 km discontinuity is thought to represents a major

boundary separating a less dense upper mantle from a denser lower
mantle.

3.Core:

✓ seismic velocities gradually increase with depth in the mantle

✓ at arc distances of between about 103° and 143° no P waves are recorded.
✓ no S waves are record beyond about 103°.
✓ Gutenberg (1914) explained this as the result of a molten core beginning at a
depth of around 2900 km. Shear waves could not penetrate this molten layer
and P waves would be severely slowed and refracted (bent). → known as
Gutenberg discontinuity
✓ Lehman Seismic Discontinuity / The Inner Core
o Between 143° and 180° from an earthquake another refraction is
recognized (Lehman, 1936) resulting from a sudden increase in P
wave velocities at a depth of 5150 km.
o This velocity increase is consistent with a change from a molten outer
core to a solid inner core.
✓

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 ✓ The Core is Made of:

o it must be denser than the mantle

o It must account for the observed seismic velocities.
o It should also be a material with magnetic properties to account for the
Earth's magnetic field.
o outer core is a fluid layer about 2,400 km thick and composed of mostly
iron and nickel

o that lies above Earth's solid inner core and below its mantle

o Its outer boundary lies 2900 km beneath Earth's surface.

o The transition between the inner core and outer core is located
approximately 5,150 km beneath the Earth's surface

o Earth's inner core is the innermost geologic layer of the planet Earth.

o It is primarily a solid ball with a radius of about 1,220 km which is

about 20% of Earth's radius

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 2.9 Miscellaneous

1. Fore shocks and after shocks

✓ Th earth’s crust is being slowly displaced at the margins of
plates in the upper mantle
✓ The differential displacements give rise to the elastic strains
✓ These elastic strains eventually exceed the strength of rock
✓ Hence fault is developed
✓ Shearing stress is gradually build-up
✓ Initially movement may occur over a small area of the fault
plane → causes foreshocks
✓ Followed by a movement over much larger surface
→earthquake/ main shock
✓ The displacement of rock masses involved in this relieves the
stress →but sets new set of stresses in adjacent areas→ this
leads to the re adjustment of fault plane→causes aftershocks

2. Elastic rebound theory

✓ The tectonic earthquakes can be explained by elastic rebound

theory
✓ Tectonic earthquake occurs when there is sufficiently stored
elastic strain energy to drive fracture propagation (widening/
spreading of fracture) along a fault plane.
✓ Faults generally have irregular surfaces → this causes stick-
slip behaviour (spontaneous jerking) during the relative
movement of rocks along the faults
✓ Once the faults have locked, continued relative motion
develops stress
✓ Due to this stress→ strain energy is stored in volumes around
the fault plane→ continues until the stress has rises
sufficiently enough break through the rough surface →
sudden sliding occurs → release stored energy
✓ This energy is released as a combination of
o radiated elastic strain seismic waves,
o frictional heating of the fault surface,
o and cracking of the rock
✓ this process of gradual build-up of strain and stress
punctuated by occasional sudden earthquake is referred as
elastic rebound theory

3. measurement of earthquake
two popular ways to assess the severity of earthquake are
a) magnitude
✓ measure of the energy released during an
earthquake

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 ✓ earthquakes are rated on the basis of amplitude of
seismic waves recorded on a seismograph
✓ does not vary place to place
✓ magnitude scales are logarithmic scales →
because amplitude ranges can be extreme
✓ E.g. Richter scale

b) Intensity
✓ Indicates how much ground shaking occurred at a
particular location
✓ Isoseismal- Lines joining locations of equal
earthquake intensity
✓ Seismic waves damps over travelling distances
✓ Hence the intensity varies with the location where
the observer and the observer’s sensibility
✓ E.g. Mercalli scale

4. Accelerograph
✓ Also known as accelerometer
✓ Consist of a:
o Self-contained box
o 3 accelerometer sensing heads (usually micro
machined chips) that are sensitive to the
measurements in one direction→ hence measure full
motion in 3 directions
✓ Most useful in case of strong ground movements
✓ Accelerometers are used to monitor structures for earthquake
response

5. Earthquake forecasting
✓ Probabilistic assessment of general earthquake hazard
✓ Includes the assessment of frequency and magnitude of
damaging earthquakes in a given area over a period of years/
decades

6. Prediction of earthquake:
a. From animal behaviour
✓ Animals are more sensitive to the P-waves (first arriving
waves)
✓ The mice, snakes, beetles leave their region
✓

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 b. From changes in Vp /or/ Vs
✓ Velocity of P and S waves changes when a rock is near the
point of fracture

c. From radon emissions

✓ Rocks contains small amount of gases that are isotopically
distinguished from the atmospheric gases
✓ Their concentration is reported to be spiking prior to major
earthquake

d. From electromagnetic variations

e. Change in water level

✓ Ground water first decrease then fluctuates

f. Look for the trends that lead to earthquake

7. Location of epicenter
✓ It can be located as the intersection of 3 circles in a point:
a. In any recording station, P waves arrives first and then s waves
b. From the seismograph, exact time of arrival can be obtained
c. The time lag between the arrival of P wave and S waves is directly
proportional to the distance of the station to the focus
𝑡
d. If the velocities are known, the distance, D = 1 1
( − )
𝑉𝑠 𝑉𝑝
e. Amplitude of the strongest wave is measured
f. The distance of the point of origin of an earthquake is measured from
atleast 3 stations
g. Circles with radius=calculated distance to the focus
h. All of the circles should overlap and intersect over appoint=approximate
epicenter

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