The document provides an overview of earthquakes and seismology. It discusses key topics such as: - Seismology is the study of earthquakes and seismic waves. Earthquakes are caused by the sudden movement of tectonic plates. - The movement of tectonic plates is driven by convection currents in the earth's mantle. As plates move against each other, strain builds up at plate boundaries and is released through earthquakes. - India experiences earthquakes due to its location in a seismically active zone where the Indian plate is moving northward into the Eurasian plate. The country is divided into several seismic zones based on expected earthquake intensities.

1. Introduction to
Earthquakes

2. Seismology
• Seismology is the study of the generation, propagation and measurement
of seismic waves through earth and the sources that generate them.The
word seismic is commonly used to qualify anything related to an
earthquake, such as seismic waves, seismic intensity, seismic zoning,
seismic region and so on.

3. Causes of
Earthquakes
Natural
• Tectonic
• Volcanic
• Landslides
• Microseism
Man-Made
• Controlled explosions
• Reservoir failure
• Mining accidents
• Cultural, Industrial,Traffic

4. Earth
• The earth’s shape is an oblate
spheroid with a diameter along
the equator of about 12740 km
with the polar diameter as
12700km.
• The interior of the earth can be
classified into three major
categories as Crust, Mantle and
Core

5. Continental DriftTheory
• German scientist Alfred
Wegener, in 1915, proposed
the hypothesis that the
continents had once formed a
single landmass before
breaking apart and drifting to
their present locations.
• He proposed that a large
continent termed Pangea
existed in earth around 200
million years ago and was
surrounded by an ocean called
Panthalassa.

6. • This theory requires a source that can generate
tremendous force is acting on the plates.The
widely accepted explanation is based on the force
offered by convection currents created by thermo-
mechanical behavior of the earth’s subsurface.
• The variation of mantle density with temperature
produces an unstable equilibrium.The colder and
denser upper layer sinks under the action of gravity
to the warmer bottom layer which is less dense.The
lesser dense material rises upwards and the colder
material as it sinks gets heated up and becomes
less dense.These convection currents create shear
stresses at the bottom of the plates which drags
them along the surface of earth.

7. Theory of PlateTectonics
• Presented in early 1960’s, explains that the lithosphere is broken into seven large (and several
smaller) segments
• The upper most part of the earth is considered to be divided into two layers with different
deformation properties.The upper rigid layer, called the lithosphere, is about 100 km thick
below the continents, and about 50 km under the oceans, and consists of Crust and rigid upper-
mantle rocks.
• The lower layer, called the asthenosphere, extends down to about 700 km depth.
• The rigid lithospheric shell is broken into several irregularly shaped major plates and a large
number of minor or secondary plates.
• The lithospheric plates are not stationary, on the contrary, they float in a complex pattern, with a
velocity of some 2-10 cm/year on the soft rocks of the underlying asthenosphere like rafts on a
lake.

8. PlateTectonics

9. Epicentres of 30000 earthquakes recorded during
1961-1967

10. Movement of Plate Boundaries
• Divergent/Spreading Ridges - Spreading
ridges or divergent boundaries are areas
along the edges of plates move apart
from each other.
• Convergent boundaries - The convergent
boundaries are formed where the two
plates move toward each other. In this
process, one plate could slip below the
other one or both could collide with each
other.
• Transform boundaries - Transform
boundaries occur along the plate margins
where two plate moves past each other
without destroying or creating new crust

11. Convergence Boundaries
• Oceanic-continental convergence - When
oceanic lithosphere subducts beneath
continental lithosphere
• Ocean-ocean convergence - These boundaries
are created when either oceanic lithosphere
subducts beneath oceanic lithosphere.
• Continental-continental convergence - When
two plates with continental lithosphere
collide, subduction ceases and a mountain
range is formed by squeezing together and
uplifting the continental crust on both plates,

12. The movement of Indian Plate
• After splitting with ‘Pangea’
• Indian plate started moving towards Eurasian plate at a
speed of 9 m a century.
• Both continents had same rock density so one plate could not
be subducted under the other.
• Immense Himalayan mountain range started forming 40 to
50 million years ago.

13. Fault
• A fault is a fracture within some particular rocky mass within the earth's crust.The
depth and length of faults vary greatly. Faults may range in length from few meters
to many kilometers and are drawn on a geological map as continuous or broken
lines.
• Earthquakes are caused by active faults, that is, faults along which the two sides of
the fracture move with respect to each other. So, an earthquake is caused by the
sudden movement of the two sides of a fault with respect to another .

14. Types of Faults

15. EarthquakeTerminology
• Earthquake is the vibration of earth’s
surface caused by waves coming from a
source of disturbance inside the earth.
Most earthquakes of engineering
significance are of tectonic origin and is
caused by slip along geological faults.
• The point on the fault where rupture
initiates is referred to as the focus or
hypocenter of an earthquake.
• The hypocenter of an earthquake is
described by its depth in kilometers, its
map location in latitude and longitude,
its date and time of its occurrence, and
its magnitude.
• The term epicenter is the point on the
earth’s surface directly above the
hypocenter

16. Elastic Rebound theory
• As the plate try to move relative to each
other, strain energy gets built up along the
boundaries.When the stress buildup
reaches the ultimate strength of rock, rock
fractures and releases the accumulated
strain energy
• The nature of failure dictates the effect of
the fracture. If the material is very ductile
and weak, hardly any strain energy could
be stored in the plates due to their
movement.
• But if the material is strong and brittle, the
stress built up and subsequent sudden
rupture releases the energy stored in the
form of stress waves and heat.The
propagation of these elastic stress waves
causes the vibratory motion associated
with earthquakes.

17. SeismicWaves
• Earthquake vibrations originate from the point of initiation of rupture and
propagates in all directions.These vibrations travel through the rocks in the form of
elastic waves.
Seismic
Waves
BodyWaves
Primary (P)
waves
Secondary (S)
waves
SH SV
Surface waves
(LWaves)
Love waves
(LQ)
Rayleigh
waves (LR)

18. Primary (P) waves
• These waves propagate by longitudinal or compressive action, which mean that
the ground is alternately compressed and dilated in the direction of propagation
• Particle motion for P-wave is in the direction of wave propagation.
• P waves are the fastest among the seismic waves and travel as fast as 8 to 13 km
per second

19. Secondary (S) waves
• These are also called shear waves, secondary waves, transverse waves, etc. Compared to P
waves, these are relatively slow.These are transverse or shear waves, which mean that the
ground is displaced perpendicularly to the direction of propagation.
• In nature, these are like light waves, i.e., the waves move perpendicular to the direction of
propagation. Hence, transverse particle motion is characteristic of these waves.

20. RayleighWaves
• Lord Rayleigh(1885) described the propagation of wave as retrograde ellipse
in the vertical plane with its major axis vertical and minor axis in the
direction of wave propagation.
• The particle motion can regarded as the combination of P and SV-waves.

22. LoveWaves
• A.E.H. Love (1911)
• When the angle of reflection at the base of the soil layer is more than the critical angle, SH
waves are trapped in the soil layer.
• The particle motion is in the horizontal plane and transverse to the direction of wave
propagation.

24. Recording Earthquakes
• The vibratory motion produced during an earthquake could be measured in
terms of displacement, velocity or acceleration. A seismologist is interested
in even small amplitude ground motions (in terms of displacement) that
provides insight into the wave propagation characteristics and enables him
to estimate the associated earthquake parameters.

25. Intensity of Earthquake
• The intensity of an earthquake refers to the degree of destruction caused by it. 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

27. 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.
• Richter scale
𝑀 = 𝑙𝑜𝑔10 𝐴
Where, A = A denotes the
amplitude in micron (10-6m) recorded
by theWood-Anderson (WA)
seismograph instrument located at an
epicentral distance of 100 km

29. Relationship between Magnitude and intensity of
Earthquake
• Comparisons between magnitude and intensity are fraught with difficulty.
Firstly, intensity varies with distance from the epicentre.
• 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 favoured 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.

30. Seismic Zoning of
India
• Under the initiative of the Ministry
of Urban Development, a
Vulnerability Atlas of India was
prepared in which the earthquake,
cyclone and flood hazard maps for
every state and UnionTerritory of
India
• The seismic zoning map was
periodically updated and the latest
(2007) map is shown in figure.

31. Seismic Zones of India
Zone 5 Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity MSK IX or
greater.The IS code assigns zone factor of 0.36 for Zone 5. Structural designers use this factor for
earthquake resistant design of structures in Zone 5.The zone factor of 0.36 is indicative of
effective (zero period) level earthquake in this zone. It is referred to as theVery High Damage Risk
Zone.The region of Kashmir, the Western and Central Himalayas, North and Middle Bihar,
the North-East Indian region, the Rann of Kutch and the Andaman and Nicobar group of
islands fall in this zone.
Zone 4 This zone is called the High Damage Risk Zone and covers areas liable to MSKVIII.The IS code
assigns zone factor of 0.24 for Zone 4 Jammu and Kashmir, Himachal Pradesh, Uttarakhand,
Sikkim, the parts of Indo-Gangetic plains (North Punjab, Chandigarh, Western Uttar
Pradesh, Terai, North Bengal, Sundarbans) and the capital of the country Delhi fall in Zone 4.
In Maharashtra, the Patan area (Koynanagar) is also in zone no-4. In Bihar the northern part of the
state like Raxaul, Near the border of India and Nepal, is also in zone no-4.
Zone 3 This zone is classified as Moderate Damage Risk Zone which is liable to MSKVII. and also 7.8The
IS code assigns zone factor of 0.16 for Zone 3. Mumbai comes in Zone 3
Zone 2 This region is liable to MSKVI or less and is classified as the Low Damage Risk Zone.The IS code
assigns zone factor of 0.10 (maximum horizontal acceleration that can be experienced by a
structure) for Zone 2.
Zone 1 Since the current division of India into earthquake hazard zones does not use Zone 1, no area of
India is classed as Zone 1. Future changes in the classification system may or may not return this
zone to use.