This document discusses earthquakes, including their causes and global distribution. It begins by defining earthquakes and describing the different types of seismic waves generated. There are two main types of body waves (P and S waves) and two main types of surface waves (Love and Rayleigh waves). Earthquakes are primarily caused by tectonic plate movement and faulting, as well as volcanic activity. They most commonly occur along plate boundaries and zones of historical mountain building. India has been divided into different seismic zones based on earthquake risk, with Zone V representing the highest risk.

1. EARTHQUAKES: CAUSES AND
DISTRIBUTION
Presented by :
Marvi Khan
Guided by
Prof. Lal Singh Solanki
Dr. Aarti Dawar
MSc. 1 Semester
Govt. Holkar Science college
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2. Contents
◦ Introduction
◦ Earthquake
◦ Seismic waves
◦ Measurement of seismic waves
◦ Magnitude of earthquake
◦ Types of earthquakes
◦ Causes of earthquakes
◦ Distibution of earthquakes worldwide
◦ Indian areas prone to earthquake
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3. Introduction
◦ Earthquake is a major demonstration of power of tectonic
forces caused by endogenetic thermal conditions of the
Earth.
◦ It is the shaking of surface of the Earth.
◦ High Magnitude earthquakes are harbingers of destruction
and loss of lives.
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4. Earthquake
An earthquake is a motion of ground surface ranging from faint
tremor to a wild motion capable of shaking buildings apart and
causing gaping fissures to open.
It is form of energy of wave motion transmitted through surface
layer of Earth from a point of sudden energy release.
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5. Components of earthquake
◦ Place of origin of earthquake is called focus or hypocenter
◦ Place on ground surface, which is perpendicular to focus, recording the seismic waves for first
time is called epicentre.
◦ Body and surface waves generated by an earthquake, together are called seismic waves.
◦ Science dealing with seismic waves is called seismology.
◦ Seismic waves are recorded by instrument called seismograph or seismometer at the epicentre.
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6. Fig 1 components of earthquake
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7. Seismic waves
There are two types of seismic waves :
◦ Body waves : seismic wave which move through interior of
earth. Higher frequency than surface waves.
◦ Surface waves: travel just below the surface of the ground.
Larger in amplitude, often most destructive type of seismic
wave.
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8. Body waves
Two types of body waves :
◦ P- waves / compressional waves / primary waves
The fastest wave, compresses and ultimately expands material in same direction of
of travelling. Speed ranges from 4-8 km/sec. These waves shakes the ground in the
direction they are propagating. It can move through solid rock and fluids(mantle and
inner core).
◦ S- waves / shear waves / secondary waves
It is slower than P wave and can only move through solid rock. It shakes the ground
perpendicularly or transverse to the direction of propagation.
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9. Fig 2 P wave and S waves
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10. Surface waves
◦ Love waves
It is fastest surface waves which move the ground side
to side . Confined to the surface, they produce purely
horizontal in motion. They are mainly responsible for
destruction.
◦ Rayleigh waves
It moves ground both vertically and horizontally in the
same direction as the waves moves. They are mainly
responsible for shaking of ground.
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11. Fig 3 Love waves and Rayleigh waves
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12. Seismograph
◦ A seismograph detects, amplifies, and records earthquakes as well as ground motion.
◦ A seismometer is a ground motion detector part of the seismograph system.
◦ Seismographs operate on principle of inertia.
◦ Simple seismograph includes spring and weight suspended from frame that moves along with
the earth’s surface. A recording system is installed such as a rotating drum attached to the mass,
this relative motion between weight and the Earth can be recorded to produce a history of
ground motion
◦ Modern seismometers are electronic, the relative motion between weight and frame generates
electrical voltage that is recorded by a computer to produce a digital seismogram.
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13. Seismogram
◦ Record written by a seismograph in respond to ground motions produced by an earthquake,
explosion, or other ground motion sources.
◦ The horizontal axis represents time (in seconds) and vertical axis represents ground
displacement (usually measured in millimeters).
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14. Scales for measuring earthquake
◦ Richter scale : Based on magnitude of earthquake. Magnitude is expressed by amplitude and
distance between seismometer and epicenter. It is a logarithmic scale.
◦ Moment magnitude scale: Introduced by Hanks and Kanamouri in 1979. Moment magnitude
relates to amount of movement by rock along fault or fracture and area of the fault per fracture
surface.
◦ Modified Mercalli Intensity Scale: Developed by Harry wood and Frank Neumann in 1931.
Scale composed of increasing level of intensity that range from shaking to catastrophic
destruction, is designated by Roman numerals. It is not based on mathematical basis, instead is
an arbitrary ranking based on observed effects.
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15. Classification of Earthquakes
◦ Natural earthquakes( caused due to endogenetic forces)
1. Volcanic earthquake : confined to volcanic areas and depends upon intensity and magnitude
of volcanic eruption. Examples include explosions of Etna volcano in 1968 and Krakatoa
volcano in 1883.
2. Tectonic earthquakes: caused due to dislocation of rock blocks during faulting activity.
Examples include 1906 earthquake of California, 2001 earthquake of Gujarat.
3. Isostatic earthquakes: triggered due to sudden disturbance isostatic balance due to
imbalance in geological processes. Earthquakes of active zone of mountain building are
included in this category
4. Plutonic earthquakes: deep focus earthquakes which occur at greater depths. Focus of these
earthquakes are generally located within depths ranging from 240 km to 670 km.
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16. Classification of Earthquakes
◦ Artificial or man- induced earthquakes or anthropogenic earthquakes
These earthquakes are caused by human activities such as pumping of water deep
deep underground mining, blasting of rocks by dynamites for constructional purpose
nuclear explosion, storage of huge volume of water in big reservoirs etc.
Examples include Koyna earthquake of 1967 due to reservoir, 1931 earthquake of
Greece etc.
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17. Causes of Earthquakes
Main causes of earthquakes are the following :
1. Vulcanicity
2. Faulting and elastic rebound theory
3. Hydrostatic pressure and anthropogenic causes
4. Plate tectonic theory
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18. Vulcanicity
◦ Vulcanicity and seismic events are so closely related that they become cause and effect for each
other.
◦ Earth tremors are major precursor events of vulcanicity.
◦ Severe disequilibrium in crustal surface due to magma injection or escaping of gases upwards
invite severe earth tremors.
◦ Examples include the violent eruption of Krakatoa volcano caused such a earthquake the impact
which was experienced as far away as Cape Horn. This earthquake generated high tsunamis
waves along coast of Java and Sumatra.
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19. Faulting
◦ Most of earthquakes are a result of active faulting or reactivation of certain faults.
◦ Sudden dislocation of rock blocks caused by both tensile and compressive forces trigger
immediate earth tremors due to sudden maladjustment of rocks blocks.
◦ Examples include 1950 earthquake of Assam and 1934 earthquake of Bihar.
◦ Investigation of 1906 earthquake of San Francisco by H. F Reid led to famous elastic rebound
theory.
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20. Elastic rebound theory
◦ According to theory, underground rocks are like rubber and expand when stretched.
◦ This stretching and pulling of crustal rocks due to tensile forces is slow process.
◦ When tensile forces exceed the rocks elasticity, latter are broken and they tend to occupy their
previous positions. Broken rocks tend to adjust themselves.
◦ All these processes occur so rapidly that it results in disequilibrium of crustal surface which
causes earth tremors.
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21. Anthropogenic causes
◦ Human activities such as pumping of groundwater and oil, deep underground mining, huge
reservoirs, dams, nuclear explosion etc. also cause earth tremors of serious consequences.
◦ Introduction of additional artificial superincumbent load through construction of dams and big
reservoirs cause huge Hydrostatic pressure which, ultimately, results in earthquakes.
◦ Examples include Koyna earthquake of 1967 due to reservoir, Earth tremors around Hoover
dam(USA) due to creation of Mead lake in 1936
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22. Plate tectonics
◦ Moderate earthquakes are caused along constructive plate boundaries. Shallow focus
earthquakes are caused along these boundaries or along mid-oceanic ridges. Depth of focus
associated ranges between 25 km to 35 km usually.
◦ High Magnitude earthquakes and deep focus earthquakes are caused along destructive plate
boundaries. Here mountain building, faulting and violent volcanic eruptions cause severe and
disastrous earthquakes having focus at depth up to 700 km.
◦ Creation of transform faults along conservative plate boundaries explains occurrence of severe
earthquakes of California
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23. Distribution of earthquakes
Most of the world earthquakes occur in certain zones of the globe which are weaker and
isostatically disturbed areas. These zones are:
1. Zones of young folded mountains
2. Zones of faulting and fracturing
3. Zones representing junction of continental and oceanic margins
4. Zones of active volcanoes
5. Zones along different plate boundaries
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24. World distribution of earthquakes
Circum-Pacific Belt
Mid- continental Belt (epicenters located along the
Alpine-Himalayan Chains of Eurasia and northern Africa
and epicenters of East African Fault Zones)
Mid – Atlantic Belt
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25. Fig 4 Distribution of earthquakes
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26. Seismic zones of India
On the basis of magnitude of damage risk, India is divided into damage risk zones.
1. Zone II
2. Zone III
3. Zone IV
4. Zone V
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27. Fig 5 seismic zones of India 27

28. References
◦ Physical geography by Savindra Singh (pg no. 129 to 139)
◦ Physical geology by Mahapatra
◦ www.usgs.com
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29. Thank you
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