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Earthquake:
An earthquake is the shaking of the Earth's crust caused by the sudden release of energy,
which triggers waves that travel in all directions. This energy can be generated by a sudden
dislocation of segments of the crust, a volcanic eruption, or even manmade explosions.
However, most destructive quakes are caused by dislocations of the crust. The crust may
bend and then, when the stress exceeds the strength of the rocks, break and "snap" to a new
position. In the process of breaking, vibrations called "seismic waves" are generated. These
waves travel outward from the source of the earthquake along the surface and through the
Earth at varying speeds depending on the material through which they move. Some
vibrations are of high enough frequency to be audible, while others are of very low
frequency. These vibrations cause the entire planet to quiver or ring like a bell or tuning fork.
Earthquakes occur most often along geologic faults, narrow zones where rock masses move
in relation to one another, located at the fringes of the huge tectonic plates that make up
Earth’s crust.
Seismology: is the scientific study of earthquakes and the propagation of seismic waves
through the Earth or through other planet-like bodies.
Seismologist: A seismologist is a scientist who studies earthquakes and seismic waves.
Seismic wave: seismic waves are the waves of energy caused by the sudden breaking of
rock within the earth.
Hypocenter: The point where the energy is released is called the focus of an earthquake or
hypocenter.
Epicenter: The epicenter is the point on the earth's surface that is directly above the
hypocenter. The intensity of the earthquake is highest at the epicenter and decreases with
distance from the epicenter.
FORESHOCKS: these are the smaller shocks come before the main shoucks.
Main shock: the largest shocks is called the main shock.
After shocks: these are smaller shocks comes after the main shocks.
Earthquake swarms: a long series of small shocks with no main shock, these sequences are
called earthquake swarms.
Types of seismic wave:
Earthquake waves are of two types — body waves and surface waves.
Body waves: A body wave is a seismic wave that travels into the earth’s interior.
Types of body wave: body waves are of two types, p wave and s wave.
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p wave:
 P-waves or primary waves are the first waves to arrive at the earth’s surface.
 The characteristics of P-waves are like sound waves.
 Primary waves travel through all mediums-solid, liquid, and gas.
 They vibrate parallel to the direction of the wave propagation.
 They are fastest waves.
 They are also known as compressional and longitudinal wave.
S waves:
 S-waves or secondary waves are those body waves that arrive after the earthquake.
 Secondary waves travel only through the solid medium.
 The direction of vibration of this S-wave is perpendicular to the direction of wave
propagation.
 This wave moves rock up and down, or side-to-side.

Surface waves:
When the body waves interact with surface rocks, a new set of waves is generated called as
surface waves. These waves move along the earth surface. Surface waves are also transverse
waves in which particle movement is perpendicular to the wave propagation. Hence, they
create crests and troughs in the material through which they pass. Surface waves are
considered to be the most damaging waves.
Types of surface waves: surface waves are of two types, Love Waves and Rayleigh Waves.
Love Waves:
 The first kind of , named after A.E.H. Love, a British mathematician.
 This kind of surface waves causes horizontal shifting of the earth during an
earthquake.
 They have much slower than body waves but are faster than Rayleigh.
 Love waves cause rock to move side-to-side and perpendicular to the direction in
which the waves are traveling.
Rayleigh Waves:
 The other kind of surface wave is the Rayleigh waves, named after Lord Rayleigh.
 A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an
ocean.
 Because it rolls, it moves the ground up and down and side to side in the same
direction that the wave is moving.
 Most of the shaking felt from an earthquake due to the Rayleigh wave, which can be
much larger than other waves.
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Distribution of earthquake:
There are three large zones (belts) where earthquake frequently occur.
1. Circum-Pacific belt
2. Alpide Earthquake belt
3. Mid-Atlantic Ridge

Circum-pacific belt: The world's greatest earthquake belt, is the circum-Pacific seismic belt,
it is found along the rim of the Pacific Ocean, where about 81 percent of our planet's largest
earthquakes occur.It is also known as “Ring of Fire”. The belt exists along boundaries of
tectonic plates, where plates of mostly oceanic crust are sinking (or subducting) beneath
another plate. Earthquakes in these subduction zones are caused by slip between plates and
rupture within plates. Earthquakes in the circum-Pacific seismic belt include the M9.5 Great
chilean Earthquake Great Chilean Earthquake (1960) and the M9.2 Great Alaska
Earthquake (1964). This belt extends through the coasts of Alaska, Japan, Philippines, New
Zealand, North and South America.
The Alpide earthquake belt: This earthquake belt is begins from Alps mountain range and
passes through Turkey, Caucasus Range, Iran, Iraq, Himalayan mountains and Tibet to
China. One of its branches passes through Mongolia and Lake Baikal and another branch
extends to Myanmar. About 17% of world's earthquakes are located in this region, including
some of the most destructive, such as the 2005 M7.6 shock in Pakistan that killed over
80,000 and the 2004 M9.1 Indonesia earthquake, which generated a tsunami that killed over
230,000 people
Mid-Atlantic Ridge: Mid -World belt extends from the Alps with their extension into
Mediterranean, the Caucasus, and the Himalayan region and continues to Indonesia. 21% of
earthquakes are occurring in this belt.

Types of Earthquakes: Based on the Depth of Focus

An earthquake depth range of 0 - 700 km is divided into three zones: shallow focus
intermediate focus, and deep focus earthquake.
1. Shallow focus: Shallow earthquakes are between 0 and 70 km (43 miles) deep.
2. intermediate focus: Such earthquakes range from 70 to 300 Km (43 to 186 miles).
3. Deep focus earthquake: Deep focus earthquakes range from 300 - 700 km (186 to
434.96 miles).
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Measurements of earthquake
Seismometer: The instrument used to measure/detect seismic waves is a seismometer.
Seismograph: The instrument used for the measurement of vibrations is known as
Seismograph.

Seismogram : The paper record, or computer image of Earth vibration is called a

Seismogram. The recorded geographical output from a seismometer/seismograph is called
as a seismogram.
Note: Seismograph is an instrument while Seismogram is the recorded output.
Modified Mercalli scale: The scale represents the intensity of earthquake by analyzing the
after effects like how many people felt it, how much destruction occurred etc. The range of
intensity is from 1-12.
Richter scale: The scale represents the magnitude of the earthquake. The magnitude is
expressed in absolute numbers from 1-10. Each whole number increase in Richter scale
represents a ten times increase in power of an earthquake.
Effects Of Earthquake:
 Ground Shaking - It is a vibration of the ground observed during an Earthquake.
 Differential ground settlement - Only part of the floor is affected by the ground
failure and causes more severe damage than uniform or tilt settlement.
 Land and mudslides - The shaking of the Earth's surface can disrupt land and mud.
 Fires - The quakes can cause fire or volcanic eruption.
 Ground lurching -The parallel movement of soil, sediments, or fills found on vertical
embankments that create irregular ground cracks.
 Avalanches - Tremors can affect a large amount of snow by causing them to slide
quickly down the side of a mountain.
 Ground displacement- It is another effect of an Earthquake that can cause the ground
to change position in horizontal and vertical directions.
 Floods from dam and levee failures - Dams and levees are hydraulic structures that
may break because of defaults or unexpected events. When an Earthquake occurs,
water can be suddenly released and create a flood.
 Structural collapse - Excessive weight placed on a faulty structure can cause a
building to collapse.
 Tsunamis are ocean waves activated by large Earthquakes that occur near or under the
ocean.
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Types of earthquake:
1. Tectonic Earthquake
2. Explosion Earthquake
3. Collapse Earthquake
4. Volcanic Earthquake

Tectonic Earthquake: The most common form of earthquake, is caused by the movement
of loose fragmented pieces of land on the earth crust known as tectonic plates.
Explosion Earthquake: The occurrence of this type of earthquake is artificial. High-density
explosion such as nuclear explosions is the primary cause.
Collapse Earthquake: This earthquake occurs in underground mines. The main cause is the
pressure generated within the rocks.
Volcanic Earthquake: The less prevalent compared to the tectonic variety, these
earthquakes happen before or after the eruption of a volcano.
Shadow Zone: Shadow zones are zones where earthquake waves are not reported. It has
been observed that for each earthquake, there exists a different shadow zone Seismographs
have recorded P and S waves within a distance of 105° from the epicentre The seismographs
also recorded P-waves beyond 145° from the epicentre. S-waves were not recorded beyond
105°. Therefore, a shadow zone between 105° and 145° was created for both of the waves .It
is observed that the shadow zone of P-waves appears as a band around the earth.
Earthquake Fault Types
Normal, reverse (thrust), as well as strike-slip faults are the three primary fault types that can
all result in an interplate earthquake. Examples of dip-slip faulting include normal and
reverse faulting when movement on the faults contains a vertical component and
displacement all along fault is in the plane of dip.
Norma: Normal faults primarily appear along divergent boundaries or other regions in
which the crust is extending.
Reverse: In regions where the crust is shortening, like near a convergent boundary, reverse
faults develop.
Strike-slip: Transform boundaries are a specific kind of strike-slip fault. Strike-slip faults
are steep formations in which the opposing sides of the fault slip past one another
horizontally. Movement on faults with both dip-slip and strike-slip components is a common
cause of earthquakes.
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Intraplate and Interplate Earthquakes
• The world’s seismicity map, however, shows that earthquakes do take place within the
plate, far from the plate boundaries as well.
• These earthquakes are called intraplate earthquakes, and they arise from the localised
system of forces.
• The recurrence interval of intraplate earthquakes is generally much longer than those
of the interplate earthquakes.(Earthquake that occurs along the boundaries of plates)
• Hypocentres of the earthquakes in the intraplate zones rarely occur deeper than 15-20
km.