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Earthquake - Geography for UPSC CSE

 Table of contents Earthquake Causes and Effects Terminology Used in the Study of Earthquakes Focus and Epicenter World Distribution of Earthquakes Earthquake Causes Damage caused by earthquakes Earthquake warning system

Earthquake Causes and Effects

• An earthquake is the shaking of the Earth's surface, resulting from the sudden release of energy in the Earth‘s lithosphere that creates seismic waves.
• Earthquake is the form of energy of wave motion transmitted through the surface layer of the earth.
• It may be due to faulting, folding, plate movement, volcanic eruptions and anthropogenic factors like dams and reservoirs.
• Earthquake is by far the most unpredictable and highly destructive of all the natural disasters.
• Minor earth tremors caused by gentle waves of vibration within the earth’s crust occur every few minutes. Major earthquakes usually caused by movement along faults can be very disastrous, particularly in densely populated areas.

Terminology Used in the Study of Earthquakes

• Earthquake intensity
• Earthquake magnitude
• Richter Scale
• Mercalli Scale
• Fault
• Focus
• Epicentre
• Seismic wave
• Seismograph

Question for Earthquake
Try yourself:Consider the following factors

1. Dams

2. Reservoirs

Which of the above is/are the anthropological factors responsible for earthquake?

Focus and Epicenter

The point within Earth where faulting begins is the focus or hypocenter.

The point directly above the focus on the surface is the epicentre.

Richter Scale

• Richter magnitude scale is the scale to measure the magnitude of energy released by an earthquake.
• Charles devised this scale. F. Richter in the year 1935.
• The number indicating magnitude ranges between 0 to 9

An earthquake that registers 5.0 on the Richter scale has a shaking amplitude ten times that of an earthquake that registered 4.0, it corresponds to a release of energy 31.6 times that released by the lesser earthquake.

Mercalli scale

• The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake.
• It measures the effects of an earthquake.
• The number indicating intensity ranges between  1 to 12

Seismic Waves

• Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth.
• They are the energy that travels through the earth and is recorded on seismographs.
• The two main types of waves are body waves and surface waves.
Body waves
(i) Primary waves ( P-waves)
(ii) Secondary waves ( S-waves)
Surface Waves
(i) Love Waves (L-waves)
(ii) Rayleigh waves

Primary waves (longitudinal wave)-

• The first kind of body wave is the P wave or primary wave.
• This is the fastest kind of seismic wave.
• The P wave can move through gaseous, solid rock and fluids, like water or the earth's liquid layers.
• It pushes and pulls the rock; it moves through just like sound waves push and pull the air.

Secondary waves (transverse wave)

• The second type of body wave is the S wave or secondary wave.
• An S wave is slower than the P wave and can only move through solid rock.
• This wave moves rock up and down, or side-to-side.
• S-waves arrive at the surface with some time Lag.

Love Waves

The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British

mathematician.

It’s the fastest surface wave and moves the ground from side-to-side.

Rayleigh Waves

• The other kind of surface wave is the Rayleigh wave, named after Lord Rayleigh.
• A Rayleigh wave rolls along the ground just like a wave moves across a lake or an ocean.
• Because it rolls, 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 is due to the Rayleigh wave, which can be much larger than the other waves.

Classification of Earthquake

1. Based on causative factors
• Natural
(i) Volcanic
(ii) Tectonic
(iii) Isostatic
(iv) Plutonic
• Artificial
2. Based on the depth of focus
• Moderate(0-50km)
• Intermediate(50-250km)
• Deep focus( 250-700km)
3. Based on human casualties
• Moderate (deaths<50,oo)
• Highly hazardous(51,000-1,00,00)
• Most hazardous(>1,00,00)

World Distribution of Earthquakes

• The world’s distribution of earthquakes coincides very closely with that of volcanoes.
• Region of greatest seismicity are Circum-Pacific areas, with the epicentres and the most frequent occurrences along the ‘Pacific Ring of Fire’.
• It is said that as many as 70% of earthquakes occur in the Circum-Pacific belt.
• Another 20% of earthquakes occur in the Mediterranean-Himalayan belt, including Asia Minor, the Himalayas and parts of north-west China.
• Remaining occur in the interiors of plates and on spreading ridge centres.

Earthquake Causes

Earthquakes are caused mainly due to dis-equilibrium in any part of the crust of the earth.

Several causes have been assigned to caused dis-equilibrium or isostatic imbalance in the earth’s crust such as-.

• Volcanic eruption
• Faulting and folding
• Upwarping and down warping
• Gaseous expansion and contraction inside the earth.
• The hydrostatic pressure of human-made water bodies like reservoirs and lakes.
• Plate Movement

Plate tectonics provides the most logical explanation of volcanoes and earthquakes.

There are three types of plate boundaries along which earthquake occurs.

• Convergent
• Divergent
• Transform

Damage caused by earthquakes

• Slope instability and landslides
• Damage to human structures
• Damage to towns and cities
• Loss of human lives
• Fires
• Deformation of the ground surface
• Flash floods
• Tsunamis

Earthquake warning system

The document Earthquake | Geography for UPSC CSE is a part of the UPSC Course Geography for UPSC CSE.
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FAQs on Earthquake - Geography for UPSC CSE

 1. What causes earthquakes?
Ans. Earthquakes are caused by the sudden release of energy in the Earth's crust. This energy is typically generated by the movement and interaction of tectonic plates, which are large sections of the Earth's surface that float on the semi-fluid asthenosphere beneath them. When these plates move, they can become stuck due to friction, and the accumulated stress causes the rocks to break or slip, resulting in an earthquake.
 2. How are earthquakes measured and classified?
Ans. Earthquakes are measured and classified using various scales. The most commonly used scale is the Richter scale, which measures the amplitude of seismic waves produced by the earthquake. It is logarithmic, meaning that each whole number increase on the scale represents a tenfold increase in the amplitude and approximately 31.6 times more energy released. Other scales used include the moment magnitude scale (Mw), which measures the total energy released, and the modified Mercalli intensity scale, which assesses the damage caused by an earthquake at specific locations.
 3. What are the primary effects of earthquakes?
Ans. The primary effects of earthquakes include shaking and ground rupture. Shaking can cause buildings, bridges, and other structures to collapse, leading to injuries, loss of life, and widespread property damage. Ground rupture occurs when the Earth's surface breaks due to the movement of the fault, creating visible cracks or displacements. This can result in damage to infrastructure and the alteration of landscapes.
 4. How does the distribution of earthquakes vary globally?
Ans. The distribution of earthquakes is not uniform globally. The majority of earthquakes occur along plate boundaries, where tectonic plates interact. These regions include the Pacific Ring of Fire, which encircles the Pacific Ocean and is known for its high seismic activity, and the Alpide Belt, which extends from the Mediterranean region to Southeast Asia. However, earthquakes can also occur within plates, known as intraplate earthquakes, although they are less frequent.
 5. Are there warning systems for earthquakes?
Ans. Yes, there are earthquake warning systems that can provide advance notice of an impending earthquake. These systems use seismic monitoring networks to detect the initial, less destructive primary waves (P-waves) that travel faster than the more damaging secondary waves (S-waves). By analyzing the P-wave data, warning systems can estimate the magnitude and location of an earthquake and issue alerts to areas that will experience the shaking later. While these systems can provide valuable seconds to minutes of warning, their effectiveness depends on the proximity of the monitoring network to the earthquake source and the speed of communication to the affected areas.

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