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IAS (Civil Services) 2017 Geography Notes: Earthquakes

Last Updated - August 23, 2017

Geography in Civil Services is one of the most chosen subject owing to its limited syllabus and interesting topics. ‘Earthquakes’ in Geography is among the most important sections generally asked in Civil Services. Candidates appearing for upcoming Civil Services exams are advised to go through the notes and important pointers mentioned in the article about pattern of IAS and it’s section.

Civil services examination is a nationwide competitive exam, conducted by union public service commission (UPSC)  for recruitment to various Civil Services of the Government of India such as including the Indian Administrative Service (IAS) among others. The Civil Services Examination is considered to be one of the most difficult and competitive examinations in India. Only 50% of those who apply for the exam, appear for the preliminary. The examination consists of the following three stages:

  • Stage I: Preliminary examination
  1. General Test 1 - 100 Questions
  2. General Test 2 - 80 Questions
  3. Note: The merit list is created solely based on the score of GS Paper I. General Studies Paper II is just a qualifying paper only with cut off score as 33%.
  • Stage II: Main examination
  1. Written exam of nine papers
  • Stage III: Personality Test (interview)
  1. candidate can give preference of the language in which they may like to be interviewed.

IAS Geography Notes: Earthquake

Geography is a field of science which focuses on the study of the lands, the physical features, the inhabitants, and the phenomena of Earth. Earthquake is one of those natural phenomena of mother Earth. Let us study more on earthquakes.

What is an Earthquake?

Earthquake is a natural phenomenon which is caused by the release of sudden energy in the earth's lithosphere that creates seismic waves. These waves can cause shaking of ground and other experiences based on the frequency/power of the seismic waves.

Causes of an Earthquake

The definition of an earthquake pretty much explains how an earthquake is caused. There are two main causes of an earthquake.

  • First Being, Explosive volcanic eruptions. Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes worldwide are of this type.
  • Second, Majority of earthquakes worldwide are of this type. They can be triggered by the movement of tectonic faults & plate margins.

Important Terms & Notes

Seismograph: An instrument that measures and records details of earthquakes, such as force. (older term, New term Seismometer)

Focus/Hypocenter: The location where the earthquake begins. Origin of the earthquake, within the earth's crust.

Epicenter: The point on the earth's surface vertically above the focus of an earthquake.

  • Epicenter usually lies within 60km's beneath the earth's surface.
  • The intensity of an earthquake is the highest near the epicenter, which decreases as one moves away.

Lithosphere: It is that rigid outer layer of the earth, consisting of the crust and upper mantle.

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Earthquake waves or Seismic waves

Earthquake is measured by a seismograph, which displays different types of waves caused by an earthquake. There are two types of waves:

  • Body Waves
  1. Primary (P-Waves)
  2. Secondary (S-Waves)
  • Surface Waves
  1. Love (L Waves)
  2. Rayleigh (R Waves)

Body waves

A body wave is a seismic wave that moves through the interior of the earth and that is why it is called body waves. Body waves are faster than surface waves and hence they are the first to be detected on a seismograph. There are two types of body waves,  primary waves, and secondary waves.

Primary waves (p-waves):

P wave A.K.A compressional wave is a seismic body wave that causes ground to shake back and forth in the same direction and the opposite direction as the direction the wave is moving. Primary waves are twice as fast as s-waves and are the first to reach during an earthquake. P- Waves are longitudinal, in which particle movement is in the same direction of wave propagation. They are the reason which create density differences in the earth material leading to stretching and squeezing.

Secondary waves (s-waves):

Since they are slower than primary waves. They are second to arrive on the surface after P-Wave. . S- Waves can pass only through solid materials. This very property of s-waves led seismologists to conclude that the earth’s outer core is in a liquid state. (the entire zone beyond 105o from the epicenter does not receive S-waves). Secondary waves are transverse, in which directions of particle movement and wave propagation are perpendicular to each other.

Surface Waves

A surface wave is a seismic wave that is trapped near the surface of the earth. Basically, when the body waves interact with surface rocks, it is called a surface wave. They create crests and troughs in the material through which they pass. Surface waves are considered to be the damaging waves. There are Two types of surface waves, Love waves and Rayleigh waves.

Love waves:

L- Wave is the kind of surface waves which causes horizontal shifting of the earth during an earthquake. They move in between of semi-infinite medium & an upper finite thickness. Love waves are much slower than body waves but are faster than Rayleigh. 

Rayleigh waves:

The majority of ground shakes in an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. These waves follow an elliptical motion & flow along the ground just like a wave flow across an ocean. That very flow, makes the ground move up and down and side-to-side in the same direction that the wave is moving.

Shadow regions of waves

Even though p-waves pass through all mediums, it causes reflection when it enters from one medium to another. The area where the seismograph records no waves is called as ‘shadow zone’ of that wave. Accordingly, it is observed that the area beyond 1050 does not receive S-waves and the area in between 1050 to 1400  does not receive P-waves.


Measuring earthquakes

Seismometers are the instruments which are used to measure the motion of the ground, which including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. The graphical representation which is recorded by seimometer/seisomograph is called seismogram. There are two main scales used in the seismometers:

  1. Mercalli Scale
  2. Richter Scale.

Mercalli Scale:

Mercalli scale analysis the intensity of the earthquake by analyzing, by evaluating the damage done and the number of people felt it.

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.

Distribution of Earthquakes

There are two well-defined belts where earthquakes frequently occur

  1. The Circum-Pacific Belt
  1. 68% of earthquakes in the world occur in the Circum-Pacific Belt.
  1. The Mid-World Mountain Belt
  1. Mid-World Mountain belt extends from the Alps with their extension into Mediterranean, the Caucasus, and the Himalayan region and continues to Indonesia.
  2. 21% of earthquakes are occurring in this belt.
  • The remaining 11% occur in the other parts of the world.

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