Earthquakes and Tsunami

Earthquakes and Tsunami

Earthquake- The vibrations or oscillations of the Earth caused by a transient disturbance of the elastic or gravitational equilibrium of the rock at or beneath the surface of the Earth is known as Earthquake. 

Cause of the Earthquake -

Natural causes

• Plate tectonic movements
• Volcanic activities
• Subsidence of landforms
• Landslides
• Meteoritic impact

Anthropogenic causes

• Mining
• Nuclear Explosion
• RIS (Reservoir induced seismicity) - If the crust below the reservoir is not strong enough then it will produce some vibrations. Example - Koyna reservoir

Focus - Focus is the point where energy is released below the surface. It is also known as Hypocentre.

Epicentre - It is the the point on the surface nearest to the focus.

Earthquake waves - The waves generated by the release of energy during an Earthquake is known as Earthquake waves.

It is of two types -

Body waves - These are the Earthquake waves generated from the focus and moves through the interior of the Earth.

These are of two types -

1. P Waves - They are also known as Primary Waves or Compressional Waves. They oscillates back and forth in the direction of propagation and they depends on the compressibility of the material. P Waves can through all mediums, i.e., Solid, Liquid and Gases and hence crosses the core and have a smaller Shadow zone. P Waves are the first recorded waves.
2. S Waves - These are also known as Secondary Waves or Transverse Waves. The oscillation in the S Wave occurs perpendicular to the direction of the material and therefore can't pass through liquid and gases and hence can't travel through the core and have a larger Shadow zone. S Waves are recorded after P Waves.

Surface Wave - These are the Earthquake waves generated from the epicentre (when the Body Waves hits the surface, Surface Waves are generated) and are confined to the surface of the Earth. They are much slower than the Body Wave but can cause more damage. 

Example of Surface Waves - Love Wave and Rally Wave

S Waves travel slower as compared to P Waves.

Shadow Zone - The seismic waves (Earthquake waves) undergo reflection, bending (refraction) and change in speed while passing from one medium to another.

The zone in the interior of the Earth where the Earthquake waves are not reported is called Shadow zone.

P Waves are not detected between 105 degree to 145 degree from the focus which is called as P-Wave Shadow zone. This is generated due to bending and slowing down of P-Waves as it enters the outer core.

S-Waves can't pass through outer liquid core. Hence, beyond 105 degree, S-Waves are not reported resulting in S-Wave Shadow zone.

Where ever an Earthquake has occurred, the waves are recorded across all the different stations on the surface of the Earth and by analyzing these recording, i.e., the time of arrival, speed of wave and the direction, we can conclude about the interior of the Earth.

(Asthenosphere 🠊 Semi solid 🠊 Speed of wave is low 🠊 Therefore, it is also known as Low velocity zone)

Note - The absence of P wave between 104 - 140 degree doesn't indicate the nature of Outer core. It indicates the change in direction of waves while traveling from one medium to another.

Note - P waves travels parallel to the direction of motion of Earthquake waves while all other waves (S waves and Surface waves) travel perpendicular to the direction of motion of Earthquake waves.

Types of Earthquake

1. Shallow Earthquake (0-70 KM)
2. Intermediate Earthquake (70-350 KM)
3. Deep Seated Earthquake (350-700 KM)

Distribution of Earthquake

Measurement of Earthquake - Earthquake waves are recorded by an instrument called Seismograph and the record produced by Seismograph is called Seismogram.

Intensity vs Magnitude of the Earthquake

Intensity

• It is the visible damage caused by the Earthquake
• It is subjective assessment of damage
• It varies (decreases) as we move from epicentre
• It can be reduced by constructing Earthquake resistant structures
• Measured by Mercalli Scale (Range - 1 to 12)

Magnitude

• It is the amount of energy released at the focus
• It does not depend on epicentre
• It does not varies/ depends on location
• It is measured by Richter Scale (Range - 1 to infinity)
• Maximum magnitude ever recorded - 9.5 @ Chile

Note - Richter Scale has a lower limit of 1 but has no upper limit. It is a logarithmic scale with each unit being 10 times more in magnitude than the previous one. 

Effect/Impact of the Earthquake -

• Loss of life and property
• Landslide and Avalanches
• Flash flood
• Tsunami, etc

Tsunami

Tsunami - It is a Japanese word which means Harbor Waves. It is also called Seismic Sea Waves.

Cause of Tsunami -

• Earthquake along normal and reverse fault
• Volcanic activity
• Subsidence
• Meteoritic impact

Tsunami is generally preceded by the sudden vertical displacement of Water (Sea/Ocean water).

Normal Fault/Reverse Fault 🠊 Sudden vertical displacement of water 🠊 Tsunami

Transverse Fault 🠊 No vertical displacement 🠊 No Tsunami

Stages in Tsunami -

1. Generation of Tsunami waves
2. Propagation in deep water
3. Propagation in shallow water
4. Landfall

Generation of Waves - Caused by sudden vertical displacement of water

Note - Amplitude of wave is inversely proportional to the depth & the speed of wave is directly proportional to the depth, i.e., waves travel faster in open ocean than along the shores.

Shoaling Effect - The building up of wave height near to the coastal region is known as Shoaling Effect.

Propagation in deep water - The speed of the wave will be very high and the amplitude, i.e., height of the wave is very low. Therefore, Tsunami will bot be visible in open ocean and will not cause any damage in open ocean.

Propagation in shallow water - As the wave approaches the coast, the depth of the water decreases; the speed of the water also decreases but the amplitude, i.e., height of the wave increases. Due to low speed of water, the water piles up near to the coast resulting in generation of very high waves, which is known as Shoaling effect.

Landfall - When the wave hits the land.

Previous Article - Geomagnetism

Next Article - Exogenic movements

Notes on other subjects 

Ancient History

Medieval History

Modern History

Art & Culture

Polity

Geography

International Relations

Society

Optional Subject 

Public Administration

Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own. 

Hope! It will help you to achieve your dream of getting selected in Civil Services Examination 👍