All about Earth Quake

WHAT is EARTHQUAKE ??

 They are known as tremors (or) shaking of earth’s crust.

 Evidence of present day earth’s movement.

 They are actions of internal forces of earth.

CAUSES –  

- Crustal plates are continually in motion interacting with neighbouring plates-causes

strain and deformation at their edges.

- When the deformation exceeds the elastic limit, the rocks emit some form of waves

called earthquakes.

- Earthquakes of lesser magnitude are caused by human activities also.

PLACE OF ORIGIN –

- Most of the earthquake – origin – 60kms below the surface of earth.

 Seismic focus – point of origin of earthquake in the interior of earth.

 Epicentre –    focus where earthquake is 1st felt on the surface of earth.

 Coseismal lines – lines joining the places experiencing the earthquake at the                                          

 Isoseismal lines – lines joining places experiencing same intensity of the

EARTHQUAKE WAVES – 

 The energy released during earthquake moves from the seismic focus in different directions

in the form of seismic waves .

- TYPES – P-waves, S-waves, L-waves

 P-waves          –    Primary waves

 S-waves          –     Secondary waves (or) transverse waves

 L-waves          –    Surface waves

MEASURED IN ?

- Intensity      –   Richter scale

- Instrument   –   seismograph

REASONS FOR EARTHQUAKE:-

a) Elastic rebound theory

b) Theory of plate tectonics

c) Volcanoes

d) Anthropogenic factors



a) Elastic rebound theory –

- Rocks of earth pressed highly due to increasing overburden.

- Thus pushed into the zone of high density in the interior.

- However the rocks have the tendency to resume their original position, if the pressure

over them is released.

- This may cause earthquake.


b) Theory of plate tectonics –

- Earthquake occur around the margins of mobile lithospheric plates.

- 3 types  of plate movements.

 Divergent –    2 plates move away from each other.

 Transform -   2 plates slide laterally past each other.

 Convergent -  2 plates move towards each other – they collide.

 c) Anthropogenic factors –                                                      

- Pumping out excess ground water.                                  

- Oil mining.

- Nuclear explosions.

- Dam building.


d) Volcanic factors –

- Ejection of molten material from the interior of the earth to the surface.

- Caused due to convergent and divergent plate movements.

- This might result in earthquakes of varying intensity.

EARTHQUAKE SHADOW ZONE – 

- Area diametrically opposite to epicentre does not experience earthquake

- Because P-waves are deflected from liquid core and S -waves fail to reach here due to

absorption in the liquid core

- Shadow zone for S waves is beyond 103 degree celcius on both sides while Shadow zone for P waves is from 103 to 142 degree  as shown below :-

 IMPACTS OF EARTHQUAKE – 

- Landslides

- Damming of rivers

- Floods

- Fault formation

- Fall of buildings

- Loss of life & property

- Damage to oil pipelines and

electric wires

- Change in surface drainage

- Tsunami

REMEDIAL MEASURES –

- Installation of more seismic stations to issue warning.

- Long term predictions based on cyclic nature of earthquakes.

- Suitable building designs.

- Abnormal behaviour of animals.

- Special trainings to people.

- Effective disaster management in coordination with various agencies – rescue, relief,

rehabilitate



EARTHQUAKE PRONE ZONES IN INDIA :-

The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year.Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes. A World Bank & United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050.

The latest version of seismic zoning map of India given in the earthquake resistant design code of India [IS 1893 (Part 1) 2002] assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity as can be seen from below map :-

CONCLUSION –

Though man has no control over earthquakes, its proper study, early prediction and

remedial measures could help in better management of disasters and thus saving many

lives.