Class 8, Science, Chapter-12, Lecture-2, Earthquake (Notes)
EARTHQUAKE:
An unpredictable sudden shaking of the earth for a short time caused by collision or brushing of plates deep inside the earth’s crust is termed as an earthquake.
STRUCTURE OF EARTH:
- Crust: made of various large pieces of flat rocks called plates which are floating on partially molten rocks.
- Mantle: made of a mixture of hot molten and solid rocks which move slowly due to convection currents caused by heating from the core
- Core: made of iron and nickel in molten form in the outer core and solid form due to pressure in the inner core.
CAUSE OF EARTHQUAKE:
- Movement of continental plates
- When two land plates moving in opposite directions slide past each other and get entangled, pressure builds beneath plates due to convection currents of mantle. Building of high pressure gives a jolt and the plates vibrate.
- When two land plates moving towards each other collide to form mountains, they vibrate.
- Movement of oceanic plates
Collision of oceanic plates with continental plates cause vibrations of the continental plate. - Volcanic eruptions
Large volcanic eruptions also cause movement of land plates
SEISMIC ZONES:
The zones of earth’s crust where two land plates share their boundaries and which are more prone to earthquakes are called the ‘seismic zones’ or ‘fault zones’.
Seismic Zones of India:
Kashmir, Western Himalayas, Central Himalayas, North-Eastern part, Rann of Kutch, Rajasthan, Indo-Gangetic Plane
SEISMOGRAPH:
A scientific instrument which measures and record the magnitude of an earthquake is termed as a seismograph.
RICHTER SCALE:
The scale used for measurement of the magnitude of an earthquake is Richter Scale.
Protection Against Earthquakes:
- Construction of earthquake proof houses
- Use of mud and timber for construction in highly seismic areas.
- Fixing of cupboards and shelves to walls.
- Proper placement of hanging objects such as fan, clock, geysers etc.
- Proper firefighting equipment in high rise buildings
Do’s and Don’ts during Earthquake:
- Take shelter under a table till tremors stop
- Stay away from tall and heavy objects
- Don’t get up from bed. Protect your head with pillow
- Stay away from buildings, trees and overhead power transmission wires
- Remain inside car or bus while travelling. Drive slowly to a clear spot.