Notes On Earthquakes - CBSE Class 9 Geography
An earthquake is a sudden tremor or movement of the earth's crust, which usually originates at or below the surface. The outer layer of the earth is solid and is divided into many sections known as plates. These plates, float over the molten magma that flows beneath the earth’s crust. Many of the earthquakes occur on the edges of these plates along fault lines where the plates collide or try to slide past each other. Apart from natural causes like the movement of the Earth's tectonic plates and volcanic eruptions, there are other man-made causes of earthquakes. These include vibrations caused by big rocks falling down hills because of erosion, hollow parts of mines falling down due to dissolution of minerals by water and, in the present times, due to nuclear explosions. The point of origin of the earthquake within the crust or mantle is called the seismic focus. Since the focus is often deep below the surface, the location of the earthquake is often referred to as the point on the surface of the earth, vertically above the seismic focus. This point is called the epicentre. Scientists, who study earthquakes or seismology, measure earthquakes with a device called the seismometer; a device which also records is known as a seismograph. They use an instrument to measure the magnitude of the earthquake, called a ‘Richter Scale’. Such under-sea earthquakes, give rise to giant waves called the tsunami, which cause great deal of destruction along the coasts. Measures for protection against earthquakes: Proper early warning system, The information about earthquakes and their intensity should be shared with the public through radio, television and newspapers Construction of buildings based on earthquake-resistant techniques Construct buildings over pillars made of concrete and iron that are built deep in the ground Water, ration, first-aid kits, radios, flash lights, battery, blankets, jackets and fire extinguishers should be stored in safe places. Measure on the personal front: Remain calm If inside a building or a house, take cover under a solid surface like a table, or stand in the doorway If one is outside, move to an open space away from trees, electric poles and buildings Switch off the gas and electric supply in the house during a quake and do not use elevators After the quake is over, the affected people should be given immediate medical help People should be vaccinated to stop epidemics from spreading Transport and communication facilities should be restored as soon as possible Police and paramilitary forces should be deployed to protect properties which had to be abandoned from theft.

#### Summary

An earthquake is a sudden tremor or movement of the earth's crust, which usually originates at or below the surface. The outer layer of the earth is solid and is divided into many sections known as plates. These plates, float over the molten magma that flows beneath the earth’s crust. Many of the earthquakes occur on the edges of these plates along fault lines where the plates collide or try to slide past each other. Apart from natural causes like the movement of the Earth's tectonic plates and volcanic eruptions, there are other man-made causes of earthquakes. These include vibrations caused by big rocks falling down hills because of erosion, hollow parts of mines falling down due to dissolution of minerals by water and, in the present times, due to nuclear explosions. The point of origin of the earthquake within the crust or mantle is called the seismic focus. Since the focus is often deep below the surface, the location of the earthquake is often referred to as the point on the surface of the earth, vertically above the seismic focus. This point is called the epicentre. Scientists, who study earthquakes or seismology, measure earthquakes with a device called the seismometer; a device which also records is known as a seismograph. They use an instrument to measure the magnitude of the earthquake, called a ‘Richter Scale’. Such under-sea earthquakes, give rise to giant waves called the tsunami, which cause great deal of destruction along the coasts. Measures for protection against earthquakes: Proper early warning system, The information about earthquakes and their intensity should be shared with the public through radio, television and newspapers Construction of buildings based on earthquake-resistant techniques Construct buildings over pillars made of concrete and iron that are built deep in the ground Water, ration, first-aid kits, radios, flash lights, battery, blankets, jackets and fire extinguishers should be stored in safe places. Measure on the personal front: Remain calm If inside a building or a house, take cover under a solid surface like a table, or stand in the doorway If one is outside, move to an open space away from trees, electric poles and buildings Switch off the gas and electric supply in the house during a quake and do not use elevators After the quake is over, the affected people should be given immediate medical help People should be vaccinated to stop epidemics from spreading Transport and communication facilities should be restored as soon as possible Police and paramilitary forces should be deployed to protect properties which had to be abandoned from theft.

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