Facts about earthquakes we should know
What is an earthquake, why and where do they occur ?
We say an earthquake is occurring when we ourselves and everything around us begins to shack and doors and windows start to crackle. A little huge earthquake would make us feel sudden jolt and small things to move here and there in a messy way. This may continue for seconds or for little bit longer time. But major earthquake may bring disaster as well in the form buildings collapsing and infrastructure destruction and hence there can happen loss of lives of human beings living in a certain earthquake affected region.
An earthquake is actually a sudden release of huge amount of energy accumulated there inside the earth crust along the tectonic plate boundaries. How this disturbance or release of energy is caused let's try to understand.
How earthquakes happen ?
If small pieces of paper thrown into the saucepan filled with some water is put on the stove and gradually heated up, then a movement is started in the water - in which hot water from bottom of the pan comes up and cool water at the surface goes down to replace it (fig. 1). In this way a circle starts which goes on till water starts to boil. This process is called convectional transfer of heat or a convectional cycling. When this convectional cycling takes place , the pieces of paper also move here and there and collide with each other randomly.
The same dynamics works to generate earthquakes!, We generally know that inner of earth is in molten and very hot state. Convectional cycling currents - weak and strong - are continuously being generated there. Tectonic plates in the crust of earth are just like exampled pieces of paper - floating over the molten sphere. And because of convectional cycling currents the tectonic plates are in continuous disturbance or movement -- some where slow and some where faster. Because of these movements the tectonic plates collide with each other at their boundaries - or one plate goes under the other (subduction of one under the other), thus creating great thrust (fig. 2) - resulting in accumulation of huge amount of energy due to elasticity factor. Small size earthquakes may occur due to routine collision of tectonic plates, but large size earthquakes occur when a plate reaches its last elastic limit and is broken under the other -- releasing amount of energy accumulated till the time of braking.
It is like a elastic wooden sheet of which one end is broken down hard and thus creating bouncing noise and sudden rebound in the other end part. This 'bounce back' is the energy which accumulates when wooden sheet is bend hard from one end to break- and creates jolt and trembling in the other end part of the sheet. Similarly the amount of energy released due to breaking of one tectonic plate after bending hard - due to tectonic movement thrust- under the other, creates a sudden jolt and trembling in the earth's surface (upper part - crust).
This trembling of earth consists of variety of types of waves - which spread from the point of break (epicenter) in circular form travelling round the globe - dwindling down as these move away from the epicenter. Most commonly three types of waves, namely P-wave (fig. 4) and S- wave (fig. 5) and surface waves (fig. 6) - are described by the seismologists. P-wave travels at half the speed faster than S-waves and hence reach us first to S-waves. P- waves travel like compression and expansion in a coil spring. P- waves reaching make us hear roaring tumult within the earth. Then S-waves reach us, and then surface waves moving on snake trail pattern - and shakes us and our surroundings, and may cause destruction -- in other words surface waves are actually responsible for the destruction and main shake.
P-waves speed in average is 8 km/s and that and S-wave speed is 4 km/h, Though speed may slightly change depending upon the crust density i-e 'under ground' is soft or hard or some material like coal salt etc may be existing below in the crust !
Shaking due to an earthquake may be less or greater !
Shaking due to two earthquakes having same magnitude may be greater for one and less for the other. If point of occurrence of an earthquake is deep inside the crust then shanking may be less but if the occurrence point is shallow, then shaking may be greater - though magnitude may be same at both point of occurrence. Also shaking may increase if crust below certain region is soft i-e then it will act like jelly in a plate.
Richter's scale (named after a seismologist Charles Francis Richter ) is actually not a measuring device but a mathematical formula to calculate magnitude.
The World’s largest earthquake with an instrumentally documented magnitude occurred on May 22, 1960 near Valdivia, in southern Chile. It was assigned a magnitude of 9.5 by the United States Geological Survey. It is referred to as the "Great Chilean Earthquake" and the "1960 Valdivia Earthquake" (Source: http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php)
Magnitude and Intensity of an earthquake ?
Magnitude and intensity are the terms used for earthquakes which often confuse a lay man as what these mean. Let's tell you ! Magnitude, is the amount of energy released when an earthquake occurs. On measurement scale (usually Richter's scale ) it's value starts from 0 and has no end limit i-e it may be 2.0, 5.0, .... 9.5 etc or even more, but no end limit!.
Intensity on the other hand is the estimate of destruction due to an earthquake. If destruction greater/smaller the intensity is larger/smaller to be said. Even a low magnitude earthquake can have larger intensity due to heavy shaking caused because of epicenter being shallow - or crust being soft etc. Similarly large magnitude earthquake may have low intensity -- due to epicenter being deeper or crust being harder etc.It's value range is 1- 12, Here intensity level 1` is 'no destruction' but 12 is 'total destruction' (see :Modified Mercalli Intensity Scale )