Why earthquake occur? Hidden causes of earthquake and 2 main theories behind it.

In this article we are going to share the main causes of earthquake and theories behind it.

elastic rebound theory and Plate tectonic theory which explain the reason behind occurring of earthquake.

1) Elastic Rebound theory:- As we can understand from this word which itself states elastic, so somewhere it is related to stress and strain accumulated in any body which is responsible for occurring of earthquake.

Lets understand hidden details behind this:-

First we will understand "when, how and by whom" this theory is introduced between us:-

An incident of earthquake witnessed by San Francisco in the year 1906 in California. It was a very devastating incident which caused severe damage to the earth surface and a 270 miles fault/fracture was also noticed. It was found that the earth surface on both the sides of fault were not at the same level as it was earlier. It means the one side of ground surface was slipped as compared to other side, this incident attracted the scientists to know the reason behind this. So from there the research work started!

Now there was a professor who was a geologist at johns Hopkins university in America his name was H.F Reid  he introduced this theory in the year 1911. He concluded that the earthquake have involved "elastic rebound" a previously stored elastic stress in body. 

What is elastic rebound theory?

According to elastic rebound theory, earthquake are caused by gradual accumulation and release of stress and strain in any rock or mass.

Lets take an example to under this in a better way:-

Suppose we have a unit of block in which shear force is continuously acting, so within its limit of bearing this stress no strain will develop in this block and this condition will be known as "no strain condition". 

However if we increase the stress beyond its elastic limit, crack/fracture will evolve there, but due to the friction acting between the blocks there will be no movement between them (blocks) and this condition is called "strained condition". 

Now what will happen, as the shear force is increasing so block will try to re-adjust itself in order to reduce the strain and as a result, one block will tend to slip off from the other block. 

Due to this slipping of the block, the energy which was progressively getting stored in block will suddenly release causing severe vibrations which is called "Seismic Shocks", and hence responsible for earthquake. 

2) Theory of plate tectonic:- This theory was presented by the Alfred Wegener and it is based on his continental drift theory. He presented continental drift theory in one of his book named as "On the origin of continent and ocean". In his book he stated that, once all the continents were joined in a major continent which is called "Pangea" and later this split apart and moved slowly to their current position on globe.

According to this theory, the earth surface is divided into several parts as mentioned below.

1. Inner core which is at the centre of earth
2. outer core 
3. Mantle
4. Crust 

According to him, the sliding blocks take place in pieces and called tectonic plates or segments. Tectonic plates can be purely continental, oceanic or combination of both.

So according to this theory, earthquake occur due to the movement of tectonic plates. Like for an example if there is any vertical or horizontal displacement in continental plate and as it is in contact with earth surface then the surface of earth will also shake in that direction which cause earthquake.

And if any kind of plate movement happen in oceanic plates then the same effect will be there and the surface of the ocean will also shake. As the area is completely under water then along with earthquake there are chances of occurring Tsunami.

Movement of tectonic plates are broadly classified into three forms:-

1) Divergent boundaries:- Divergent boundaries is a situation where plates are moving apart. So in this situation when plates move away from each other, a drift or valley form which cause upward movement of molten magma and a new crust forms on earth surface. In this situation earthq generally occur with  low magnitude and at a shallower depth.

                                                

2) Convergent boundaries:- When two plates moves towards each other and one plate sinks under the another this condition is called convergent boundaries.     

                                                   

3) Transform Boundaries:- It is the situation when two plates moves side by side horizontally then it is called transform boundaries. In this case plate can slide in same direction as well as in opposite direction. 

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