Why Earthquake Happens?

It’s because of plate tectonics.

Now what is plate tectonics?

Our Earth is not only a solid, round sphere but it’s also divided into different parts. I am not talking about the inner mantle and the core, but the upper crust itself is divide into part like the patches (sieving of threads on the football) because of this only we have 'mid ocean ridges' and the 'Pacific ring of fire'. These divided parts of crust are called the Earths plates. And these plates are moving (I shocked when I came to know for the first time that the crust is moving). This is called ‘Plate Tectonics’ (Tectonic means motion}.

Mid Ocean Ridge - the zigzag line in the middle

And why the crust is divided into several parts and why those parts are moving?

As you know the Earth is very hot inside and it gets hotter as you travel towards the center. This trapped heat wants to go outside (don’t ask me why- because it’s the nature of the energy, heat travels toward cold and cold towards heat, till it becomes same. That’s why your Tea gets cool). Inside the Earth’s core, the molten metal conducts the heat, in mantle also the molten magma also conducts the heat through convection current (convection current in liquid is like tornado in air). But the solid crust almost blocks this because of which a very high pressure gets generated in mantle pushing the crust. That’s why the crust is divided into moving plates. The crust is cooler and denser and it tends to go down. This temperature and pressure difference keeps pushing the crust. Eventually enormously high pressure and hot magma breaks the crust and explodes in the form of Lava. This is the reason behind the Volcanism. These Volcanoes push the crust plates and when the lava cools down, it forms the rocks adds to the crust. In this way a new part of the crust is formed.

But the movement of crustal plates itself is not the Earthquake. These plates are always in slow continuous collision with each other, which generates high pressure on the rocks (rocks bigger than a city). This pressure keeps increasing on the rocks throughout centuries, till it finally breaks creating very strong shock waves. This is the moment of ‘Earthquake’. All the potential energy of that pressure gets released at that moment and we feel the Earth below our feet is moving!