What is Earth Quake. Why India is divided in to 5 Seismic Zones?

What is Earth Quake. What is seismic Force?
Is earth quake similar to wind? Is earth quake a direct force?
Many students think Earth quake is a wave coming and hitting the building.
This is not in fact true even to a certain extent. Earth quake travels as waves but its not about it hitting a building and creating a direct force.
Earth quake is inertia. Inertia due to the sudden movement of the ground.
It can be compared to the sudden movement of a bus. When the bus moves, the people in the bus experiences inertia. The joints are stressed. This is same in the case of buildings too.
The seismic forces due to inertia acts at the column beam junctions and subject the building to deflection.
Earth Quake is not a direct pressure
Earth quake is a displacement type of load
Zoning is done since the probability of seismic activity is different in different places.
The ground acceleration is not certain and hence we need to consider the acceleration based on best possible means.
Please see this video on seismic forces and why earth quake zones are needed.
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