When is an earthquake considered a major quake?

Answer 1

It is considered a major earthquake when the vibrations caused by the rock are really strong. Because rocks are constantly moving deep under the ground. Sometimes plates rub into one another. The plates have jagged edges, and so when they rub against each other, they get stuck. Since plates constantly move, the plates that are stuck create great tension, and when they are finally free, a huge amount of energy is released. Shock waves move from focus in all directions, and it causes the earth to vibrate. What_do_most_earthquakes_result_from

A2 This question draws attention to the various scales used to measure an earthquake.

The Richter Scale measures the amount of energy released at the actual place within the earth where the 'quake happened. It is an open-ended scale, in which the energy increases by a factor of about 30 for each increase in number. This scale does not even consider the depth at which the 'quake happened.

The Mercalli Scale on the other hand, is an entirely subjective one in which the amount of damage is the criterion. [Quite analogous to the Beaufort Scale used for Storms.] Items related to the damage, are construction standards (mud brick through to reinforced concrete), the susceptibility of the ground and its response to an event (swamp land vs hard rock), and so on.

It all depends whether you are an engineer/scientist, concerned with earthquake sources; or a victim/administrator/rescuer, concerned with ameliorating the destruction after the event.

You should look up both scales in your favourite search engine.

So whether or no it is a major quake depends on your interest in the subject.