What is an earthquake? What are the two main situations in which earthquakes occur?

Earthquake is the phenomenon in which the earth shakes suddenly for a very short time. It is caused by a disturbance deep inside the earth’s crust. It is the destructive natural phenomenon which cannot be predicted in advance. The earthquakes can cause floods, landslides and tsunamis. Most earthquakes are caused by the movement of earth’s plates. The magnitude (or intensity) of an earthquake is expressed in terms of Richter scale. The destructive earthquakes have magnitudes higher than 7 on the Richter scale. The outermost layer of the earth is fragmented. Each fragment is called a plate. These plates are in continual motion. When they brush past one another, or a plate goes under another due to collision they cause disturbance in the earth’s crust that shows up as an earthquake on the surface of the earth. The tremors on the earth can be caused when a volcano erupts or when a meteor hits the earth. It can also be caused by an underground nuclear explosion.