An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's crust that creates seismic waves.
The tectonic plates which comprise the Earth's Crust are in continuous movement. They press one against the other; they move apart, sometimes moving in opposite directions. As they rub against each other, this slows down their movement, causing an accumulation of an enormous amount of energy.
When the two plates finally move this energy is suddenly released and shakes with enormous vibrations known as seismic waves. These vibrations shake the Earth’s surface, causing an earthquake.
The intensity of the earthquake can be great enough to destroy an entire city, causing death and injury to many victims.