Earthquakes are caused by the collision of tectonic plates. These plates are a part of the Earth's crust. Continents float on tectonic plates that move very slowly over centuries. There are many tectonic plates all over the Earth's crust.
When tectonic plates collide, they generate massive seismic waves in the Earth's crust. These waves travel rapidly and cause severe shaking of the Earth's upper crust. This leads to the extensive damage that is often seen when an earthquake of a high magnitude strikes.
The magnitude of an earthquake is measured by the Richter scale. The range of this scale is from one to 10. The higher the number, the more severe the earthquake is.