Well you can get pretty good idea of what causes an earthquake from thinking about what happens in earthquake. During an earthquake, there is a trembling of the ground. It is the trembling of the earth which may cause buildings to fall. So an earthquake is a trembling or vibration of earth's surface.
What makes it happen? Well, the rock of the earth's crust may have a defect, a kind of break in the crust. The earth blocks shift. Sometimes the sides of the fault move up and down against each other. At other times, the sides of the fault shift lengthwise.
But when one rock mass has rubbed on another with great force and friction, we have a lot of energy being used. The vast energy that comes from the rubbing is changed to vibration in the rocks. The vibration is what we feel as an earthquake. And this vibration may travel thousands of miles.The reason earthquakes take place in certain regions frequently and almost never in other regions, is that the faults in the earth crust are located in these regions.
An earthquake occurs right from the surface of the earth known as the crust, which is separated into sections known as plates. They are constantly moving, however if one of the plates moves over another one then it will continually build up more and more energy until it needs to be released. When the energy is being released the two plates touch, which creates the basis of an earthquake. When the plates come into contact it is known as the fault line. However, an earthquake can occur from the energy released from a strong volcanic eruption as well as man-made explosions.
The mass of energy reaches its focal point which usually happens in the depths of the earth's surface and reacts with the earth's surface known as the epicentre. Once this occurs, seismic waves will begin to emerge in a number of directions and various speeds through the earth's crust with an enormous amount of energy. This ultimately creates a shaking of the earth which is the feeling of an earthquake. The strengths of earthquakes differ depending on the size and energy of the seismic waves, as you know that some cause barely any damage at all whereas others can cause buildings to crumble.