Earthquakes occur at cracks in the Earth's crust called faults. Faults are created because rock is brittle and breaks when great stress (stretching, squeezing or twisting) is exerted upon it. Stress builds up in areas of the crust because of the gradual movement of the Earth's plates.
Earthquakes happen when stress has built up in an area of rock to such an extent that sudden movement occurs. This movement can create a new fault as the rock breaks at the weakest point, or the movement causes the rock to slip along an existing fault. When this happens, an enormous amount of enorgy is given out as the stress is released. The released energy causes the surrounding rock to vibrate, which creates an earthquake
The actual point where the rock first slips or breaks, causing an earthquake, is called the focus. The place on the Earth's surface immediately above the focus is called the epicentre.
Shock waves
The vibrations of an earthquake travel out through the Earth. Scientists call them shock waves or seismic waves (from the Greek word seismos which means "trembling Earth"). Different types of shock waves are sent out from the focus, and each type makes the rock it travels through víbrate in a different way.
The main types of shock waves are called primary and secondary waves. Primary waves, or P-waves, squeeze and stretch the rock they travel through. Secondary waves, or S-waves, move the rock up and down, like a roller coaster, and also sideways at the same time. Other types of shock waves, called surface waves, have other shaking effects. These do not occur in all earthquakes, but when they do occur, they are capable of causing damage far away from the epicentre.
