Earthquakes occur when stress builds up in the Earth's crust and causes rock to break along faults. There are three main types of faults - strike-slip faults, where rock slides past each other; normal faults, where one side drops down; and reverse faults, where one side is pushed up. Seismic waves called P waves and S waves travel out from the earthquake focus and can be measured with seismographs. The intensity and magnitude of earthquakes are measured using scales like the Mercalli and Moment Magnitude scales.