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.

1. Earthquakes