Earthquakes are caused by the buildup and sudden release of stress along faults within the Earth. As tectonic plates push against each other, stress accumulates until rocks break, causing vibrations called seismic waves. There are three types of faults defined by the direction of rock movement: normal faults occur when rocks pull apart due to tension, reverse faults occur when rocks are pushed together by compression, and strike-slip faults occur when rocks slide past one another due to shear forces. Seismographs are used to measure the seismic waves and locate the epicenter, or surface point above the underground hypocenter where rock movement begins.