Earthquakes are caused by the movement of tectonic plates underneath the earth's surface. When the plates squeeze or stretch against each other, they build up stress until the crust breaks, releasing energy in waves that cause the shaking we feel. There are four major types of faults where the broken rocks of the earth's crust slide past each other. Scientists use seismographs to measure and record earthquakes to help understand seismic activity and work on predicting quakes, although they cannot be predicted with complete accuracy currently.