Earthquakes occur when two blocks of Earth's crust slip past each other suddenly. Tectonic plates fit together like pieces of a puzzle and are continually moving. The edges of tectonic plates are rough and sometimes stick, causing an earthquake when they break free.
There are many fault lines along tectonic plate boundaries. The location of an earthquake on the surface of the earth is the epicenter. However, the actual quake often happens deep in the crust below the surface at the hypocenter.
As of 2014, scientists are not able to forecast earthquakes. According to the U.S. Geological Survey, developing an accurate method of determining when to expect an earthquake is unlikely. At best, scientists are able to forecast that an earthquake is likely along a fault at some time in the future. Some earthquakes begin with foreshocks. They occur at the same location that the larger earthquake later takes place. However, until the larger earthquake occurs, scientists register foreshocks as the earthquake. After the larger earthquake occurs, the crust sometimes continues making more movement along the fault line, which causes aftershocks.
Scientists measure the magnitude of earthquakes using special instruments called seismographs. Readings from three or more seismographs in different areas around the quake help geologists determine the epicenter.