An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity, or seismic activity, of an area is the frequency, type and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling. An earthquake is a violent movement of the rocks in the Earth's crust which create waves of energy travelling through the Earth. Earthquakes are usually quite brief (and even somewhat repeated itself, known as an aftershock), but may repeat over a period of time. The study of earthquakes is called seismology. Earthquakes usually occur where tectonic plates are in constant motion, in which these areas of constant motion (or violent motion) are usually at the edges of the plates. For example, several and several earthquakes have been recorded in the Western part of South America due to the constant motion between the South American plate and the Nazca plate. Earthquakes usually occur on faults, which are breaks in the Earth's crust formed by the sliding, pushing, or pulling of tectonic plates.