An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the
Earth's crust that creates
seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured using observations from
seismometers. The
moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the
Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible and magnitude 7 and over potentially cause serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a
9.0 magnitude earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified
Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.
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At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the
epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a
tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
In its most general sense, the word
earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological
faults, but also by other events such as volcanic activity, landslides, mine blasts, and
nuclear tests. An earthquake's point of initial rupture is called its
focus or
hypocenter. The
epicenter is the point at ground level directly above the hypocenter.
