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An earthquake (also known as a quake, tremor or
temblor) is the perceptible shaking of the surface of the
Earth, resulting from the sudden release of energy in the
Earth's crust that creates seismic waves. Earthquakes
can be violent enough to toss people around and
destroy whole cities. 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 nationalseismological observatories are
measured mostly on the local magnitude scale, also
referred to as the Richter magnitude scale. These two
scales are numerically similar over their range of validity.
Magnitude 3 or lower earthquakes are mostly almost
imperceptible or weak and magnitude 7 and over
potentially cause serious damage over larger 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 2014), and it was the largest Japanese
earthquake since records began. Intensity ofshaking is measured on the modified Mercalliscale. 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 generalsense, 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 initialrupture is called its focus or hypocenter. The epicenter is the point at ground level
directly above the hypocenter