Earthquake
Earthquakes occur almost every day all over the world. Most
of the time earthquakes are not strong enough to be felt by people, but the
shaking caused by an earthquake can be recorded by a seismometer. These
machines are located all over the world to ensure detection of earthquakes of
all strengths. Only occasionally will larger magnitude earthquake strike and
cause damage to the region. There are many faults around the world and
depending on where these faults are plays a major factor in determining where
an earthquake will occur. It is these faults that are the reason for
earthquakes.
An earthquake is the perceptible shaking of the surface of
the Earth, which can be violent enough to destroy major buildings and kill
thousands of people. The severity of the shaking can range from barely felt to
violent enough to toss people around. Earthquakes have destroyed whole cities.
They result from the 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 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 causes 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 of shaking is measured on the modified Mercalli scale.
The shallower an earthquake, the more damage to structures it causes, all else
being equal.
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.
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