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 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 magnitudes 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 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.
.............By Faith Gabriella.
