earthquake

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 or seismic activity of an area refers to the
frequency, type and size of earthquakes experienced over a period of
time. Earthquakes are measured with a seismometer; a device which also
records is known as a seismograph. The moment magnitude (or the related
and mostly obsolete Richter magnitude) of an earthquake is
conventionally reported, with magnitude 3 or lower earthquakes being
mostly imperceptible and magnitude 7 causing serious damage over large
areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and
sometimes displacing the ground. When a large earthquake epicenter is
located offshore, the seabed sometimes suffers sufficient displacement
to cause a tsunami. The shaking in earthquakes can also trigger
landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any
seismic event—whether a natural phenomenon or an event caused by
humans—that generates seismic waves. Earthquakes are caused mostly by
rupture of geological faults, but also by volcanic activity,
landslides, mine blasts, and nuclear tests. An earthquake's point of
initial rupture is called its focus or hypocenter. The term epicenter
refers to the point at ground level directly above the hypocente