Earthquake is a sudden movement in Earth's crust
which produces vibration. These vibrations may be
detectable only by sensitive instruments called
seismographs, or else they may kill thousands as they
destroy cities. Many earthquakes are caused when
rocks move along faults. Severe earthquakes are most
common near the edges of the "plates" in the Earth's
crust, such as around the Pacific Ocean, along the
mid-Atlantic ridge and in the Mediterranean and South-
West Asian regions. They occur when the plates move.
So me earthquakes are caused by volcanic action,
explosions and other factors, Earthquakes under the
sea trigger off huge waves called tsunamis.