Earthquake
Earthquake means any sudden disturbance within
the Earth manifested at the surface by a shaking of the ground. This shaking,
which accounts for the destructiveness of an earthquake, is caused by the
passage of elastic waves through the Earth’s rocks. These seismic waves are
produced when some form of stored energy, such as much havoc as earthquakes.
Over the centuries, they have been responsible for damage to property. While
earthquakes lave inspired dread and superstitious awe since ancient times,
little was understood about them until the emergence of seismology at the
beginning of the 20th century. Seismology,
which involves the scientific study of all, aspects of earthquakes, has yielded
answers to such long-standing questions as why and how earthquakes occur. About
5000 earthquakes large enough to be felt or noticed without the aid of
instruments occur annually over the entire Earth. Of these, approximately 100
are of sufficient size to produce substantial damage if their centers are near
areas of habitation. Very great earthquakes occur at an average rate of about
one per year. Among the great earthquakes of the past are those of Lisbon in 1755: New Madrid, U.S., in December 1811 and January and
February 1812; sun Francesco in 1906; Tokyo- Yokohama in 1923; the coast of
Chile in 1960; south-central Alaska in 1964: Tang-Shan, china, in 1976; and Mexico
in 1985.
