FREE ESSAY ON WHAT IS AN EARTHQUAKE?

WHAT IS AN EARTHQUAKE?

Earthquakes
What is an Earthquake? 
An Earthquake is the shaking of the earth's surface caused by rapid movement of the
earth's rocky outer layer. The sudden shaking of the ground that occurs when masses of
rock change position below the Earth's surface is called an earthquake. The shifting
masses send out shock waves that may be powerful enough to alter the surface of the
Earth, thrusting up cliffs and opening great cracks in the ground. 
Earthquakes, called temblors by scientists, occur almost continuously. Fortunately, most
of them can be detected only by sensitive instruments called seismographs. Others are
felt as small tremors. Some of the rest, however, cause major catastrophes. They produce
such tragic and dramatic effects as destroyed cities, broken dams, earth slides, giant
sea waves called tsunamis, and volcanic eruptions. A very great earthquake usually occurs
at least once a year in some part of the world. 
On the average about 10,000 people die each year as a result of earthquakes. According to
a study carried out by the United Nations and covering the years 1926 to 1950, there were
350,000 deaths, and property damage losses exceeded 10 billion dollars. As cities expand
to shelter a rapidly increasing world population, it is likely that there will be even
greater losses of life and property in spite of improved methods of detection and better
warning systems. Mankind has long been concerned about earthquake hazards. The oldest
chronicle comes from the Chinese as early as the Shang Dynasty more than 3,000 years ago.

Although it is certain that violent Earth tremors in themselves are destructive, there
are often other kinds of Earth movements that are triggered by earthquake shock waves.
Thus, the violent shaking that accompanies many earthquakes often causes rockslides, snow
avalanches, and landslides. In some areas these events are frequently more devastating
than the Earth tremor itself. 
Floods and fires are also caused by earthquakes. Floods arise from tsunamis along coast
lines, from large-scale seiches in enclosed bodies of water such as lakes and canals, and
from the failure of dams. Fire produced the greatest property loss following the 1906 San
Francisco earthquake, when 521 blocks in the city center burned uncontrollably for three
days. Fire also followed the 1923 Tokyo earthquake, causing much damage and hardship for
the citizens. 
Causes 
Most of the worst earthquakes are associated with changes in the shape of the Earth's
outermost shell, particularly the crust. These so called tectonic earthquakes are
generated by the rapid release of strain energy that is stored within the rocks of the
crust, which on continents is about 22 miles thick. A small proportion of earthquakes are
associated with human activity. Dynamite or atomic explosions, for example, can sometimes
cause mild quakes. The injection of liquid wastes deep into the Earth and the pressures
resulting from holding vast amounts of water in reservoirs behind large dams can also
trigger minor earthquakes. 
The strongest and most destructive quakes, however, are associated with ruptures of the
Earth's crust, which are known as faults. Although faults are present in most regions of
the world, earthquakes are not associated with all of them. Pressures from within the
Earth strain the great rock masses beneath the Earth's surface. The strain builds until
suddenly the masses move along faults, thereby releasing energy. The masses slip and
slide in opposite directions along this fracture in the rock, shaking the ground above.
The masses may move up and down, sideways, or vertically and horizontally. On the Earth's
surface displacement of the ground may vary from several centimeters to many meters. Some
fault lines appear on the surface of the Earth. 
Shock Waves 
The shifting rock in an earthquake causes shock waves called seismic waves to spread
through the rock in all directions. In a great earthquake shocks may be felt by people
thousands of kilometers away from the center . Detection and recording devices calle
Bibliography
The shifting rock in an earthquake causes shock waves called seismic waves to spread
through the rock in all directions. In a great earthquake shocks