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......
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