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...