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An earthquake is a tremor of the earth's surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock and resulting shock waves. In spite of extensive research and sophisticated equipment, it is impossible to predict an earthquake, although experts can estimate the likelihood of an earthquake occurring in a particular region.
In 1935, American seismologist Charles Richter developed a scale that measures the magnitude of seismic waves. Called the Richter scale, it rates earth tremors on a scale from 1 to 9, with 9 being the most powerful and each number representing an increase of ten times the energy over the previous number. According to this scale, any quake that is higher than 4.5 can cause damage to stone buildings; quakes rated a magnitude of 7 and above are considered very severe. A less-known scale, the Mercalli scale, was devised by Italian seismologist Giuseppe Mercalli to measure the severity of an earthquake in terms of its impact on a particular area and its inhabitants and buildings.
Some earthquakes are too small to be felt but can cause movement of the earth, opening up holes and displacing rocks. Shock waves from a very powerful earthquake can trigger smaller quakes hundreds of miles away from the epicenter. Approximately 1,000 earthquakes measuring 5.0 and above occur yearly. Earthquakes of the greatest intensity happen about once a year and major earthquakes (7.0-7.9) occur about 18 times a year. Strong earthquakes (6.0-6.9) occur about 10 times a month and moderate earthquakes (5.0-5.9) happen more than twice daily. Most earthquakes are not even noticed by the general public, since they happen either under the ocean or in unpopulated areas. Sometimes an earthquake under the ocean can be so severe, it will cause a tsunami, responsible for far greater damage.
The greatest danger of an earthquake comes from falling buildings and structures and flying glass, stones and other objects