The jolts of an earthquake rarely last as long as a minute but the effects are devastating.In many cases, it is preceded by minor tremors before the violent shocks. It is estimated that million or more tremors occur somewhere in the earth’s crust every year. Some of them are so slight that they can be detected only with sensitive instruments and some of them are quite powerful.
We know that the mantle is in a molten state. We are familiar with the structure of the surface beneath the earth. Earth’s outer crust consists of dozens of enormous plates that float on the mantle that is below the outer crust. As they slowly but continuously jostle each other, great stress builds up along the edges. Eventually the pressure becomes so intense that the plates give way with a jolt. This slippage relieves the stress, at least temporarily but causes the earth to shake.
Most of the earthquakes result from these movements of crustal plates. Small localized earthquakes can also be set off by volcanic eruptions and landslides. Earth tremors have also been triggered by underground nuclear blasts and even by the filling of reservoirs. But these are insignificant compared to the great quakes that take place along the crust and release tremendous amounts of energy.
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