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When a stone is thrown into a pool, a series of waves spreads through the water in all directions. Similarly, when rocks are suddenly disturbed, vibrations spread out in all directions from the source of the disturbance. An earthquake is the passage of these vibration.
The innermost core of the Earth is so hot that all rock there is liquid, or molten. Compared to core rock, the cooler rock of the Earth's crust is much more solid and brittle. Molten rock is also more fluid and moves easily, while moving plates of rock in the crust tend to jam, break apart, or get stuck as they move past each other. If these plates are unable to move the way they need to, stress builds up between them and they start to bend. If enough stress builds up to break the bond between the rocks, energy is released in the form of an earthquake.
Vibrations are set up in solid bodies by a sudden blow or by the scraping together of two rough surfaces. Corresponding causes of earthquakes in the earth's crust are volcanic explosions. Perceptible tremors are set up by the passage of trains and tanks, by avalanches and landslides, by rock falls in mines and caverns and by explosions of all kinds. When a munition factory explodes, the intensity of the resulting earthquake may be comparable with that of volcanic earthquakes, while that of the shock waves through rocks set up by the underground testing of H-bombs may be far more severe.
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 (35 kilometers) thick . A small proportion of earthquakes is associated with human activities, such as the filling of new reservoirs or the pumping of fluids deep into the Earth through wells . Dynamite or atomic explosions, can sometimes cause mild quakes. The injection of liquid wastes deep into the Earth and the pressures resulting from holding vast amount of water in reservoirs behind large dams can also trigger minor earthquakes. The collapse of underground old mine in England causes sporadic small earthquakes.
Those of volcanic origin are seldom very large or destructive. Such quakes originating as magma work their way upwards, filling the chambers beneath a volcano. As the flanks and summit of the volcano swell and are tilted, rupture of the strained rocks may be signalled by swarms of small earthquakes. For example, on the North Island of New Zealand, many small quakes occur daily.
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