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Earthquakes are tremors, which are produced by the passage of vibratory waves through the rocks of the earth. Man has always feared earthquakes. The devastation caused by earthquakes to the life of man and property is sometimes more than that caused by volcanic eruption.
Causes of Earthquakes
Volcanic Activity. It was Aristotle who for the first time related the phenomenon of earthquake to physical factors. According to him when the air, compressed within the interior of the earth, escapes out, it shakes some part of the land.
The emission of volcanic material reduces this pressure but the emission of hot gases, lava, etc., creates an earthquake wave in the surrounding areas. These waves were known as earthquakes.
Faulting. The main causes of volcanic activity are considered to be faulting. When a new fault is formed or there is a movement along an old fault, an earthquake always results. There was a movement along a distance of 435 km., of San Andrea's fault.
Isostatic Adjustment. The isostatic balance between the depressed and the raised up blocks of land on the surface of the earth is not always maintained. The sediment eroded from the continents and swept up to the sea disturbs the balance. The balance is regained when the higher blocks are raised up again. During this process earthquakes are produced at zones of weakness.
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