The term earthquake is applied to any tremor or shaking of the ground. Many earthquakes are so gentle as to pass almost unrecognised, others are sufficiently pronounced to excite general alarm, while some spread enormous destruction. Destructive earthquakes are usually confined to limited regions.
The usual phenomena recorded in well-known earthquakes are first a trembling, next one or more severe shocks, and then a trembling which gradually dies away. In most cases, each shock lasts only a few seconds, but the trembling that follow may continue for days, weeks, or even months.
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Noises of various kinds usually accompany an earthquake. They have been likened to the howling of storm, the growling of thunder, the clanking and clashing of iron chains, or the rumbling of heavy wagons along a road.
Such noises are conducted through the ground, or they may travel through the sea or air, and are often heard at great distances from the place where the shock is felt.
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Some earthquakes, however, are not accompanied by these noises. At the time of the terrible shock which destroyed Riobamba in Ecuador on February 4, 1797, a complete silence reigned.
Many changes are produced on the earth’s surface by earthquakes. They cause landslips and cracks in the earth, which will sometimes alter the drainage system of a country. They are frequently accompanied by great sea waves, which will often sweep rocks and sand great distances inland.
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Permanent elevations and depressions of land are sometimes caused. After the great earthquakes of 1750, the coast of Chili was found to have been permanently raised from three to four feet.
Well-known examples of permanent depressions are those of the Run of Kutch and the coastlands near Chittagong, which suddenly sank during the Bengal earthquake of 1762.
Earthquakes are most common in volcanic and mountainous regions, and many of them are no doubt due to volcanic action. These appear to originate in the sea, and may be due to the flashing into steam of the water which finds its way down through cracks to the underlying heated rocks.
Others appear to originate in volcanoes themselves, being due to the explosion of vapours which expand. Many other causes are ascribed, of which two may be mentioned.
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Some earthquakes may be due to the collapse of hollows beneath the ground, and others again to the snapping of strata which has been subjected to too great a strain. It is noticeable that most earthquakes occur during the cold months or winter.
Among destructive earthquakes in modern times may be mentioned the one that altered the Straits of Messina between Italy and Sicily in 1908, and the terrible upheaval in Japan in 1925, which destroyed whole towns and caused the death of thousands of people.