A shaking or trembling of a portion of the Earth. A pressure wave in the Earth's crust caused by a deep-seated disturbance.
Earthquakes are sudden movements in the ground which occur when parts of the Earth's crust move or fracture.
Most earthquakes happen when the plates forming the Earth's crust push against each other or when one plate moves under another. Some earthquakes occur when molten rock rises from beneath the Earth's crust, pushing the plates apart.
According to Indian mythology the world rests on the head of a great elephant, 'Muha-pudma', and when, for the sake of rest, the huge monster refreshes itself by moving its head, an earthquake is produced. The lamas say that the earth is placed on the back of a giant frog, and when it moves its limbs or its head it shakes the earth. Other Eastern myths place the earth on the back of a tortoise. Greek and Roman mythologists ascribe earthquakes to the restlessness of the giants whom Jupiter buried under high mountains.
Earthquakes are vibrations produced in the earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can range from barely noticeable to catastrophically destructive. Six kinds of shock waves are generated in the process. Two are classified as body waves that is, they travel through the earth's interior and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. Primary or compressional waves (P waves) send particles oscillating back and forth in the same direction as the waves are traveling, whereas secondary or transverse shear waves (S waves) impart vibrations perpendicular to their direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide.
The size and extent of an earthquake is measured in units on the Richter Scale. This scale, from 0 to over 8, measures the amount of energy released. Every year there are more than 300,000 Earth tremors which have a magnitude of 2 to 2.9. However, earthquakes measuring 8.5 or higher happen only every 5 to 10 years.
The extreme force of an earthquake and its effects are measured on the Mercalli Scale. This measures the intensity of an earthquake at particular locations on the Earth's surface. Below are listed the numbers on the Mercalli Scale and the physical characteristics of earthquakes:
1 - Instrumental; animals sense tremors, tremors recorded on seismographs.
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