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Tremors or vibrations of earth’s surface produced by internal forces.

The point of origin of earthquake is called Seismic focus. Most of the earthquakes originate at the depth of 50-100 km inside the earth.

The point on the earth’s surface vertically above the earth’s surface is called Epicentre.

The passage of earthquake waves is recorded by Seismograph.

The magnitude of waves is measured on Richter’s scale. For measurement of the intensity of the earthquake (damage caused), the Modified Mercalli Intensity Scale is used.

Types of Waves Earthquakes

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  1. Primary Waves (P-Waves): Travel from the point of happening by the displacement of surrounding particles. They are transmitted through solids, liquids and gases. Travels fastest.
  1. Secondary Waves (S-Waves): Travels through solids only. Thus they cannot pass through core.
  2. Surface Waves or Long Waves (L-Waves): Travels on earth’s surface and causes maximum destruction. They are recorded after the P and S waves.

Distribution of Earthquakes

  • Around the Pacific Ocean along a belt of volcanoes known as the Ring of Fire. 68 per cent of the volcanoes are experienced in this region.
  • From the middle of Asia (Himalayas, Caspian Sea) through the Mediterranean Sea to West Indies. 21 per cent earthquakes are experienced in the region.
  • Mid-Atlantic ridge belt which accounts for 11 per cent of the earthquakes.

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