When the rocks at the joint of two plates of lithosphere are sufficiently slippery, then the convection currents just cause the plates to glide gently past one another thereby releasing the internal pressure. In some cases, however, the rocks at the joint of two plates of the lithosphere interlock into one another strongly. They do not move at all and resist pressure from within the earth’s mantle. These internal pressures go on building over hundreds of years. Ultimately the internal pressure becomes so much that it exerts an intolerable strain on the joint of the two plates of the lithosphere and breaks them apart.
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A tremendous amount of energy is released which produces shock waves through the lithosphere. The lithosphere at that place starts shaking with the intensity of these waves and we say that an earthquake has occurred. Thus, whenever there is a sudden displacement of a part of the lithosphere crust due to internal forces, it causes tremors or waves which travel in all directions from the centre of disturbance. These sudden tremors are called earthquakes.
The centre from which the earthquake waves originate is called seismic focus. The seismic focus lies within the crust of earth. The point on earth’s surface, vertically above the focus is called epicentre. Most of the earthquakes have a seismic focus at depths of less than 60 kilometres from the surface of earth. The intensity of earthquake tremors is maximum near the epicentre and decreases with distance from the epicentre. The intensity of earthquake waves and its time of occurrence is recorded by an instrument called seismograph. Earthquakes occur in unstable portions of the earth’s crust.
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Earth movements along lines of weakness and volcanic eruptions cause tremors on the earth’s surface. While hundreds of mild earthquakes occur daily, strong earthquakes which cause large scale damage to life and property are less frequent. Passage of earthquake waves may cause vertical and horizontal displacement of the earth’s surface. Cracks or fissures may appear in the earth’s surface over long distances. Due to earthquakes, river courses may be altered leading to sudden floods: Land slides may also occur due to earthquakes which may block the rivers to form lakes. Roads, railway lines, and buildings are damaged causing extensive damage to human life and property. In cities, water pipe lines and gas pipelines may get disrupted.
When an earthquake occurs, a tremendous amount of energy is released which sends shock waves all around. The intensity of earthquake is indicated by the amount of energy released when the plates of lithosphere give way (break) due to pressures from within the earth. The intensify of an earthquake is measured on Richter Scale. The earthquakes which measure upto 3 on the Richter scale are said to be very mild and do not cause any damage to life and property. The shocks of such earthquakes are so weak that sometimes they are not even noticed by the people.
An earthquake which measures 8 or more on Richter scale is very severe. In this case the affected part of the earth’s surface shakes violently. Such earthquakes which measure 8 or more on Richter scale can destroy entire cities and villages causing a great loss to life and property. The Richter scale for measuring the intensity of earthquakes was invented by a scientist called C.F. Richter.