Earthquakes may be caused by natural reasons or due to man-made activities. Natural causes are tectonic forces or volcanic eruption and man-made activities such as reservoir-associated forces.
1. Tectonic Earthquakes
Earthquakes are mainly caused due to sudden movement along faults which in turn due to tectonic origin. Such earthquakes generally result from sudden yielding to strain produced on the rocks by accumulation of stresses. Because of this the rock break along the weakest plane or otherwise and produces relative displacement of the rocks. Along the fault-planes the movement occurs after overcoming the frictional resistance along the fault-plane. Earthquakes due to fault line failure is an established fact (Parbin Singh, 2012).
2. Volcanic Earthquakes
Earthquakes associated with volcanoes are more localised. Compared to failure along faulting planes, the extent of damage and the intensity of wave produced are generally less. Volcanic earthquakes may be caused due to one of the following mechanisms:
- Explosion of volcano may take place due to the relax and expansion of gases and lavas.
- Faulting may also occur within a volcano and thereby causing high pressures in the chamber of molten rock.
- Centre of volcano may collapse and thereby extrusion of gases and molten matter.
3. Reservoir-associated Earthquakes
Only during the second half of the twentieth century, a new class of earthquake associated with reservoir has been recognised. It is believed to have caused due to impounding of water in artificially created reservoirs. Areas which were region of seismic activity (discussed elsewhere) have shown sign of disaster due to earthquake. Seismic shocks associated with filling of water in reservoirs have also been recorded in different parts of the world.
Reasons for such earthquakes have been identified due to (i) Sagging effect of the load and (ii) Increased pore pressures (Parbin Singh, 2012).
29.6.2 Magnitude of Earthquake
Magnitude of an earthquake is a measure of the amount of ground shaking based on the amplitude of elastic wave it generates. Richter’s magnitude scale, named after Prof. Charles Richter, a geologist is most often used. The Richter scale starts from 2, and there is no upper limit. Table 29.4 gives the description of an earthquake in relation to its magnitude on the Richter scale.
Table 29.4 Magnitude of an earthquake
The Richter scale is a logarithmic one; that is, an earthquake of magnitude 4 causes 10 times as much ground movement as one of magnitude 3, 100 times as much as one of magnitude 2, and so on. The Richter scale is widely used throughout the world.
Seismograph is an instrument designed to record earth motion set up by seismic waves. The actual record of motion produced by a seismograph is called a seismogram. Seismograph is designed to record both the horizontal and vertical component
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