Earthquake Hazards / Disasters
An earthquake is a major demonstration of the power of the tectonic forces caused by end genetic thermal conditions of the interior of the earth. An earthquake is motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking building apart and causing gaping fissures to open in the ground. The earthquake is a form of energy of wave motion transmitted through the surface layer of the earth in widening circles from a points of sudden energy release the focus.
The magnitude or intensity of energy released by an earthquake is measured by the RICHTER SCALE devised by Charles F Richter in 1935. The number indicating magnitude or intensity (M) on Richter scale ranges between 0 and 9 but in fact the scale has no upper limit of number because it is a logarithmic scale. It is estimated that total annual energy released by all earthquakes is about 10 25 ergs, and most of this is from a small number of earthquakes of magnitude over 7’ (A.N.Strahler and A.H. Strahler, 1976). The 1934 Bthar earthquake (India) measuring 8.4 magnitude on Richter Scale and Good Friday Earthquake of March 27, 1964. in Alaska, U.S.A. measuring 8.4 to 8.6 on Richter Scale may help in assessing the devastation caused by the energy release during earthquakes of varying magnitudes. The World’s largest and most intensive recorded earthquake was of the magnitude of 8.9 and the number of recorded earthquakes increase 10 times as magnitude decreases by one.
The quakes of magnitude 5.0 equal in energy to ordinary atomic
bomb. The atomic bomb hurled over Hiroshima (Japan) during
Second World War equaled the magnitude of 5.7 on Richter
Scale. The energy released from such earthquakes equals 8 x
Another scale of the measurement of the degree of destructiveness or intensity of earthquakes is MERCALLI SCALE. The degree of destructiveness on intensity of an earthquake depends on a variety of factors e.g. magnitude, distance from epicenter, acceleration, duration amplitude of waves, type of ground, water table, nature of geometries of the region concerned and the nature and type of constructions (such as building made of wood or bricks, earthen buildings made of muds, tin shades, huts etc.) affected by an earthquake.
The place of the origin of an earthquake is called FOCUS which is always hidden inside the earth but the depth of which varies from place to place. The deepest earthquake may have its focus at a depth of even 700 km. below the ground surface but some the major Himalayan earthquakes, such as the Bihar-Nepal earthquake of August 21,1988, had their focus around 20-30 km. deep. The place on the ground surface, which is perpendicular to the buried focus’ or hypocenter’ recording the seismic waves for the first time is called EPICENTRE. The seismic waves move away from the source of the earthquake (focus or hypocenter) in the form-of (1) primary or Pressure waves (P waves), (2) Secondary, Shear or Transverse waves (S waves) and (3) Long waves or surface waves (L waves). These seismic waves are recorded with the help of an instrument called SEIMOGRAPH or SEISMOMETER at the epicenter. The patterns of recorded seismic waves are studied and various definite information about the centre of the origin of the earthquake (focus or hypocenter), magnitude and destructive power of the earthquake etc. are received.
Magnitude and energy released during an earthquake is generally related to the effects of the earthquake in terms of human beings and his habitats. It is apparent from table that most of the earthquakes occurring on the earth are small. On an average about 80,000 earth tremors are recorded by seismographs each year but most of them are not felt by human beings. Beno Guttenburg and Charles F.Richter have stated that the size (magnitude M) and the logarithm of frequency (number of earthquakes N) of earthquakes in a given area of observation are linearly related. On the basis of this analysis one can calculate the probable return periods of earthquakes, or different magnitudes. Great earthquake, with magnitudes over 8, generally occur about once every 5 to 10 years. Earthquake of magnitude 8 or more has a probable return period of 50 years in North-East India. Real damages caused by earthquakes begin at magnitude 5 and continue to increase to nearly total destruction in the neighbouring settlements by the earthquakes with more than 8 magnitude. Maximum recorded magnitude is 8.9.