The destructiveness of an earthquake is called its intensity. Remember that the intensity theoretically decreases outwards inversely as the square of the distance.
Obviously, the destructiveness of an earthquake goes on decreasing outwards from the source of disturbance. Severity of an earthquake is measured in two ways:
(i) In physical terms, the intensity or destructiveness is determined partly by the duration and number of jerks and tremors, but mainly by the maximum rate of change of these movements of the ground, i.e. by the maximum acceleration which can be estimated from the seismograph records.
Thus, it is clear that the intensity of an earthquake in an assessment of the severity of ground motion at a given location.
(ii) The destructiveness of an earthquake is also measured in relation to its effects on the people living in the earthquake affected area. However, the damage to life and property depends on population density, building standards and the nature of the ground.
The above scale assigns relative numbers I to XII to earthquake intensity. Numbers I, II and III indicate very mild shocks felt only by a few persons, but normally are recorded in a seismograph.
The next numbers IV, V, VI and VII show sufficient intensity. The doors and windows rattle and only buildings with weak structure are damaged. The last five numbers (VIII to XII) represent most destructive earthquake shocks.
Later on, in 1935 C.F. Richter prepared a different type of Logarithmic scale which is based on quantitative measurement of the intensity of an earthquake. It is numbered 1 to 10 according to the degree destructiveness. So far the most destructive earthquakes have not exceeded 8.6 values in their magnitude.
It is well to remember that in 1965 Richter and his colleague Beno Gutenberg modified the original Richter Scale. This scale gives an idea of the energy released at the focus of an earthquake. Undoubtedly this scale measures the intensity of an earthquake.
The most important characteristic of this scale is that it has no fixed maximum or minimum. However, no earthquake rated as high as 8.4 on this scale has been exceeded until now. It may be stated that the earthquakes of magnitude 2 are the smallest and they can be easily detected by man without the aid of seismograph.
The Richter scale is logarithmic. Each whole number on this scale shows a 31.5 fold increase in the amount of energy that is released. For example, 3 on this scale demonstrate 31.5 times more energy than the number 2 and 992 times more energy than the number 1.
It is pertinent to point out that since rocks are not capable of accumulating more energy, the larger earthquakes having magnitude of more than 8.8 on the Richter scale are not likely to occur.