MAGNITUDE AND ENERGY OF AN EARTHQUAKE

 

MAGNITUDE AND ENERGY OF AN EARTHQUAKE

A record of intensities of past earthquakes at any particular site or region is extensively used to estimate the seismic risk in future, but this does not describe the possible size of an earth quake in absolute terms. For this purpose, Richter suggested that the Magnitude of an earthquake be standardised as "Logarithm (to the base 10) of the maximum amplitude 4 of the ground motion as recorded in microns at a distance of 100 km from the epicentre on a Wood-Anderson Type Torsion Seismograph having damping equal to 80% of critical, natural period of 0.8 second and magnification of 2800" i.e., M=log₁04. Since the distance of the instrument from the epicentre will.

usually not be exactly 100 km, the following equation will determine the Magnitude 'M':

 

M = logo₁₀ A - log₁₀ A₀                                                                              ...(1)

where A is the trace amplitude ) at any station and log 4, the distance correction given in Fig. 1. for near earthquakes as well as for distant earthquakes. A correction for the type of instrument or reliability of observations depending upon local conditions is further applied to get the true magnitude M. Values obtained at various stations are then compared and a mean value of magnitude is assigned to the earthquake.

Fig. 1. :Distance Correction for Magnitude Determination

Felt earthquakes may have a magnitude from 3 upwards to almost 9. but no shock smaller than 5 causes appreciable damage. The extent of damage depends upon the depth of focus and a shock of magnitude 9.0 will envelope a vast area of earth. Very shallow shocks even of small size could cause damage locally. Usually, earthquakes have their focus not shallower than about 5 km and could go deeper than 300 km.

An earthquake of magnitude 5.0 usually causes damage within a radius of about 8 km but that of magnitude 7.0 may cause damage in a radius of 80 km and that of 8.0 over a radius of 250 km. The felt areas of these earthquakes will have their radius equal to about 150, 400 and 600 km respectively. During the Koyna earthquake of Dec. 11, 1967, (M-6.5) the felt area had a radius of about 400 km and damage area was spread in a radius of about 60 km dáe to special geological features. A relationship between strain energy released by an earth quake and its magnitude is given by Richter as follows:

 

log₁₀ E = 11.4 + 1.5 M                                                ...(2)

Energy released in earthquakes of different magnitudes is presented in Table 1.which would give an idea of their relative destructive power. 

TABLE 1. MAGNITUDES OF EARTHQUAKES AND ENERGY RELEASED

M (Richter) 5.0	5.0	6.0	6.5	7.0	7.5	8.0	8.4	8.6
E(1020 ergs)	0.08	2.5	14.1	80	446	2500	10000	20000

 Note: Energy released by an earthquake of magnitude 7.0 would be of the order of 80x100²⁰ ergs.