As we know, an earthquake invariably involves the sudden release of a certain amount of energy stored in the rocks beneath the earth’s surface. The potential energy is transformed into kinetic energy in an earthquake, which is propagated into all directions from the focus in the form of elastic seismic waves. Magnitude is a measure of the energy released during the earthquake. This is determined on the basis of amplitude of seismic waves recorded as seismogram.
Professor Charles Richter of the California Institute of Technology proposed a scale of earthquake magnitude in 1935 to indicate the quantity of energy released by a single earthquake. This is a numerical scale of magnitudes from 0 to 9, with higher numbers indicating larger earthquakes. This scale is commonly called the Richter-scale.
Richter has numbered the scale in steps, with each step representing an earthquake record ten times larger than the previous step, for example an earthquake of magnitude ‘ 8’ is ten times larger than an earthquake of magnitude ‘7’ and hundred times larger than that of magnitude ‘6’ and thousand times larger than the magnitude of ‘5.’
Since the Richter scale is logarthmic, the difference between two consecutive whole numbers on the scale means an increase of ten times in the amplitude of the earth’s vibrations. The largest earthquake so far measured is 8.6. Earthquakes up to ‘6’ on the Richter scale do not cause serious damages.
Location of Epicentre
There are two principal methods to locate the epicentre of an earthquake viz. (i) Plotting of isoseismal and (ii) Determining the position of the epicentre through mathematical means using the seismograms of various recording stations for a particular earthquake.
Ploting of Isoseismals
As it has already been indicated isoseismals are lines joing points of equal intensity of an earthquake. The isoseismal lines encircle one another and the isoseismal for the highest intensity lies just around the epicentre. In this method, the location of the epicentre can be made approximately.