The intensity of the earthquake is measured by the damage done to the ground and humanity. The intensity is measured by an instrument known as seismograph, where the shocks are recorded.
Seismometers record local shocks only. The instrument consists of a steel pillar attached firmly to the floor of the building of the observatory. The pillar is connected by a wire to a cylindrical rod with a sharp point at its upper end and with a long pointer hanging a pencil point at the other end.
The pencil point touches a rotating drum and records the oscillatory movement on the paper fixed on the drum. A chronograph records the time of the arrival of the shock. The record on the sensitized paper is known as seismogram.
Rossi and Forel proposed a scale with ten divisions of the different intensities of the shocks. They are (1) Microseismic (2) Extremely feeble, (3) Very feeble, (4) Feeble, (5) Moderate, (6) Fairly strong, (7) Strong, (8) Very strong, (9) Extermely strong and (10) Shock of extreme intensity. Microseismic shocks are recorded by delicate instruments only, whereas in case of shock of extreme intensity, general destruction of the buildings and ground takes place and produces land slides in hilly terraines.
Mercalli, an Italian scientist, put forward another scale. It has twelve divisions as follows:
Table-6: Mercalli Scale
(i) Instrumental – Recorded by Seismographs
(ii) Very feeble – Perceived only by sensitive persons.
(iii) Feeble – Perceived by persons at rest
(iv) Moderate – Perceived by persons in motion
(v) Fairly strong – Wakes persons, rings bells
(vi) Strong – Slight damage to building
(vii) Very strong – Produces cracks in walls
(viii) Destructive – Throws chimneys
(ix) Ruinous – Over throws buildings
(x) Disastrous – General destruction of the buildings
(xi) Vey disastrous – Few buildings are left, fissures on the ground are produced.
(xii) Catastrophic – Total destruction of the buildings and ground Charles Richter proposed another scale to measure the intensity of the earthquakes based on the magnitude which is known as Richter’s scale.
In case of the earthquakes, the potential energy is transformed into kinetic energy in the form of elastic seismic waves. Magnitude is a measure of the energy released during the earthquake.
It is defined as the logarithm of the maximum amplitude recorded by a standard Wood Anderson seismometer kept at a distance of 100 km from the epicentre. This scale is valid upto 600 km epicentral radiuses. The magnitude of the waves is divided from 0 to 9 in this scale.
In this scale the higher step represents an earthquake record ten times the previous step. That is an earthquake of magnitude 8 is ten times larger than that of magnitude 7 and one hundred times larger than that of magnitude 6. The quakes of magnitude less than 5 do not cause much damage. The strongest earthquake so far measured is 8.9 in the Richter’s scale.