MAGNITUDE AND INTENSITY

Magnitude is a quantitative measure of the real size of the earthquake. Professor Charles Richter proposed the scale of magnitude that goes in the range 0 to 9. It is a geometric scale. This scale is now known as Richter scale. It is get from the seismograph. It based on waveform amplitude on epicentre distance. It is mention by letter M which is followed by number. An increase in magnitude by 1 implies 10 times higher waveform amplitude and approximate 31 times higher energy released. Therefore energy released in M6 and M5 earthquake have the ratio 31, and M8 to M5 is having the ratio 31 × 31 × 31. There is other magnitude scales, such as the Surface Wave Magnitude, Body Wave Magnitude and Wave Energy Magnitude.

During an earthquake Intensity is a qualitative measure of the real shaking at a location. Therefore it has different values at different places, highest value being at epicentre. It is a linear scale for the same earthquake. It is assigned as Roman Capital Numbers from I to XII.

Intensity depends upon

1.   Amount of source energy released
2.   Distance between the source and the place of interest
3.   Geographical features of the media of travel and significantly on the type of structure. Commonly Modified Mercalli Intensity (MMI) scale is used to express the intensity. MMI scale is like given below:

Very slight, felt just by instruments

1. Felt by passing traffic
2. Felt by people resting
3. Can be felt, clocks stop, outdoors ,doors swing
4. Furnitures move about, cracks appear in walls
5. Furnitures and windows rattle
6. Chimneys and monuments fall, buildings move on foundations
7. Most buildings destroyed, landslides take place, water thrown out of lakes
8. Heavy damage to buildings, large cracks open on ground
9. Utter catastrophic, no building is left standing.
10. Catastrophic, railway lines badly bent
11. People knocked over, masonry cracks and falls