Disaster Resistant Buildings

Magnitude and Intensity

Magnitude is a quantitative measure of the actual size of the earthquake. Professor Charles Richter proposed the scale of magnitude that goes from 0 to 9. It is a geometric scale. Now this scale is known as Richter scale. It is obtained from the seismograph. It depends on waveform amplitude on epicentral distance. It is denoted by letter M followed by the number. An increase in magnitude by 1 implies 10 times higher waveform amplitude and about 31 times higher energy released. Thus energy released in M6 and M5 earthquake have the ratio 31, and M8 to M5 have the ratio 31 × 31 × 31. There are other magnitude scales, like the Body Wave Magnitude, Surface Wave Magnitude and Wave Energy Magnitude.
Intensity is a qualitative measure of the actual shaking at a location during an earthquake. Hence
for the same earthquake, it has different values at different places, highest value being at epicentre. This is a linear scale. 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 importantly on the type of structure.
Modified Mercalli Intensity (MMI) scale is commonly used to express the intensity. MMI scale is as given below:

I. Very slight, felt only by instruments
II. Felt by people resting
III. Felt by passing traffic
IV. Furnitures and windows rattle
V. Can be felt outdoors, clocks stop, doors swing
VI. Furnitures move about, cracks appear in walls
VII. People knocked over, masonry cracks and falls
VIII. Chimneys and monuments fall, buildings move on foundations
IX. Heavy damage to buildings, large cracks open on ground
X. Most buildings destroyed, landslides occur, water thrown out of lakes
XI. Catastrophic, railway lines badly bent
XII. Utter catastrophic, no building is left standing.

Koyna, Great Assam, Bihar and Shillong, Kashmir earthquakes had magnitude 8. Uttarakhand, Chamoli, Jabalpur, Latur, Gujarat approached levels 5.5 to 6.5.


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