Magnitude and intensity measure various characteristics of earthquake. The former measures the energy released at the source of the earthquake. However, the latter measures the strength of shaking generated by the earthquake at a certain location.
The magnitude of earthquake is determined from measurements on seismographs, whereas the intensity is determined from effects on people, human structures, and the natural environment.
Table 1 presents the difference between magnitude and intensity of earthquake.
The correlation between intensity and magnitude of earthquake are provided in Table 4.
Table-1: Difference Between Magnitude and Intensity of Earthquake
|Magnitude of earthquake||Intensity of earthquake|
|It is a quantitative measure of the actual size of the earthquake.||It is a qualitative measure of the actual shaking at a location during an earthquake.|
|Normal numbers are used to determine the magnitude of earthquake.||Intensity is assigned as Roman Capital Numerals.|
|The Richter Scale (called Local Magnitude scale) is used to measure the magnitude. There are other magnitude scales, like the moment magnitude, Body Wave Magnitude, Surface Wave Magnitude, and Wave Energy Magnitude. These numerical magnitude scales have no upper and lower limits; the magnitude of a very small earthquake can be zero or even negative. Table 3 presents classification of earthquake based on their magnitude along with its global occurrence.||There are many intensity scales. Two commonly used ones are the Modified Mercalli Intensity (MMI) Scale and the MSK Scale. Both scales are quite similar and range from I (least perceptive) to XII (most severe).|
|The scale is obtained from the seismograms and accounts for the dependence of waveform amplitude on epicentral distance||The intensity scales are based on three features of shaking – perception by people and animals, performance of buildings, and changes to natural surroundings.|
|When an earthquake occurs, its magnitude can be given a single numerical value on the Richter Scale.||When an earthquake occurs, its intensity is variable over the area affected by the earthquake, with high intensities near the epicenter and lower values further away. These are allocated a value depending on the effects of the shaking.|
|The magnitude of is not the base of design of structures since the same magnitude would have various intensities at different locations.||Structures are designed to withstand particular levels of intensity of shaking, and not so much the magnitude. The peak ground acceleration (PGA) is one way of quantifying the severity of the ground shaking which is used in the earthquake resistance of structures. Table 3 provide correlation between PGA and earthquake intensities|
|An increase in magnitude (M) by 1.0 implies 10 times higher waveform amplitude and about 31 times higher energy released.||The level of earthquake intensities is discussed blow.|
|Seismographs used to record earthquakes magnitudes.||The intensity value is determined from the observable effects of the shaking on people, on manmade structures and their contents, and on the landscape.|
Table 2: Global occurrence of earthquakes
|Group||Magnitude||Annual Average Number|
|Great||8 and higher||1|
|Major||7 – 7.9||18|
|Strong||6 – 6.9||120|
|Moderate||5 – 5.9||800|
|Light||4 – 4.9||6,200 (estimated)|
|Minor||3 – 3.9||49,000 (estimated)|
|Very Minor||< 3.0||M2-3: ~1,000/day; M1-2: ~8,000/day|
Table 3: PGAs during shaking of different intensities
Intensities that are typically observed at locations near the epicenter of earthquakes of different magnitudes.
Table 4: Correlation between Magnitude and Intensities of Earthquakes
|Magnitude|| Typical Maximum|
Modified Mercalli Intensity
|1.0 – 3.0||I|
|3.0 – 3.9||II – III|
|4.0 – 4.9||IV – V|
|5.0 – 5.9||VI – VII|
|6.0 – 6.9||VII – IX|
|7.0 and higher||VIII or higher|
Modified Mercalli scale of earthquake intensity
- I. Not felt. Marginal and long-period effects of large earthquakes.
- II. Felt by persons at rest, on upper floors, or otherwise favourably placed to sense tremors.
- III. Felt indoors. Hanging objects swing. Vibrations are similar to those caused by the passing of light trucks. Duration can be estimated.
- IV. Vibrations are similar to those caused by the passing of heavy trucks (or a jolt similar to that caused by a heavy ball striking the walls). Standing automobiles rock. Windows, dishes, doors rattle. Glasses clink, crockery clashes. In the upper range of grade IV, wooden walls and frames creak.
- V. Felt outdoors; direction may be estimated. Sleepers awaken. Liquids are disturbed, some spilled. Small objects are displaced or upset. Doors swing, open, close. Pendulum clocks stop, start, change rate.
- VI. Felt by all; many are frightened and run outdoors. Persons walk unsteadily. Pictures fall off walls. Furniture moves or overturns. Weak plaster and masonry cracks. Small bells ring (church, school). Trees, bushes shake.
- VII. Difficult to stand. Noticed by drivers of automobiles. Hanging objects quivering. Furniture broken. Damage to weak masonry. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices. Waves on ponds; water turbid with mud. Small slides and caving along sand or gravel banks. Large bells ringing. Concrete irrigation ditches damaged.
- VIII. Steering of automobiles affected. Damage to masonry; partial collapse. Some damage to reinforced masonry; none to reinforced masonry designed to resist lateral forces. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed pilings broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.
- IX. General panic. Weak masonry destroyed; ordinary masonry heavily damaged, sometimes with complete collapse; reinforced masonry seriously damaged. Serious damage to reservoirs. Underground pipes broken. conspicuous cracks in ground. In alluvial areas, sand and mud ejected; earthquake fountains, sand craters.
- X. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, and so on. Sand and mud shifted horizontally on beaches and flat land. Railway rails bent slightly.
- XI. Rails bent greatly. Underground pipelines completely out of service. XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into air.