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Difference Between Magnitude and Intensity of Earthquake

What is the difference between intensity and magnitude of earthquake

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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 distanceThe 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.
Fig. 1: The distribution of intensity at different places during an earthquake is shown graphically using isoseismal
Fig. 2: Seismograph

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

MMI V VI VII VIII IX X
PGA (g)0.03-0.040.06-0.070.10-0.15 0.25-0.30 0.50-0.55>0.60

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 – 3.0I
3.0 – 3.9II – III
4.0 – 4.9IV – V
5.0 – 5.9VI – VII
6.0 – 6.9VII – IX
7.0 and higherVIII or higher

Modified Mercalli scale of earthquake intensity

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