Earthquake Magnitude

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Earthquake Magnitude Scale

Earthquake magnitude is the measure of the “size” or amplitude of the seismic waves generated by the sources of the earthquake and recorded by the seismograph. As the magnitude of the earthquake changes tremendously, it is necessary to compare in order to compress the range of amplitude of waves measured on seismograph using the Mathematical device. 

In 1933, the American seismologist Charles. F. Righter introduced the earthquake magnitude scale (known as the Richter magnitude scale) as the logarithm to the base 10 of the maximum seismic waves amplitude reported on a standard seismograph (in thousandths of ml) at a distance of 60 miles or 100 km from the earthquake epicentre. 

Reductions in amplitude are observed at various distances to the expected amplitude at a standard distance of 100 km is formulated based on the empirical tables. Righter earthquake magnitude scales are computed based on the assumption that the ratio of the amplitude of the maximum wave at two given distances is equal for all earthquakes and is different from the azimuth.

Due to the various shortcomings of the Richter scale, most seismologist authorities now use another scale known as the moment magnitude scale to record the magnitude of an earthquake.


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What is the Magnitude of Earthquakes?

The earthquake magnitude is a measure of the amount of seismic energy released by it, so it is a quantitative scale. Magnitude is the most commonly used measure to describe the overall strength or size of an earthquake. The magnitude of an earthquake is expressed in decimal fractions and whole numbers. For example, a magnitude of 5.3 is considered a moderate earthquake whereas a magnitude of 6.3 is a strong earthquake. Due to the logarithm basis of scale, each whole number increases in magnitude represents a 10-fold increase in measured amplitude as measured on a seismograph.

In modern times, several different magnitude scales for measuring the relative size of an earthquake are used by scientists and engineers. An earthquake scale for measuring magnitude has no lower or upper bounds. Sensitive seismographs can even record earthquake magnitudes of negative values and have reported magnitude up to about 9.0 (For example, the 1906 San Francisco earthquake had a Richter magnitude of 8.5).


Earthquake Frequency

An earthquake frequently defines how often a given earthquake with a certain magnitude comes about. On average, an earthquake with a magnitude of 2 or less comes about several hundred times a day throughout the world.

However, a major earthquake range with a magnitude of 7.0 - 7.9 occurs more than once per month throughout the world. Whereas, an earthquake ranges to a great extent with a magnitude of 8.0 or greater or comes about only once a year.

Knowing the earthquake frequency is important for engineers as they not only strengthen a building against earthquake shock but also minimize the force a building is subjected to. To minimize the loss, they install a base isolator that helps to isolate the base of the buildings from the earth's movement. 


Earthquake Range in Magnitude Scale

Magnitude

Descriptor

Earthquake Effects 

Estimated Earthquake Per Year

2.5 or less

Very Minor

Usually not felt, but can be reported by a seismograph

900,000

2.5-5.4

Minor

Often felt, but cause only minor damages

30,000

5.5 - 6.0

Moderate

Slight damage to buildings and other structures

500

6.1 - 6.9

Strong

May cause a considerable amount of damage in an overpopulated area 

100

7.0-7.9

Major

Serious damage as earthquake is major

20

8.0 or greater 

Great

An earthquake to a great extent. Can destroy the communities near the epicentre  completely 

Once in 5-10 years


How Strong is a 3.5 Magnitude Earthquake?

An earthquake of magnitude 3.5 on the scale is considered to be minor. A 3.5 magnitude earthquake is often felt by people, but rarely causes any damage to the buildings. You can even observe the shaking of an object inside the buildings.  


Did You Know?

  • The largest-ever earthquake with a magnitude of 9.6 occurred in China in 1916.

  • The largest earthquake in the US with a magnitude of 9.2 struck the Prince Willian studio, Alaska on March 28, 1964, UTC.

  • An earthquake can occur in any type of weather.

  • It is estimated that approximately 500,000 earthquakes are detected in the world each year. 100,000 among those can be felt, and 100 of the earthquakes can cause damage.

  • The most deadly earthquake occurred in Shaanxi, China in 1556. It is estimated that 850,000 people were killed in that period.

  • Florida and North Dàkota are the places with the least earthquakes.

FAQs (Frequently Asked Questions)

1. What is an Earthquake?

Ans. An earthquake is the sudden, and instant shaking of the ground, caused by the sudden release of energy in the  Earth lithosphere that creates seismic waves. Earth ranges in size from those that are so weak that they cannot be felt to those that are so severe that they destroy the cities. 

2. Where Do Earthquakes Occur Most Frequently?

Ans. The uppermost layer of the Earth is made up of many tectonic plates that either slide towards each other or away from each other or under each other. Severe earthquakes usually come about along the boundaries of tectonic plates. The regions commonly affected by the earthquake include the west coast of North and South America, Indonesia, Japan, Central Asia and parts of China and Turkey, etc.

3. Why Does the Moment Magnitude Earthquake Scale are More Preferred?

Ans. The moment magnitude scale abbreviated as Mw is more preferred because they provide accurate measurements for the wide range of earthquake size as compared to the Richter scale, and is applicable globally. The calculation of earthquake size using this scale is based on earthquake seismic movement rather than the amplitude of seismic waves recorded by a seismograph. The moment magnitude scale is the only reliable scale,  capable of measuring the largest and the destructive earthquake (that is greater than magnitude 8).

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