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Size and frequency of occurrence of an earthquake

Size and frequency of occurrence of Earthquake:

Small earthquakes occur nearly constantly around the world in places like California and Alaska in the US as well as in Chile, Peru, Indonesia, Iran, the Azores in Portugal, New Zealand, Greece and Japane. Large Earthquakes occur less frequently. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are:

  • An earthquake of 3.7 – 4.6 every year
  • An earthquake of 4.7 – 5.5 every ten years
  • An earthquake 5.6 or larger every hundred years

The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past because of the vast improvement in instrumentation (not because the number of earthquakes has increased). It is estimated since 1900, there have been an average of 18 major earthquakes (magnitude 7.0 – 7.9) and one greate earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.

Earthquake Magnitude Scale

Magnitude Earthquake Effects Estimated Number
Each Year
2.5 or less Usually not felt, but can be recorded by seismograph. 900,000
2.5 to 5.4 Often felt, but only causes minor damage. 30,000
5.5 to 6.0 Slight damage to buildings and other structures. 500
6.1 to 6.9 May cause a lot of damage in very populated areas. 100
7.0 to 7.9 Major earthquake. Serious damage. 20
8.0 or greater Great earthquake. Can totally destroy communities near the epicenter. One every 5 to 10 years

Earthquake Magnitude Classes

Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude.

Class Magnitude
Great 8 or more
Major 7 – 7.9
Strong 6 – 6.9
Moderate 5 – 5.9
Light 4 – 4.9
Minor 3 -3.9

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