earthquake classification according to its magnitude

Earthquakes are result of a sudden release of energy that creates seismic waves. Energy released from earthquakes includes (1) energy dissipated as heat through friction and (2) energy elastically radiated through the earth. Only the radiated energy can be measured since it propagates and shakes the earth surface.

Depending on their magnitude earthquakes are classified from minor to great:

CLASS          MAGNITUDE

 Great         -->    8 or more

 Major        -->    7 to 7.9

 Strong       -->    6 to 6.9

 Moderate  -->    5 to 5.9

 Light         -->    4 to 4.9

 Minor        -->    3 to 3.9

An approximated estimation of the number of earthquakes per year, its magnitude and effects:
Magnitude       Earthquake Effects         Estimated Number
2.5 or less           Usually not felt,                       900,000
2.5 to 5.4             Often felt, minor damage.        30,000
5.5 to 6.0             Slight damage to structures.        500
6.1 to 6.9             Damage in  populated areas.       100
7.0 to 7.9             Serious damage.                            20
8.0 or greater      Collapse of towns.             One every 5 to 10 years