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
