Earthquake Magnitude and Expected World Incidence

Earthquake vary enormously in strength. And to measure these wide variations, geologists used a logarithmic scale to express magnitude, which is called as Richter scale where a tenfold increase in wave amplitude corresponds to an increase of 1 on the magnitude scale. Thus, the amount of ground shaking for a 5-magnitude earthquake is 10 times greater than that produced by an earthquake having a Richter magnitude of 4.

Expected World Incidence

A major earthquake with a magnitude of 8.5 releases millions of times more energy than the smallest earthquakes felt by humans. The December 26, 2004 Indian Ocean tsunami was caused by an earthquake that is thought to have had the energy of 23,000 Hiroshima-type atomic bomb. The epicenter of the 9.0 magnitude quake was under the Indian Ocean near coastal areas of Aceh. Over 170,000 people were killed by tsunami in Aceh alone.

                         Sources: Edward J. Tarbuck. Earth: An Introduction to Physical Geology
                                       http://en.wikipedia.org/wiki/Aceh#Tsunami_disaster