The type of earthquake determines the potential for generating a tsunami. Earthquakes below subduction zones can have large vertical movement (reverse fault) that generates tsunamis. This animation from the BBC demonstrates what happens. Earthquakes on a strike-slip fault have mostly horizontal movement and tend not to generate tsunamis. For example, the magnitude 8.1 Macquerie Island earthquake near New Zealand at 1:59 AM local time on 23 December 2004 did not generate a tsunami. If you are not familiar with these types of fault motion, see these animations.
The size of a tsunami depends partly on the size of an earthquake, usually given as a Richter Scale magnitude.
An increase of one unit of magnitude (for example, from 8.1 to 9.1) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released.
Where are earthquakes common?
Earthquakes are not randomly distributed around the earth. They occur most frequently along plate boundaries. Very large, deep earthquakes occur at convergent boundaries where ocean plates move beneath other plates, at regions called subduction zones.
The U.S. Advanced National Seismic System has maps fhowing distribution of earthquakes and cross-sectional plots of earthquake depth as a function of distance from trenches in three regions of the Pacific.
Large earthquakes, greater than magnitude 8.5 are rare. The Indian Ocean earthquake of 2004 was the third largest since 1900. A rough idea of frequency of occurrence of large earthquakes is given by the following table from the National Earthquake Information Center:
This table is based on data for a recent 47 year period. The rate of earthquake occurrence is highly variable and some other 47 year period could give quite different results.Ms Earthquakes per year ---------- ----------- 8.5 - 8.9 0.3 8.0 - 8.4 1.1 7.5 - 7.9 3.1 7.0 - 7.4 15 6.5 - 6.9 56 6.0 - 6.4 210
Revised on: 11 March, 2009