Tsunami warning center scientists usually measure an earthquake's "size" with the moment magnitude scale rather than the older but more famous Richter magnitude scale. The moment magnitude scale is better suited for measuring the "sizes" of very large earthquakes and its values are proportional to an earthquake's total energy release, making this measurement more useful for tsunami forecasting. This measurement has been used since the 1970's with the introduction of more modern broadband seismometers. To read more about the Richter vs. Moment magnitude scales, click here. or watch a video here.
Moment magnitude numbers scale such that energy release increases by a factor of about 32 for each whole magnitude number. For example, magnitude 6 releases about 32 times as much energy as magnitude 5, magnitude 7 about 32 times as much as magnitude 6, and so on.
This animation graphically compares the relative "sizes" of some 20th and 21st century earthquakes by their moment magnitudes. Each circle's area represents its relative energy release, and its label lists its moment magnitude, its location, and the year it happened.
You can view a YouTube version of this animation here.
Today the magnitudes of large earthquakes (M > 7.0) are reported as moment magnitudes.
Every whole number increase in moment magnitude represents an increase of about 32x in energy release, and two whole numbers represent an increase of 1000x in energy release. Therefore a M8.0 earthquake releases about 32 times as much energy as a M7.0 earthquake and 1000 times as much energy as a M6.0 earthquake.
The great majority of earthquakes occur at tectonic plate boundaries.
The majority of great earthquakes (magnitude 8.0 or larger) are megathrust earthquakes that occur at convergent plate boundaries, also called destructive margins or subduction zones.
For an earthquake to pose a tsunami hazard it needs to vertically move the seafloor; therefore it needs to be large (typically 8.0 or larger), under or near the ocean, and shallow within the earth (less than 100 km deep).